﻿ Power Transmission - Encyclopedia

To determine the scale on which a central station plant should be designed is frequently a difficult matter. The rate of growth of the expected demand for the power is an important factor, but it has been clearly established that the reduction of working expenses resulting from the increase of size of an undertaking proceeds in a diminishing ratio. Increase in output is in fact sometimes accompanied by more than a proportionate increase of expenses. During recent years there have been causes at work which have raised considerably the price of labour, fuel, other items of expense, and the law of the " diminishing ratio " has been masked.

On the diagram (fig. 3) of the costs of the London undertaking and the amount of power supplied, have been plotted points marking the total expenses of each year in relation to the output of power. These points for the years 1884-1899, and for output of from 50 to 700 million gallons followed approximately a straight line. Since 1899, however, though the output has increased from 708 millions to 1040 million gallons, the costs per unit of output have been always considerably above the preceding periods. The details of the London supply given in table I partly explain this by the relatively high price of fuel, but an equally important factor has been the rise in the local rates, which in the period1899-1909have risen from 2d. up to 3d. per moo gallons. If the cost of fuel, rates and wages had remained constant the plotting of expenses in relation to output would have been approximately along the extension of the line AB. This line cuts the vertical axis at A above the origin 0, and the line OA indicates the minimum amount of the expenses, and by implication the initial size of the first central station erected in London. The curve in this diagram gives the cost per moo gallons.

Whether it is more economical to have several smaller stations in any particular system of power transmission, or a single centre of supply, is mainly governed by the cost of the mains and the facilities for laying them in the area served. No general rule can, however, be formulated, for it is a question of balance of advantages, and the .t...m.e, ¦ ¦ ¦¦¦¦¦¦¦¦¦¦¦¦i¦¦¦¦¦ ' 'SS ' '? ' '?1¦¦ ¦¦¦¦¦ /i¦ ¦¦¦¦¦ ¦¦¦¦¦ t° ¦%¦¦¦¦ 111¦/iM¦¦¦¦¦¦¦¦ ¦¦¦ ¦¦¦¦ FIG. 3.

solution must be obtained by consideration of the special circumstances of each case. It has been found desirable as the demand for the power and the area within which it is supplied has enlarged, not only to increase the number of central stations but also their capacity. The first pumping station erected was installed with 4 pumping engines of 200 h.p. each. The pumping capacity of this station has been increased to 7 units. The station at Rotherhithe completed in 1904 has 8 units together 1600 h.p., and the plant at the new station at Grosvenor Road has 8 units equalling 2400 h.p. The pumping stations are situated about 3 m. apart and concurrently with the increase in their size it has been found desirable to introduce a system of feeder mains (see below).

There are in all five central stations at work in connexion with the public supply of hydraulic power in London, having an aggregate of 7000 i.h.p. All the stations and mains are connected together and worked as one system. There are 14 accumulators with a total capacity of 4000 gallons, most of them having rams 20 in. diameter by 23 ft. stroke. The pumping engines are able together to deliver 11,000 gallons per minute Details of the London supply are given in fig. 3 and in table .

TABLE I.

 E? a`"i.?.`l . Year. Gallons ? w E o o ° [ o o o ? ? d .a Pumped„ ? a ?

The load-factors are calculated on the actual recorded maximum output, and not on the estimated capacity of the plant running or installed. The daily periods of maximum output are shown in fig. 2. The table shows that the load-factors have not been much affected either by the increase of the area of supply or by the increased consumption of power. The coal used has been principally Durham small. The capital cost of the London undertaking has been about £950,000. In the central station at Wapping, erected in 1891, there are six sets of triple-expansion, surface-condensing vertical pumping engines of 200 i.h.p. each; six boilers with a working pressure of 150 lb per square inch, and two accumulators with rams 20 in. diameter by 23 ft. stroke loaded up to Boo lb per square inch. The engines run at a maximum piston speed of 250 ft. per minute, and the pumps are single-acting, driven directly from the piston rods. The supply given from this station in 1909 was approximately 6,800,000 gallons per week, and the cost for fuel, wages, superintendence, lighting, repairs and sundry station expenses 4.28d. per moo gallons, the value of the coal used being 14s. 11.3d. per ton in bunkers. The capital cost of the station, including the land, was £70,000. The load-factor at this station for 1909 was 49, and the supply was maintained for 168 hours per week. The conditions are exceptionally favourable, and the figures represent the best result that has hitherto been obtained in hydraulic power central station work, having regard to the high price of fuel.

The installation in Hull differs little from the numerous private plants at work on the docks and railways of the United Kingdom. The value of the experiment was chiefly commercial, and the large public hydraulic power works established since are to be directly attributed to the Hull undertaking. In Birmingham gas engines are employed to drive the pumps. In Liverpool there are two central stations. The working pressure is 850 lb per square inch. There are 27 m. of mains, and about 1100 machines at work. In Manchester and Glasgow the pressure adopted is Imo lb per square inch. In Manchester this pressure was selected principally in view of the large number of hydraulic packing presses used in the city, and the result has been altogether satisfactory. The works were established by the corporation in 1894, the central station being designed for 1200 i.h.p. Another station has since been built of equal capacity, and nearly 5 million gallons per week are being supplied to work about 2100 machines. Twenty-three miles of mains are laid.

In Antwerp a regular system of high-pressure hydraulic power transmission was established in 1894 specially to provide electric light for the city. The scheme was due to von Ryssleburgh, an electrical engineer of Ghent, who came to the conclusion that the most economical way of installing the electric light was to have a central hydraulic station, and from it transmit the power through pipes to various sub-stations in the town, where it could be converted by means of turbines and dynamos into electric energy. The coal cost of the electricity supplied-0.88d. per kw. hour - compares favourably with most central electric supply stations, although the efficiency of the turbines and dynamos used for the conversion does not exceed 40%. Von Ryssleburgh argued that hydraulic pumping engines would be more economical than steam-engines and dynamos, and that the loss in transmission from the central station to the, consumer would be less with hydraulic converters than if the current were distributed directly. The loss in conversion, however, proved to be twice as great as had been anticipated, owing largely to defective apparatus and to under-estimation of the expense of maintaining the converting stations; and the net result was commercially unsatisfactory.

At Buenos Aires hydraulic mains are laid in the streets solely for drainage purposes. Each of the sumps, which are provided at intervals, contains two hydraulic pumps which automatically pump the sewage from a small section of the town into an outfall sewer at a higher level. The districts where this system is at work lie below the general drainage level of Buenos Aires. The average efficiency (pump h.p. to i.h.p.) is 41%, which is high, having regard to the low heads against which the pumps work. In this application all the conditions are favourable to hydraulic power transmission. The work is intermittent, there is direct action of the hydraulic pressure in the machines, and the load at each stroke of the pumps is constant. The same system has been adopted for the drainage of Woking and district, and a somewhat similar installation is in use at Margate.

Hydraulic power is supplied from the hydraulic mains on a sliding scale according to the quantity consumed. The minimum charge in London except for very large quantities is is. 6d. per moo gallons. In moo gallons at 750 lb per square inch there is an energy of 10,000 X1730 8.74 h.p. hours; thus Is. 6d. per moo gallons = 2d. 33, 000 X 60 per h..p. hour nearly. This amount is made up approximately of 9d. per 1000 gallons for the cost of generation, distribution and general expenses including rates and 9d. for capital charges. The average rate charged to consumers in 1908 was about 2s. 4d. per moo gallons. Even under the most favourable circumstances it does not appear probable that hydraulic power at 750 lb per square inch can be supplied from central stations in towns on a commercial basis over any considerable areas at less than Is. per moo gallons. Allowing 70.0 0 75% as the efficiency of the motor through which the power is utilized, this rate would give I. 83d. per brake or effective h.p. hour. This cost seems high, and it is difficult to believe that it is the best hydraulic power transmission can accomplish having regard to the well-established fact that the mechanical efficiency of a steam pumping engine is greater than any other application of a steam-engine, and that the power can be conveyed through mains without any material loss for considerable distances. Still, no other system of power transmission except gas seems to be better off, and there is no method of transmission by which energy could, at the present time, be supplied retail in towns with commercial success at such an average rate when steam is employed as the prime mover. The average rate charged for hydraulic power in London and elsewhere FIG. 4.

is much the same as the average rate charged for the supply of electrical energy to the ordinary consumer. Gas is undoubtedly cheaper, but in a large number of cases is mechanically inconvenient in its application. Hydraulic pressure, electrical energy and compressed air (with reheating) can all be transmitted throughout towns with approximately the same losses and at the same cost, because the power is obtained in each system from coal, boilers, and steam-engines, and the actual loss in transmission can be kept down to a small percentage. The use of any particular system of power does not, however, primarily depend upon the cost of running the central station and distributing the power, but mainly upon the mechanical convenience of the system for the purpose to which it is applied. One form of energy is, in practice, found most useful for one purpose, another form for another and no one can command the whole field.

FIG. 5.

When water is employed as the fluid in hydraulic transmission the effects of frost must usually be provided against. In London Precautions and other towns, the water, before being pumped u Precat into the mains, is passed through the surface condensers against of the engines, so as to raise its temperature. The mains Frost. are laid 3 ft. below the surface of the ground. Exposed pipes and cylinders are clothed, and means provided for draining them when out of use. When these simple precautions are adopted damage from frost is very rare. In special cases oil having a low freezing point is used, and in small plants good results have been obtained by mixing glycerin and methylated spirit with the water.

A few gas jets judiciously distributed are of value where there is a difficulty in properly protecting the machinery by clothing. From the central station the hydraulic power must be transmitted through a system of mains to the various points at which it is to be used. In laying out a network of mains it is first neces- Distr96u- sary to determine what velocity of flow can be allowed. tion. Owing to the weight of water, the medium usually employed for hydraulic transmission, a low velocity is necessary in order to avoid shocks. The loss of pressure due to the velocity is 3 -'‘41141 FIG. 6. - Half section and elevation at AB. Detail of Io" steel pipe.

independent of the actual pressure employed, and at moderate velocities of 3 to 4 ft. per second the loss per woo yds. is almost a negligible quantity at a pressure of 700 lb per square inch. For practical purposes Box's formula is sufficiently accurate Loss of head - gallons' X length in yards There is a further (diameter of pipes in inches X3)5 loss due to obstruction caused by valves and bends, but it has been found in London that a pressure of 150 lb at the central accumulators is sufficient to ensure a pressure of 700 lb throughout the system. The greatest distance the power is conveyed from the central stations in London is about 4 m. The higher the initial velocity the more variable the pressure; and in order to avoid this variation in any large system of mains it is usual to place additional accumulators at a FIG. 6. - Half back elevation, half front elevation. Detail of Io" steel pipe.

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\?O ?' distance from the central station. They act in the same way as air-vessels. The mains should be laid in circuit, and valves placed at intervals, so that any section can be isolated for repairs or for making connexions without affecting the supply at other points. The main valves F adopted in London are shown in fig. 4. Valves are also fixed to control all branch pipes, while relief valves, washouts and air valves are fixed as required.

The largest pipes used in London are 10 in. internal diameter, and the smallest laid in the streets are 2 in. The pipes from 8 in. and below are usually made in cast iron, flanged and provided with spigots and faucets. The joint (fig. 5) is made with a gutta-percha ring, though sometimes asbestos and leather packing rings are used. Cast iron pipes for hydraulic power transmission have been standardized by the Engineering Standards Committee. Fig. 6 shows the u p in. steel main as used in London. The main was laid in 1903 from the Rotherhithe Pumping Station of the London Hydraulic Power Company through the Tower Subway, and is used as a feeder main for supply to the City. It is the first instance of the use of feeder mains in hydraulic transmission. The velocity of flow is 6 ft. per second, and is automatically disconnected from the general system should the pressure in this main fall below that of accumulator pressure. Other mains, similarly controlled, are now in use. Ellington's system of hydraulic feeder mains has been developed by the laying of a 6-in. steel main from the Falcon Wharf Station at Blackfriars to the Strand, over Waterloo Bridge.

In public supplies the power used is in all cases registered by meters, and since 1887 automatic instruments have been used at the central stations to record the amount supplied at each instant during the day and night. The ratio between the power registered at the consumers' machines and the power sent into the mains is the commercial efficiency of the whole system. The loss may be due to leakage from the mains or to defects in the meters; or if, as is often the case, the exhaust from the machines is registered, to waste on the consumers' premises. The automatic recorders give the maximum and minimum supplies during 24 hours every day, the maximum record showing the power required for a given number and capacity of machines, and the minimum giving an indication of the leakage. It has been found practicable to obtain an efficiency of 95% in most public power transmission plants over a series of years, but great care is required to produce so good a result. In some years 98% has been registered. Until 1888, no meters were available for registering a pressure of 700 lb per square inch, and all that could be done was to register the water after it had passed through the machines and lost its pressure. This method is still largely adopted; but now high-pressure meters give excellent results, exhaust registration is being superseded to a considerable extent by the more satisfactory arrangement of registering the power on its entry into the consumers' premises. In Manchester Kent's high-pressure meters are now used exclusively. Venturi meters have also been used with success for registering automatically the velocity of flow, and, by integration, the quantity in hydraulic power mains, and form a most useful check on the automatic recorders. The water after the pressure has been eliminated by passage through the machines, may run to a drain or be led back to the central station in return mains; the method adopted is a question of relative cost and convenience.

We proceed to the machines actuated by hydraulic power, and by a comparison of the useful work done by them with the work done by the engines and boilers at the central station Machinery. the mechanical efficiency of the system as a whole can be gauged. At the central station and in the distribution there is no great difficulty in determining the efficiency within narrow limits; it should be 80% at the point of entry to the machine in which the pressure is used.

Where feeder mains are in use the efficiency of the system is necessarily reduced, owing to the higher velocities allowable in the feeder mains. Mechanical efficiency is then sacrificed for the sake of economy. The mechanical efficiency of the machines is a very uncertain quantity; the character of the machines and the nature of the conditions are so variable that a really accurate general statement is impossible. In most cases the losses in the machine are practically constant for a given size and speed of working; consequently the efficiency of a given machine may vary within very wide limits according to the work it has to do. For instance, a hydraulic pump of a given capacity, delivering the water to an elevation of loo ft., will have an efficiency of 80%; but if the elevation of discharge is reduced to 15 ft., even though the hydraulicpressure rams may be proportioned to the reduced head, the efficiency falls below 50%. The ultimate efficiency of the system, or pump 1143"i.h.p., in the one case is 64%, and in the other under 40%. In crane or lift work the efficiency varies with the size of the apparatus, with the load and with the speed. Efficiency in this sense is a most uncertain guide. Some of the most useful and successful applications of hydraulic power - as, for instance, hydraulic capstans for hauling wagons in railway goods yards - have a very low efficiency expressed on the ratio of work done to power expended. Hydraulic cranes for coal or grain hoisting have a high efficiency when well designed, but it is now very usual to employ grabs to save the labour of filling the buckets, and their use lowers the efficiency, expressed in tons of coal or grain raised, by 33% or even 50%. When hydraulic machines are fully loaded, 50% to 60% of the indicated power of the central station engine is often utilized in useful work done with a radius of 2 or 3 m. from the station. In very favourable circumstances the efficiency may rise to over 70% and in a great many cases in practice it no doubt falls below 25%. If, however, energy in any form can be obtained ready for use at a moderate rate, the actual efficiency of the machines (i.e. the ratio of the useful work done to the energy absorbed in the process) is not of very great importance where the use is intermittent.

Hydraulic pressure is more particularly advantageous in cases where the incompressibility of the fluid employed can be utilized, as in hydraulic lifts, cranes and presses. Hydraulic machines for these purposes have the peculiar and distinct advantage of direct action of the pressure on the moving rams, resulting in simplicity of construction, slow and steady movement of the working parts, absence of mechanical brakes and greatest safety in action. When the valve regulating the admission of the pressure to the hydraulic cylinder is closed, the water is shut in, and, as it is FIG. 7.

incompressible, the machine is locked. Thus all hydraulic machines possess an inherent brake; indeed, many of them are used solely as brakes.

Hydraulic power transmission does not possess the flexibility of electricity, its useful applications being comparatively limited, but the simplicity, efficiency, durability and reliability of typical hydraulic apparatus is such that it must continue to occupy an important position in industrial development.

Sometimes a much higher pressure than 700 lb or moo lb per square inch is desirable, more particularly for heavy presses and for machine tools such as are used for riveting, for punching, shearing, &c. The development of these applications has been largely due to the very complete machinery invented and perfected by R. H. Tweddell. One of the principal installations of this kind was erected in 1876 at Toulon dockyard, where the machines are all connected with a system of mains of 21-in. bore and about 1700 yds. long, laid throughout the yard, and kept charged at a pressure of 1500 lb per square inch by engines of 100 h.p. with two large accumulators. Marc Berrier-Fontaine, the superintending engineer of the dockyard, stated that the economy of the system over the separately-driven geared machines formerly used is very great. But while pressures so high as 3 tons per square inch (as in the 12,000-ton ArmstrongWhitworth press) have been used for forging and other presses, it is not desirable, in the distribution of hydraulic power for general purposes, that 1000 lb per square inch should be much exceeded; otherwise the rams, which form the principal feature in nearly all hydraulic machines, if proportioned to the work required, will often become inconveniently small, and other mechanical difficulties will arise. The cost of the machinery also tends to become greater. In particular cases the working pressure can be increased to any desired extent by means of an intensifier (fig. 8).

An important application of hydraulic power transmission is for ship work, the system being largely adopted both in H.M. navy and for merchant vessels. Hydraulic coal-discharging machinery was fitted by Armstrong as early as 1854 on board a small steamer, and in 1868 some hopper barges on the Tyne were supplied with hydraulic cranes. A. Betts Brown of Edinburgh applied hydraulic power to ship work in 1873, and in the same year the first use of this power for gunnery work was effected by G. M.Rendel on H.M.S. " Thunderer." The pressure usu ally employed in H.M. navy is moo lb per square inch. Accumulators are not used and *W K the engines have to be fully equal to supply directly the whole demand. The distance through which the power has to be trans mitted is, of course, very short, and the high FIG. 8. velocity of 20 ft. per second is allowed in the main pipes. The maximum engine-power required under these conditions on the larger ships is very considerable. A recent development of hydraulic power on board ship is the Stone-Lloyd system of closing bulkhead doors. In hydraulic transmission of power it is usually the pressure which is employed, but there are one or two important cases in which the velocity of flow due to the pressure is utilized in the machine. Reference has already been made to the use of turbines working at 750 lb per square inch at Antwerp. The Pelton wheel has also been found to be adapted for use with such high pressures. Another useful application of the velocity due to the head in hydraulic transmission is in an adaptation of the well-known jet pump to fire hydrants. The value of the system of hydraulic transmission for the extinction of fire can hardly be overestimated where, as in London and most large towns, the ordinary pressure in the water mains is insufficient for the purpose.

## AuTxoRITIEs

Armstrong, Proc. Inst. C.E. (1850 and 1877), Proc. Inst. Mech. E. (1858 and 1868); Blaine, Hydraulic Machinery (1897); Davey, Pumping Machinery (1905); Dunkerley, Hydraulics (1907); Ellington, Proc. Inst. C.E. (1888 and 1893), Proc. Inst. Mech. E. (1882 and 1895), Proc. Liverpool Eng. Sic. (1880 and 1885); Greathead, Proc. Inst. Mech. E. (1879); Marks, " Hydraulic Power," Engineering (1905); Parsons, " Sanitary Works, Buenos Aires," Proc. Inst. C.E. (1896); Robinson, Hydraulic Power and Hydraulic Machinery (1887); Tweddell, Proc. Inst. C.E. (1883 and 1894), Proc. Inst. Mech. E. (1872 and 1874); Unwin, Transmission of Power (1894), Treatise on Hydraulics (1907). (E. B. E.) III. - Pneumatic Every wind that blows is an instance of the pneumatic transmission of power, and every windmill or sail that catches the breeze is a demonstration of it. The modern or technical use of the term, however, is confined to the compression of air at one point and its transmission to another point where it is used in motors to do work. The first recorded instance of this being done was by Denis Papin (b. 1647), who compressed air with power derived from a water-wheel and transmitted it through tubes to a distance. About 1800 George Medhurst (1759-1827) took out patents in England for compressing air. He compressed and transmitted air which worked motors, and he built a pneumatic automobile. William Mann in 1829 took out a patent in England for a compound air compressor. In his application he states: " The condensing pumps used in compressing I make of different capacities, according to the densities of the fluid to be, compressed, those used to compress the higher densities being proportionately smaller than those previously used to compress it to the first or lower densities,". &c. This is a very exact description of the best methods of compressing air to-day, omitting the very important inter-cooling. Baron Van Rathen in 1849 proposed to compress air in stages and to use inter-coolers between each stage to get 750 lb pressure for use in locomotives. For the next forty years inventors tried without success all manner of devices for cooling air during compression by water, either injected into the cylinder or circulated around it, and finally, with few exceptions, settled down to direct compression with no cooling worthy of mention. Only in the last ten years of the 19th century were the fundamental principles of economical air compression put into general practice, though all of them are contained in the patent of William Mann and the suggestion of Van Rathen.

The first successful application of compressed air to the transmission of power, as we know it, was at the Mont Cenis Tunnel in 1861. The form of compressor used was a system of water rams - several of them in succession - in which water was the piston, compressing the air upwards in the cylinder and forcing it out. Although the air came in contact with the water, it was not cooled, except slightly at the surface of the water and around the walls of the cylinders. The compressors were located near the tunnel, and the compressed air was transmitted through pipes to drilling machines working at the faces in the tunnel. Rotary drills were tried first, but were soon replaced by percussion drills adapted from drawings in the United States Patent Office, copied by a French and Italian commission from the patent of J. W. Fowle of Philadelphia. H. S. Drinker (Tunneling, Explosive Compounds and Rock Drills, New York, 1893) states positively that the first percussion drill ever made to work successfully was patented by J. J. Couch of Philadelphia in 1849. Shortly afterwards Fowle patented his drills, in which the direct stroke and self-rotating principle was used as we use it now. The first successful drill in the Hoosac Tunnel was patented in 1866 by W. Brooks, S. F. Gates and C. Burleigh, but after a few months was replaced by one made by Burleigh, who had bought Fowle's patent and improved it. Burleigh made a compressor, cooling the air during compression by an injected spray of water in the cylinders. The successful work in the Mont Cenis and Hoosac Tunnels with the percussion drilling machines caused the use of compressed air to spread rapidly, and it was soon found there were many other purposes for which it could be employed with advantage.

Dr Julius G. Pohle, of Arizona, patented in 1886, and introduced extensively, the use of compressed air for lifting water directly, by admitting it into the water column. His plan is largely adopted in artesian wells that do not flow, or do not flow as much as desired, and is so arranged that the air supply has a back pressure of water equal to at least half the lift. If it is desired to lift the water 30 ft. the air is admitted to the water column at least 30 ft. below the standing water surface. The air admitted being so much lighter than the water it displaces, the column 60 ft. high becomes lighter than the column 30 ft. high and is constantly released and flows out at the top. The efficiency of this method is only 20 to 40%, depending on the lift, but its adaptation to artesian wells renders it valuable in many localities.

The actual transmission of power by air from the compressor to the motor i., simple and effective. The air admits of a velocity of 15 to 20 ft. per second through pipes, with very slight loss by friction, and consequently there is no necessity for an expensive pipe system in proportion to the power transmitted. It is found in practice that, allowing a velocity as given above, there is no noticeable difference in pressure between the compressor and the motor several miles away. Light butt-welded tubing is largely used for piping, and if properly put in there is very slight loss from leakage, which, moreover, can be easily detected and stopped. In practice, a sponge with soap-suds passed around a joint furnishes a detective agency, the escaping air blowing soap bubbles. In good practice there need not be more than 1% loss through leakage and 1% possibly through friction in the pneumatic transmission of power.

Air develops heat on compression and is cooled by expansion, and it expands with heat and contracts with cold. For the purpose of illustration suppose a cylinder 10 ft. long containing io cub. ft. of air at 60° F., with a frictionless piston at one end. If this piston be moved 71 ft. into the cylinder, so that the air is compressed to onequarter of its volume, and none of the heat developed by compression be allowed to escape, the air will be under a pressure of 90 lb per square inch and at a temperature of 460° F. If this air be cooled down to 60° F. the pressure will be reduced to 45 lb per square inch, showing that the heat produced in the air itself during compression gives it an additional expansive force of 45 lb per square inch. The average force or pressure in compressing this air without loss of heat is 21 lb per square inch, whereas if all the heat developed during compression had been removed as rapidly as developed the average pressure on the piston would have been only i I lb per square inch, showing that the heat developed in the air during compression, when not removed as fast as developed, caused in this case an extra force of to lb per square inch to be used on the piston. If this heated air could be transmitted and used without any loss of heat the extra force used in compressing it could be utilized; but in practice this is impossible, as the heat is lost in transmission. If the piston holding the 22 cub. ft. of air at 45 lb per square inch and at 60° F. were released the air expanding without receiving any heat would move it back within 33 ft. of the end only, and the temperature of the air would be lowered 170° F., or to 110° F. below zero. If the air were then warmed to 60° F. again it would move the piston the remaining 33 ft. to its starting point.

It is seen that the ideal air-compressing machine is one which will take all the heat from the air as rapidly as it is developed during compression. Such " isothermal compression " is never reached in practice, the best work yet done lacking io % of it. It follows that the most inefficient compressing machine is one which takes away no heat during compression - that is, works by " adiabatic compression," which in practice has been much more nearly approached than the ideal. It also follows that the ideal motor for using compressed air is one which will supply heat to the air as required when it is expanding. Such " isothermal " expansion is often attained, and sometimes exceeded, in practice by supplying heat artificially. Finally, the most inefficient motor for using compressed air is one which supplies no heat to the air during its expansion, or works by adiabatic expansion, which was long very closely approached by most air motors. In practice isothermal compression is approached by compressing the air slightly, then cooling it, compressing it slightly again, and again cooling it until the desired compression is completed. This is called compression in stages or compound compression. Isothermal expansion is approximately accomplished by allowing the air to do part of its work (as expanding slightly in a cylinder) and then warming it, then allowing it to do a little more and then warming it again, and so continuing until expansion is complete. It will be seen that the air is carefully cooled during compression to prevent the heat it develops from working against compression, and even more carefully heated during expansion to prevent loss from cold developed during expansion. More stages of compression of course give a higher efficiency, but the cost of machinery and friction losses have to be considered. The reheating of air is often a disadvantage, especially in mining, where there are great objections to having any kind of combustion underground; but where reheating is possible, as W. C. Unwin says, " for the amount of heat supplied the economy realized in the weight of air used is surprising. The reason for this is, the heat supplied to the air is used nearly five times as efficiently as an equal amount of heat employed in generating steam." Practically there is a hotair engine, using a medium much more effective than common air, in addition to a compressed-air engine, making the efficiency of the whole system extremely high. (A. DE W. F.) IV. - Electrical Though the older methods of power transmission, such as wire ropes, compressed air and high-pressure water, are still worked on a comparatively small scale, the chief commercial burden has fallen upon the electric generator and motor linked by a transmission line. The efficiency of the conversion from mechanical power to electrical energy and back again is so high, and the facility of power distribution by electric motors is so great, as to leave little room for competition in any but very exceptional cases. The largest single department of electrical power transmission - that is, transmission for traction purposes - is at present almost wholly carried on by continuous currents. The usual voltage is 500 to 600, and the motors are almost universally series-wound constant-potential machines. The total amount of such transmission in daily use reaches probably a million and a half horse power. In long distance power transmission proper continuous currents are not used to any considerable extent, owing mainly to the difficulty of generating continuous currents at sufficient pressure to be available for such work, and the difficulty of reducing such pressure, even if it could be conveniently obtained, far enough to render it available for convenient distribution at the receiving end of the line. Single continuous current machines have seldom been built successfully for more than about 2000 to 3000 volts, if at the same time they were required to deliver any considerable amount of current. About 300 to Soo kilowatts per machine at this voltage appears to be the present limit, although it is by no means unlikely that the use of commutating poles and.

xxrr. 8 a other improvements may considerably increase these figures. For distances at which more than this very moderate voltage is desirable one must either depend on alternating currents or use machines in series. In American practice the former alternative is universally taken. On the continent of Europe a very creditable degree of success has been achieved by adopting the latter, and many plants upon this system are in use, mostly in Switzerland. In these generators are worked at constant current, a sufficient number in series being employed to give the necessary electromotive force.

## Power Transmission at Constant Current

In this system, which has been developed chiefly by M. Thury, power is transmitted from constant current generators worked in series, and commonly coupled mechanically in pairs or larger groups driven by a single prime mover. The individual generators are wound for moderate currents, generally between 50 and 150 amperes, and deliver this ordinarily at a maximum voltage of 2000 to 3500, the output per armature seldom being above 300 kw. For the high voltages needed for long distance transmission as many generators as may be required are thrown in series. In the Moutiers-Lyons transmission of i ro m., the most considerable yet installed on this system, there are four groups, each consisting of four mechanically-coupled generators. The common current is 75 amp., and the maximum voltage per group is about 15,000 volts, giving nearly 60,000 volts as the transmission voltage at maximum load. In the St Maurice-Lausanne transmission of about 35 m. the constant current is 150 amp. and the voltage per armature is 2300, five pairs being put in series for the maximum load voltage of 23,000.

Regulation in such plants is accomplished either by varying the field strength through an automatic governor or by similarly varying the speed of the generators. Either method gives sufficiently good results. The transmission circuit is of the simplest character, and the power is received by motors, or for local distribution by motor generators, held to speed by centrifugal governors controlling fieldvarying mechanism. For large output the motors, like the generators, are in groups mechanically coupled and in series. In the MoutiersLyons transmission motor-generators are even designed to give a three-phase constant potential distribution, and in reverse to permit interchange of energy between the continuous current and several polyphase transmission systems.

The advantages of the system reside chiefly in easier line insulation than with alternating currents and in the abolition of the difficulties due to line inductance and capacity. It is probably as easy to insulate for 100,000 volts continuous current as for 50,000 volts alternating current. Part of the difference is due to the fact that in the latter case the crest of the E.M.F. wave reaches nearly 75,000 volts, and in addition static effects and minor resonant rise of voltage must be reckoned with. There is some possibility, therefore, of the advantageous use of continuous current in case very great distances, requiring enormous voltages, have to be covered. In addition, a constant current plant is at full voltage only at brief and rare periods of maximum load instead of all the time, which greatly increases the average factor of safety in insulation.

On the other hand, the constant current generators are relatively expensive and of inconveniently small individual output for large transmission work, and require very elaborate precautions in the matter of insulation. Their efficiency is a little less than that of large alternators, but the difference is partially off-set by the transformers used with the latter for any considerable voltage. A characteristic advantage of the constant current system is the extreme simplicity and cheapness of the switching arrangements as compared with the complication and cost of the ordinary switch-board tor a polyphase station at high voltage. Comparing station with station as a whole it is at least an open question whether the polyphase system would have any material advantage in cost per kw. in an average case. The principal gains of the alternating systems appear in the relative simplicity of the distribution. In dealing with a few large power units the constant current system has the best of the argument in efficiency, but in the ordinary case of widespread distribution for varied purposes the advantage is quite the other way.

The high-voltage constant-current plant lends itself with especial ease to operation, at least in emergency, over a grounded circuit. In some recent plants, e.g. Moutiers-Lyons, provision is made at the sub-stations for grounding the central point of the system and either line in case of need, and in point of fact the voltage drop in working grounded is found to be within moderate and practicable limits.

The possibilities of improvement in the system have by no means been worked out, and although it has been overshadowed by the enormous growth of polyphase transmission it must still be considered seriously.

## Transmission by Alternating Current

The alternating current has conspicuous advantages. In the first place, whatever the voltage of transmission, the voltage of generation and that of distribution can be brought within moderate limits at a very high degree of efficiency by the use of transformers; and, in the second place, it is possible to build alternating-current generators of any required capacity, and for voltages high enough to permit the abolition of raising transformers except in unusual circumstances. At present such generators, giving ro,000 to 13,500 volts directly from the armature windings, are in common and highly successful use; and while the use of raising transformers is preferred by some engineers, experience shows that they cannot be considered essential, and are probably not desirable for the voltages in question, which are as great as at the present time seem necessary for the numerical majority of transmission plants. Polyphase generators, especially in large sizes, can be successfully wound up to more than double the figures just mentioned. The plant at Manojlovac, Dalmatia, has been equipped with four 30,000 volt three-phase generators, giving each about 5000 kw. at 42 with 420 revolutions per minute, the full load efficiency being 94%. But for very large transmission work to considerable distances where much higher voltages are requisite such transformers cannot be dispensed with. Alternating currents are practically employed in the polyphase form, on account both of increased generator output in this type of apparatus and of the extremely valuable properties of the polyphase induction motors, which furnish a ready means for the distribution of power at the receiving end of the line. As between twoand three-phase apparatus the present practice is about equally divided; the transmission lines themselves, however, are, with rare exceptions, worked three-phase, on account of the saving of 25% in copper secured by the use of this system. Inasmuch as transformers can be freely combined vectorially to give resultant electromotive forces having any desired magnitudes and phase relations the passage from twophase to three-phase, and back again, is made with the utmost ease, and the character of the generating and receiving apparatus thus becomes almost a matter of indifference. As regards such apparatus it is safe to say that honours are about even: sometimes one system proves more convenient, sometimes the other. The difficulty of obtaining proper single-phase motors for the varied purposes of general distribution has so far prevented any material use of single-phase transmission systems.

## Generators for Power Transmission

The generators are usually large twoor three-phase machines, and in the majority of instances they are driven by water-wheels. Power transmission on a large scale from steam plant has, up to the present, made no substantial progress, save as the networks of large electrical supply stations have in some cases grown to cover radii of many miles. The size of these generators varies from 100 or 200 kw. in small plants, up to 10,000 or more in the larger ones. Their efficiency ranges from 92% or thereabouts in the smaller sizes up to 96% or a fraction more in the largest, at full load. The voltage of these generators varies greatly. When raising transformers are used it is usually from 500 to 2500 volts; without them the generators are usually wound for 10,000 to 13,500 volts. Intermediate voltages have sometimes been employed, but are rather passing out of use, as they seem to fulfil no particularly useful purpose. The tendency at the present time, whatever the voltage, is towards the use of machines with stationary armatures and revolving field magnets, or towards a pure inductor type having all its windings stationary. At moderate voltages such an arrangement is merely a matter of convenience, but in high-voltage generators it is practically a necessity. Low-voltage machines are usually provided with polyodontal windings, these windings having several separate armature teeth per pole per phase, while the high-voltage machines are generally monodontal; in both classes the edges of the pole pieces are usually chamfered away in such form as to produce at least a close approximation to the sinusoidal form for the electromotive force. For this purpose, and to obtain a better inherent regulation under variations of load, the field magnets are, or should be, particularly powerful. In the best modern generators the variation of electromotive force from no load to full load, non-inductive, is less than to % at constant field excitation. Closeness of inherent regulation is an important matter in generators for transmission work.

inasmuch as there is as yet no entirely successful method of automatic voltage regulation on very large units; and the less hand regulation the better. Moreover, the design which secures this result also tends to secure stability of wave form in the electromotive force, a matter of even greater importance. There has been much discussion as to the best wave form for use on alternating circuits, it having been conclusively shown that for a given fundamental frequency the sinusoidal wave does not give the most economical use of iron in the transformers. For transmission work, however, particularly over long lines, this is a matter of inconceivably small importance compared with the stability and the freedom from troubles from higher harmonics that result from the use of a wave as nearly sinusoidal as can possibly be obtained. In every alternating circuit the odd harmonics are considerably in evidence in the electromotive force, either produced by the structure of the generator or introduced by the transformers and other apparatus. These are of no particular moment in work upon a small scale, but in transmission on a large scale to long distances, or especially through underground cables, they are, as will be seen in the consideration of the transmission line itself, a serious menace. Inasmuch as the periodicity of an alternating circuit must be maintained sensibly constant for successful operation, great care is usually exercised to secure such governing of the prime movers as will give constant speed at the generators. This can now be obtained, in all ordinary circumstances, by several forms of sensitive hydraulic governors which are now in use. The matter of absolute periodicity has not yet settled itself into any final form. American practice is based largely upon 60 cycles per second, which is probably as high a frequency as can be advantageously employed. Indeed, even this leads to some embarrassment in securing good motors of moderate rotative speed, and the tendency of the frequency is rather downward than upward. An inferior limit is set by the general desirability of operating incandescent lamps off the transmission circuits. For this purpose the frequency should be held above 30 cycles per second; below this point, flickering of the lamps becomes progressively more serious, especially with lamps having the very slender metallic filaments now commonly employed - so serious, indeed, as practically to prohibit their successful use - and plants installed for such low frequencies are generally confined to motor practice, or to the use of synchronous converters, which are somewhat easier to build in large units at low than at high periodicities. Occasional plants for railway and heavy motor service operate at as low as 15 -, and more at 25 Nearly all the general work of power transmission, however, is carried on between 30 and 60 -. The inferior limit at which it is possible successfully to operate alternating arc lamps is about 40 -; and if these are to be an important feature in transmission systems the indications are that practice will tend towards a periodicity above 40 - at at which all the accessory apparatus can be successfully operated. European practice is based generally upon a frequency of 50 -, which admirably meets average conditions of distribution.

With respect to line construction the introduction of high voltages, say 40,000 and upwards, has made a radical change in the situation. The earlier transmission lines were for rather low voltages, seldom above 10,000. Insulation was extremely easy, and the transmission of any considerable amount of power implied heavy or numerous conductors. The line construction therefore followed rather closely the precedents set in telegraph and telephone construction and in low tension electric light service. In American practice the lines were usually of simple wooden poles set 40 to 50 to the mile, and carrying wooden cross-arms furnished with wooden pins carrying insulators of glass or porcelain. The poles were little larger than those used in telegraph lines, a favourite size being a 40-ft. pole about 8 in. in diameter at the top and 15 in. at the butt, set 6 to 7 ft. in the earth. Such poles commonly bore two crossarms, the lower and longer carrying 4 pins, and the shorter upper a'rm 2 pins, so disposed that the upper pin on each side of the pole would form with the nearer pins below an equilateral triangle 18 to 24 in. on the side. The poles therefore carried two threephase circuits one on either side, one or both circuits being spiralled. In European practice iron poles have been more frequently used, again following rather closely the model of telegraph practice, with similar spacing of poles, and with insulators, usually of porcelain, somewhat enlarged and improved over telegraph and electric light insulators, and spaced somewhat more widely. As between wooden and steel poles, the latter are of course the more durable and much the more costly. The difference in cost depends largely on the locality, and ultimately on the life of the wooden poles. This ranges from two or three up to ten or fifteen years, the latter figures only in favourable soils and when the lower ends of the poles have been thoroughly treated with some preservative. Under such conditions wood is often ultimately the cheaper material.

The use of very high voltages results in, for all moderate powers, the use of small and consequently light wires and in the necessity for heavy, large and costly insulators. For security against leakage and failure it becomes desirable to reduce the numer of insulation points, and with the resulting lengthening of span to design the line as a mechanical structure. A transmission line is subject to three sets of stresses. The most considerable are those due to the longitudinal pull of the catenary depending on the weight and tension of the wires. Under ordinary conditions these strains are balanced and come into play only when there is breakage of one or more wires and consequent unbalancing. It has been the common practice to give the poles sufficient strength to withstand this pull without failing. The maximum amount of the pull may be safely taken at the sum of the elastic limits of the wires, since it is unsafe so to design the spans as to be subject to larger stresses.

There is also lateral stress on a line due to wind acting upon the poles and wires, the latter amounting to little unless their diameter is increased by a coating of sleet, a condition which gives maximum stresses on the line. Wind then tends to push the line over, and it also increases the longitudinal stresses, being added geometrically to the catenary stress. The actual possibility of wind pressure is very generally over-estimated, and has resulted in much needlessly costly construction. In the first place, save for actual tornadoes, for which no estimates can be given, even the highest winds at the level of any ordinary transmission line are of modest actual velocity. It is probable that no transmission line save on mountain peaks at a very high elevation is ever exposed to an actual wind velocity of 75 m. per hour, and only at intervals of years is a velocity of even 60 m. reached near the ground level. Further, the maximum wind velocities are practically never reached at very low temperatures when the line is under its maximum catenary stress, and sleet formation, which takes place only within a very limited temperature range, is practically unknown under conditions of maximum wind.

The relation of wind velocity to pressure in case of a suspended wire or cable may be approximately expressed by the equation P=oo025V 2, where P is the pressure per square foot of projected area of cable, and V is the actual wind velocity in miles per hour. Except for sleet conditions the wind pressure is, then, a matter of little concern. At times sleet may accumulate on bare wires to a thickness of half an inch to an inch. Even under these conditions the lateral stability of the line is a matter of less concern than the added component of stress in the catenary. The third element of line stress, the actual crushing stress of the wire load, is of no consequence in high voltage transmission work.

In scientific line design the best example has been set by the Italian engineers, who, realizing that the longitudinal strains, which are very severe in case of breakage of spans rigidly supported from pole to pole, are immediately relieved by a slight increase in catenary drop, have introduced the principle of longitudinal flexibility. The poles or towers of structural steel are so designed as to be fairly stiff against lateral pressure and are given secure foundation against overturning, but are deliberately designed to deflect lengthwise the line in the extreme case of breakage of wires so as at once to relieve the catenary tension without passing their elastic limit. In this way complete security is attained with a minimum of material and expense.

In recent construction both in America and Europe the tendency is to use steel poles or towers of ample height, 40 to 60 ft. and spans ranging from 300 to 600 ft., occasionally more. The catenary drop allowed is considerable, often 3 to 4% of the span length. Crossarms and pins, when used, are commonly of iron or steel, and the interiors of the insulators are therefore fairly at earth potential. The insulators are of dense and hard-baked porcelain, built up of three or four shells cemented together to form a whole, with several deep petticoats to protect the inner surfaces from wetting. Such insulators may be 12 to 18 in. in diameter over all, and from top groove to base a little more. If well designed and made, insulators of this type can endure even under very heavy precipitation alternating voltages of 60,000 to 100,000 effective without flashing over, and double these figures when dry. For line voltages above 60,000 to 70,000 it is apparent that the insulating factor of safety would be seriously reduced, and some recent lines have been equipped with suspension insulators. These are in effect porcelain bells from 10 in. diameter upward strung together like a string of Japanese gongs. The bells are all the same size and are spaced about a foot apart, the suspensions being variously designed. These insulating groups can be as large as need be, and it is easy to push the aggregate insulation resistance, both dry and wet, far beyond the figures just mentioned. This suspension requires higher poles than the ordinary, but allows a considerable amount of longitudinal back lash, in case a wire burns off. Too extensive slip along the line is checked by guys fitted with strain insulators, like the suspension ones, at suitable intervals. The suspension insulator gives promise of successful use of voltages much higher than 100,000 volts. The wires on high voltage systems are generally widely spaced: very seldom less than 2 ft. between centres, and for the higher voltages something like 1 ft. for each 10,000 volts.

## Voltage

The most important factor in the economy;pf the conducting system is the actual voltage used for the transmission. This varies within very wide limits. For transmissions only a few miles in length the pressures employed may be from 2000 to 5000 volts, but for the serious work of power transmission less than 10,000 volts are now seldom used. This pressure, under all ordinary conditions and in all ordinary climates, can be and is used with complete success, and apparently without any greater difficulty than would be encountered at much lower voltage. It is regarded as the standard transmission voltage in American practice for short distances up to Jo or 15 m. Beyond this, and sometimes even on shorter lines, it is greatly increased; up to 20,000 volts there seems to be no material difficulty whatever in effecting and maintaining a sufficient insulation of the line. In the higher voltages there were in 1908 more than fifty plants in regular operation at 40,000 volts and above. Of these more than a score are operated at 60,000 volts and above. The highest working voltage employed in 1909 was 110,000 volts, which was successfully used in two American plants: that of the Grand Rapids - Muskegon (Michigan) system,and in the transmission work of the Central Colorado system. These both employ suspension insulators with five bells in series, and operate with no more trouble than falls to the lot of systems using ordinarily high voltages. The Rio de Janeiro transmission system, operates at 88,000 volts with large porcelain insulators, 17.5 in. in over-all diameter and 19.75 in height, carried on steel pins; the Kern River (California) plant at 75,000 volts with similar construction; the Missouri River Power Co. (Montana) at 70,000 volts, using glass insulators on wooden pins saturated with insulating material. There is no especial difficulty in building transformers for still higher pressures, the real problem lying in the insulation of the line. Taken as a whole these high voltage lines have given good service, those near the upper limit doing apparently as well as those near the lower, owing to more careful precautions in construction. Likewise the distances of transmission have steadily risen. There are, all told, nearly a score of power transmissions over 100 m. in length, the longest distance yet covered being from De Sabla to Sausalito (California), a distance of 232 m. This, like most other long American transmissions, is at 60 -, and it is interesting to note that even over such distances there seems to be very little evidence of trouble due to frequency. In point of fact, those who have had the most experience with long distance transmission are the last to worry about the difficulties of using alternating current. Some unusual phenomena turn up in high voltage work, but they are rather interesting than alarming. The lines become self-luminous from "coronal " discharge at a little above 20,000 volts, and at 40,000 or 50,000 volts the phenomenon, which is sometimes aggravated by resonance, becomes of a striking, not to say startling, character. At above: 100,000 volts this coronal discharge must be given serious consideration.

 Size No. Diameter. L. C. inch. 0000 0.460 0.00312 0.0167 000 0.410 0.00322 0.0164 00 0.365 0.00328 0.0160 0 0.325 0.00336 0.0157 I 0.289 0.00338 0.0154 2 0.258 0.00347 0.0151 3 0.229 0.00351 0.0148 4 0.204 0.00358 0.0145

Resonance, in substance, is. due to synchronism of the periodic electromotive force, or a harmonic thereof, with the electro-magnetic time-constant of the system. The frequency of the currents actually employed in transmission work is so low that resonance with the fundamental frequency must be extremely rare; resonance with the harmonics is, however, common - much commoner than is generally supposed. In every electromotive force wave the odd harmonics are more or less in evidence, particularly the third, fifth and seventh. If the electromotive force wave departs notably from a sinusoidal form, traces of harmonics up to at least the 15th may generally be found; the third, seventh and the alternate higher harmonics are manifest in flattening the crest of the wave. Supposing, what is seldom quite true, that the harmonics are symmetrically disposed in phase with the fundamental, all the harmonics tend somewhat to elevate the shoulders of the wave; a wave, therefore, with peaked shoulders and a depression in the centre is certain to be affected by harmonics, while if it has a high central crest, there is evidence of great predominance of the fifth and higher harmonics. Generally the harmonics are slightly out of phase with the fundamental, so that the wave is both deformed and unsymmetrical. As to the amplitude of these harmonics, the third is usually the largest, and may sometimes in commercial machines amount to as much as 20% of the amplitude of the fundamental, and frequently 10%. In machines giving nearly sinusoidal waves it is of course much less, but it is not difficult to find even the seventh and higher harmonics producing variations as great as 5%. Since, other things being equal, the rise in electromotive force due to resonance is directly proportional to the magnitude of the harmonics, and the chance of getting it increases rapidly with the presence of those of the higher orders, the desirability of using the closest possible approximation to a sinusoidal wave is self-evident. The greater the inductance and capacity of the system and the less its ohmic resistance, the greater the chance of getting serious resonance. As regards the distributed capacity and inductance due to the line alone, the ordinary conditions are not at all formidable; the general effect of such distributed capacity and inductance is to produce in the system a series of static waves, their length varying inversely with the frequency. At commercial frequencies the wave length is very great, so great that even in the longest lines at present employed only a small fraction of a single wave length appears; the total length of the line is generally much less than one quarter the complete wave length, and the only notable effect is a moderate rise of potential along the line. The time-constant of the alternating circuit is T - 00629 (LC), where L is the absolute self-induction in henrys and C the capacity in microfarads; and if the frequency, or a marked harmonic thereof, coincide with this time-period, resonance may safely be looked for, and resonance of the harmonics may appear conspicuously in lines of ordinary lengths. The following table gives the values, both L and C, per mile of three-phase circuit, of the sizes (American wire-gauge) ordinarily employed for transmission circuits, the wires being assumed to be strung 48 in. apart and about the height already indicated: - In cases where underground cables form a part of the system, the above values of C are very largely increased, and the probability of resonance is in proportion enhanced. A still further complication is introduced by the capacity and inductance of the apparatus used upon the system, which may often be far greater than that due to the entire line, even if the latter be of considerable length. In point of fact, it is altogether probable that resonance due to the distributed capacity and inductance of the overhead line along is of rare occurrence and generally of trivial amount, while it is equally probable that resonance due to localized capacity and inductance other than that of the line conductors may, and often does, cause very serious disturbances upon the system. The subject has never been adequately investigated, but the tendency towards formidable sparking and arcing at various points on long-distance transmission systems is generally far greater than can be accounted for by consideration of the nominal voltages alone. The conditions may be still further complicated by the effect of earths or open circuits, which sometimes may produce, temporarily, appalling resonance phenomena, through bringing into action the capacity and inductance of the apparatus and introducing surges. In ordinary working the resonance of the harmonics is not very conspicuous, and the fact that it occurs not systematically, but only in special ways and under special conditions, indicates more strongly than anything else that the vital point is not the time-constant of the line alone, but those of the apparatus connected thereto. A definite and persistent tendency towards resonance may sometimes be effectively checked by the introduction of suitable inductance in the parts of the system most seriously affected, but the best general policy is to avoid as far as possible the presence of the higher harmonics, which are the chief sources of danger.

Closely allied to and connected with resonance is the phenomenon known as " surging," which is due to the discharge of the electromagnetic energy stored in a circuit containing inductance and capacity when that circuit is broken. This discharge is an oscillatory one, going on with decreasing amplitude until it is frittered away by resistance and other sources of loss. Its frequency is that of the system affected, and the surge may get reinforcement from resonance proper. It is sufficiently serious on its merits, however, since the resulting rise of voltage increases directly with the current and may produce terrific results when the break comes as the result of a short circuit. Minor surging occurs when there is a sudden and violent change in the conditions of the circuit even without an actual break. Such a change produces an impulsive redistribution of energy that may give a sharp rise in voltage. Every point of abrupt variation in the electrical constants on the system is liable to be affected by minor surges. Such disturbances when trivial are commonly referred to as " static." Surging, depending as it does on the current ruptured, may, and indeed often does, give particularly formidable effects on circuits of moderate voltage, while on high voltage transmission circuits the usually moderate current and the large margin of safety in the insulation are important ameliorating influences.

## Maintenance

Transmission lines are, when practicable, laid out through open country, and along roads which furnish easy access for inspection and repairs. The chief sources of danger in temperate climates are mechanical injury from the falling of branches of trees across the circuits, sleet and wind storms, and lightning. The firstmentioned difficulty may be avoided by keeping clear, so far as possible, of wooded country, and it should be remembered that, at the voltages customarily used for transmission, a twig the size of a lead-pencil falling across the wires may set up arcing, and it will end by burning the wires completely off - not directly by fusion, but by persistent arcing. A properly constructed overhead line is practically safe against all storms, save those of most extraordinary violence, and with care may be made secure even against these. As a matter of practice, interruptions of service upon transmission systems are very rarely due to trouble upon the main line itself, but are far more likely to occur in some part of the distributing system. The most dangerous combination of circumstances is a sleet storm sufficient to coat the wires with ice, followed by heavy winds; if the line, however, is constructed with proper factors of safety, bearing this particular danger in mind, there need be very little fear of serious results. Lightning is a much more formidable enemy. The lightning discharges observed upon electric circuits are of two general descriptions: first, a direct discharge of lightning upon the line, more or less severe, and always to be dreaded; and secondly, induced discharges due to lightning flashes which do not hit the line, or to static disturbances which may or may not produce actual lightning. Discharges of the former class are vastly more severe than those of the latter, and, fortunately, are somewhat rare. They may actually shatter the line, or may distribute themselves along it for a considerable distance, leaping from wire to pole, and thence to earth, without actually damaging the line to any marked degree. The induced discharges are felt principally in the apparatus, causing many of the burn-outs observed in transformers and generators. There is no complete protection against the effects of lightning upon the apparatus. Even the best lightning arresters are palliatives rather than preventives. If, however, a number of arresters are put in parallel, with reactance coils between them on the way towards the apparatus, the vast majority of lightning discharges, to whatever cause they may be due, will be deflected harmlessly to earth. Moreover, the apparatus itself teas a considerable power of resistance, due to its high insulation. The ends of the line should be very thoroughly protected by such lightning arresters, and other points, such as prominent elevations along the line, should receive similar additional protection. In some cases a substantial steel-wire cable stretched along the tops of the poles several feet above the line wires and well grounded at frequent intervals has been found very advantageous. With the best protection at present available, lightning is not a serious menace to continuity of service, and the apparatus of the distributing system is far more difficult to protect than the main line and its apparatus.

## Sub-stations

In most long-distance transmission work the transmission line itself terminates in a sub-station, which bears to the general distribution system precisely the same relations which are borne by a central electric supply station to its distributing lines. Such a sub-station should be treated, in fact, as a central station, receiving its electric energy from a distance instead of employing local generators driven by prime movers. The design of the substation, however, is somewhat different from that of the ordinary central station. The transmission lines terminate generally in a bank of reducing transformers, bringing the voltage from the 10,000 or higher voltage employed upon the line to the 2000 or more generally used in the distribution. These transformers are usually large, and their magnitude should be determined by the same considerations which apply to determining the size of the units to be employed in a generating station. The general rule to be followed is that the separate units shall be of such size that one of them may be dispensed with without serious inconvenience. In the case of transformers, the unit in twoor three-phase working is the bank of transformers, which must be used together. In Continental practice three-phase reducing transformers are frequently made to include all three phases in a single structure; this practice is less frequently followed in American plants, separate transformers being more often used in each phase. In this case, two or three transformers, according as the twoor three-phase system is used, constitute a single transformer unit in the sense just mentioned. If a change is to be made from three-phase line to two-phase distribution, the change is made by the appropriate vector connexion of the transformers. The full-load efficiency of large sub-station transformers is commonly 97 to 98%. In any case, the sub-station is furnished with voltage regulating appliances, to enable the voltage upon the distribution lines to be held constant and uniform. These regulators are, in practice, transformers with a variable transformation ratio. This is obtained in divers ways - sometimes by changing the inductive relations of the primary and secondary coils, sometimes by changing the relative number of effective turns in primary and secondary. Sets of these inductive regulators enable the voltage to be controlled over a sufficiently wide range to secure uniform potential on the system, and .with a degree of delicacy that obviates any undesirable changes in voltage. The regulation is usually manual, no automatic regulator yet having proved entirely satisfactory. In very large systems it is worth noting that the so-called " skin effect " in alternating current conductors may become conspicuous. In the transmission circuits themselves the wires are, in practice, never large enough to produce any sensible difference in conductivity for continuous and for alternating currents. In the heavy omnibus-bars of a large sub-station this immunity may not be continued, but in such cases flat strips are frequently employed. If these are not more than, say, a centimetre in thickness, the " skin effect " is practically insignificant for all frequencies used commercially. Not infrequently the sub-station also contains devices for the changing of alternating to continuous current, usually synchronous converters feeding either traction system or electric lighting mains. Beyond these converters the system becomes an ordinary continuous-current system, and is treated as such. When very close regulation is necessary, motorgenerators are often preferred to synchronous converters. Series arc lighting from transmission circuits is a much more serious problem. At the present time two methods are in vogue: first, the operation of continuous-current series-arc machines by synchronous or induction motors driven from the transmission system; and, secondly, series alternating apparatus for feeding alternating arcs. This apparatus consists either of constantcurrent transformers with automatically moving secondaries, or of inductive regulators, also automatic in their action, supplemented by transformers to supply them with the necessarily rather high voltage employed for arc distribution. As between these two systems practice is at present divided; electrically, the alternating apparatus gives a rather higher real efficiency, but involves the use of alternating arcs, which are somewhat less efficient, watt for watt, as light producers than the continuous-current arcs. The apparatus, however, requires practically no care, while the arc machines, driven by motors, require the same amount of care as if they were driven by other power. Arc light transformers, however, are likely to have low power factors, hardly above o8 at full load, and rapidly falling off at lower loads. Synchronous rectifiers changing the alternating current into a unidirectional current, suitable for use with arc lights, have been employed with some success, but not to any considerable extent. They, are satisfactory in avoiding the use of alternating currents in the arc, and consume but little energy in the transformation from one form of current to the other, but involve the use of static transformers automatically giving constant current, which are somewhat objectionable on the score of lowpower factor. Mercury rectifiers are now used rather extensively and give excellent results, although they are as yet of somewhat uncertain life, and, like the synchronous rectifiers, require special transformers when worked at constant current. In Continental practice arc - lights are almost universally worked off constant potential circuits, and hence the difficulties just considered are for the most part peculiar to American systems.

## Distances of Transmission

The ultimate determining factor in the distance to which power can be commercially transmitted is the economic side of the transmission, the maximum distance being the maximum distance at which the transmission will pay. As a mere engineering feat the transmission of power to a distance of many hundred miles is perfectly feasible, and, judging from the data available, the phenomena encountered in increasing the length of lines have not been of such character as to cause any hesitation in going still farther, provided the increase is commercially feasible. In American practice, it is within the truth to say that nearly all transmissions of reasonable size (say a few hundred kilowatts) to distances of twenty miles, or less, are pretty certain to pay. At distances up to fifty miles, in a large proportion of cases power can be delivered at prices which will enable it to compete with power locally generated by steam. From fifty to one hundred miles (on a large scale - several thousand kilowatts) the chances for commercial success are still good. The larger the amount of power transmitted, the better on the whole is the commercial outlook. The longest one yet operated has already been noted, and may be regarded as a commercial success. In certain localities where the cost of fuel is extremely high, transmissions of several hundred miles may prove successful from a commercial as well as an engineering standpoint, but the growth of industry, which indicates the necessity for such a transmission, may go on until, through improved facilities of transport, the cost of fuel may be greatly lowered and the economic conditions entirely changed. Such a modification of the conditions sometimes takes place much more quickly than would be anticipated at first sight, so that when very long distance transmissions are under consideration, the permanence of the conditions which will render them profitable should be a very serious subject of consideration. (L. BL.)

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