Pierre Le Roy

Pierre Le Roy (1717–1785) was a French clockmaker. He was the inventor of the detent escapement, the temperature-compensated balance and the isochronous balance spring. His developments are considered as the foundation of the modern precision clock. Le Roy was born in Paris, eldest son of Julien Le Roy, a clockmaker to Louis XV who had worked with Henry Sully, in which place Pierre Le Roy succeeded his father. He had three brothers: Julien Le Roy (1686-1759) a clockmaker and watchmaker, Julien-David Le Roy (1724–1803) an architect, and Charles Le Roy (1726–1779), a physician and Encyclopédiste.

In 1748, he invented a pivoted detent type of escapement, or detached escapement, which makes him the inventor of the detent escapement: "The invention of the detached escapement belongs to P. Le Roy".

He was distinguished principally in his mastery and improvement of the clock and chronograph, above all of the marine chronometer, in which he carried forward the pioneering work of John Harrison. He took a different approach from that of Harrison, believing that the way to achieve seaworthiness was to detach the escapement from the balance. He also differed from Harrison regarding his temperature compensation method, which used the variation of the rotation radius of the balance by modifying the diameter of the balance through bi-metallic components, a method which would become a standard in chronometers. His technique for temperature compensation was highly efficient in that it worked without changing the length of the spiral balance spring, which he had discovered to be isochronous only at a precise given length (i.e. when frequency is independent of amplitude, so that a mechanical clock or watch runs at the same rate regardless of changes in its drive force, so it keeps correct time as the mainspring unwinds).

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