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.
After having designed plans in 1754, he constructed his first chronometers by 1756, and accomplished his masterpiece in 1766. This remarkable chronometer incorporated a detached escapement, a temperature-compensated balance and an isochronous balance spring, innovations which would be adopted in subsequent chronometers. Harrison demonstrated a reliable chronometer at sea, but these developments by Le Roy are considered by Rupert Gould to be the foundation of the modern chronometer. Pierre Le Roy's chronometer had a performance equivalent to that of the Harrison H4 chronometer. In 1769, after his chronometre underwent testing aboard the corvette Aurore, he was awardedthe double prize offered by the Académie française for the best method of measuring time at sea. He succeeded in giving his instruments the greatest possible regularity by the discovery of the isochronous spiralspring, in which he was in competition with Ferdinand Berthoud, but which he published first. He was the author of several valuable publications on the art and science of clock-making and chronography, among them the Étrennes chronométriques of 1760. He also became Horloger du Roi in 1760. The work of Le Roy was not fully recognized in France however, and his contemporary Ferdinand Berthoud became more famous, obtaining the prestigious title of Horloger de Marine, which left Le Roy disillusioned and led him to retire. He died in Vitry in 1785.
