John Arnold (1736 – 11 August 1799) was an English watchmaker and inventor. John Arnold was the first to design a watch that was both practical and accurate, and also brought the term "chronometer" into use in its modern sense, meaning a precision timekeeper. His technical advances enabled the quantity production of marine chronometers for use on board ships from around 1782. The basic design of these has remained, with a few modifications unchanged until the late twentieth century. His legacy includes, together with Abraham-Louis Breguet, as one of the inventors of the modern mechanical watch. One of his most important inventions, the overcoil balance spring is still used in most mechanical wristwatches. It was from around 1770 that Arnold developed the portable precision timekeeper, almost from the point where John Harrison ended his work in this field. But, compared to Harrison's complicated and expensive watch, Arnold's basic design was simple whilst consistently accurate and mechanically reliable. Importantly, the relatively simple and conventional design of his movement facilitated its production in quantity at a reasonable price whilst also enabling easier maintenance and adjustment. Three elements were necessary for this achievement: A detached escapement, which gave minimal interference with the vibrating balance and balance spring A balance design that enabled compensation for the effect of temperature on the balance spring A method for adjusting the balance spring, so that the balance oscillates in equal time periods, even through different degrees of balance arc Early life and work John Arnold was apprenticed to his father, also a clockmaker, in Bodmin in Cornwall. He probably also worked with his uncle, a gunsmith. Around 1755, when he was 19, he left England and worked as a watchmaker in the Hague, Holland, returning to England around 1757. In 1762, whilst at St Albans, Hertfordshire, he encountered William McGuire for whom he repaired a repeating watch. Arnold made a sufficient impression so that McGuire gave him a loan, enabling him to set up in business as a watchmaker at Devereux Court, Strand, London. In 1764, Arnold obtained permission to present to King George III an exceptionally small half quarter repeating watch cylinder escapement watch mounted in a ring. A similar repeating watch by Arnold has survived; it is of interest that the basic movement is Swiss in origin but finished in London. The escapement of this watch was later fitted with one of the first jewelled