PANTOGRAPH, or Pantagraph (from the Greek rearm, all, and -ypbÆCV, to write), an instrument for making a reduced, an enlarged, or an exact copy of a plane figure.
In its commonest form it consists of' two long arms, AB and AC (fig. I), jointed together at A, and two short arms, FD and FE, jointed together at F and with the long arms at D and E; FD is made exactly equal to AE and FE to AD, so that Adfe is a parallelogram whatever the angle at A. The instrument is supported parallel to the paper on castors, on which it moves freelyA tube is usually fixed vertically at c, near the extremity of the long arm AC, and similar tubes are mounted on plates FIG. I.
which slide along the short arms BD and FD; they are intended to hold either the axle pin on a weighted fulcrum round which the instrument turns, or a steel pointer, or a pencil, interchangeably. When the centres of the tubes are exactly in a straight line, as on the dotted line bfc, the small triangle bfD will always be similar to the large triangle bcA; and then, if the fulcrum is placed under b, the pencil at f, and the pointer at c, when the instrument is moved round the fulcrum as a pivot, the pencil and the pointer will move parallel to each other through distances which will be respectively in the proportion of bf to be; thus the pencil at f draws a reduced copy of the map under the pointer at c; if the pencil and the pointer were interchanged an enlarged copy would be drawn; if the fulcrum and pencil were interchanged, and the sliders set for f to bisect bc, the map would be copied exactly. Lines are engraved on the arms BD and FD, to indicate the positions to which the sliders must be set for the ratios 2, 3.. ., which are commonly required.
The square pantograph of Adrian Gavard consists of two graduated arms which are pivoted on a plain bar and connected by a graduated bar sliding between them throughout their entire length, to be set at any required distance from the plain bar; a sliding plate carrying a vertical tube, to hold either the axle of the fulcrum, the pencil, or the pointer, is mounted on one of the arms and on a prolongation of the plain bar beyond the other arm, and also on the graduated connecting bar; and an additional arm is provided by means of which reductions below or enlargements above the scales given on the instrument can be readily effected.
The eidograph (Gr. eiOor, form) is designed to supersede the pantograph, which is somewhat unsteady, having several supports and joints. It is composed of three graduated bars, one of which is held over a fulcrum and carries the others, which are lighter, one at each extremity. The three bars are movable from end to end in boxsockets, each having an index and a vernier in contact with the graduated scale. The box-socket of the principal bar turns round the vertical axle of the fulcrum; that of each side bar is attached to a vertical;axle, which also carries a grooved wheel of large diameter and turns in a collar at either end of the principal bar. The two wheels are of exactly the same diameter and are connected by a steel band fitting tightly into the grooves, so that they always turn together through identical arcs; thus the side bars over which they are respectively mounted, when once set parallel, turn with them and always remain parallel. A pointer is held at the end of one of the side bars and a pencil at the diagonally opposite end of the other. The bars may be readily set by their graduated scales to positions in which the distances of the pencil and the pointer from the fulcrum will always be in the ratio of the given and the required map scales.
Numerous other modifications have been proposed from time to time; many forms are described in G. Pellehn's Der Pantograph (Berlin, 1903).
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