A physics demonstration in the 1870s of the isochronism of the cycloidal pendulum. An ordinary pendulum consisting of a weight swinging on a string is not isochronous; meaning its period is not quite independent of the width of the swing but gets slightly longer as the maximum angle of swing increases. In 1673 Christiaan Huygens showed that this was due to the circular arc of the pendulum; if the pendulum bob swung in a cycloid curve it would be isochronous. This demonstration shows a pendulum where the cord is limited by curving cycloid-shaped pieces of wood "(EOD)" forcing the pendulum bob to follow a cycloid curve. By timing its swing, it can be shown that this pendulum is isochronous; it has the same period for different width swings "(A and B)".