Pendulum Clocks

Abstract

In modern usage ‘clock’ refers to any device for measuring and displaying time, but not intended for carrying on the person. A pendulum clock has five essential features. Firstly, a timekeeping element: this is its pendulum which oscillates at a constant frequency. Secondly, a power source, usually either a falling weight, or a coiled spring called a mainspring: this replaces energy lost due to inevitable friction etc. Thirdly, a controlling device called the escapement: this has the dual function of providing impulses to replace energy lost due to friction etc., and converting the pendulum oscillations into a series of pulses to measure time. In an escapement, one tooth of the escape wheel (usually called the ‘scape wheel) is released at regular intervals. Fourthly, a counter train, this is the going train of gears: this counts and sums the pulses into hours and minutes etc. Finally, an indicating device, usually a dial with hands that indicate hours, minutes and seconds. Huygens is usually credited with the invention of the pendulum clock in the seventeenth Century, but he did not claim to be the inventor, and there is evidence that he was not the inventor. The invention resulted in a dramatic improvement in timekeeping. Obviously, the accuracy of a pendulum clock depends upon the accuracy with which its pendulum keeps time. Essential features of a pendulum clock are that the pendulum oscillation must be stable in the presence of small disturbances, and that just enough energy must be supplied to compensate for the inevitable losses due to friction etc. In mechanical engineering terms a pendulum clock is a dynamical system that must be considered as a whole. What Huygens did do was to provide the first theoretical and practical account of the application of pendulums to clocks. In the three and a half centuries since then the history of the pendulum clock is very much the history of refinements to improve timekeeping. Refinements, and attempted refinements, are so numerous that it is impossible to write a concise history. As often happens in technological developments, some refinements were made by trial and error with theoretical understanding coming later. Some aspects of clock pendulums are discussed. These include; pendulum quality, the effect of pendulum suspensions, and the effect of escapements.