Pantograph is an instrument for reproducing a drawing on a different scale from the original. A very usual form of it consists of four rods, A f, A E, ob, o c, jointed at B A c o.
Points E and F are chosen (according to the requirements of the drawing) such that E o F is a, straight line. The point O is fixed and, however the parallelogram may be moved, the ratio of OE to OF is unaltered, since it is equal to the fixed rate EC/CA. If a pointer be fixed at E and made to travel over a drawing, a pencil at F will trace out an exactly similar drawing, the linear dimensions of the two drawings being in the ratio of EO/OF.