Eidograph has the simple function of copying maps and plots to a different scale. It works on the same principles as a pantograph with the only difference of mechanism. The pantograph uses linkages arranged in a parallelogram while the eidograph might have either a string and pulley system or parallelogram linkages.
There are two pens attached. If you move the primary pin on the map, after setting a desired scale, the secondary pen draws the same map scaled to the scale you set.