Timed exposure of a simple pendulum. Mechanically, a simple pendulum is defined as a small heavy body (in this case a metal sphere) suspended by a light inextensible string. One complete to & fro motion of a pendulum is called its oscillation: the time taken for a complete oscillation is the periodic time. The maximum displacement of the pendulum from its rest position is called the amplitude. Galileo is said to have observed a lamp swinging on a chain in Pisa Cathedral & noticed that the time of swing (period) remained constant as the swings (oscillations) were dying away. He realised such a pendulum might govern a clock & at his death in 1642 had made plans for its construction.