Pantograph

A pantograph is a device realizing a homothety. If a user draws a figure with one of the pens, the other pens will reproduce it dilated by a fixed ratio. This feature is a consequence of similar triangles. Before the emergence of computer controlled machines, it was commonly used to manipulate a milling machine: one the pen was replaced by the milling cuter.

The pantograph on the picture was made by Elena Willis during an internship under my supervision at the math department.

For other mechanical linkage see also the Sylvester machine