Amsler is also credited with conceiving a design for a planimeter that restricted the movement of the end of the tracer arm opposite the tracer point to a straight line. The collections suggest that Americans were particularly interested in further developing linear planimeters, with examples patented by John Coffin, Edward Jones Willis, and Frederick R. Williams. Another linear planimeter sold by an American company and an instrument that does not fit any of the categories of planimeters are also included on this page.
In 1875, Danish mathematician and cavalry officer Holger Prytz came up with a final form of planimeter—the hatchet planimeter. It simply consists of a rod with a tracer point at one end and a chisel edge at the other. As the tracer goes around the drawing, the chisel makes a zigzag path. The product of the length of the path and the length of the rod gives the area of the drawing. The result, though, is not as accurate as those provided by more complex forms of planimeters. At present, no examples of this instrument are known to be among the mathematics collections.
