John Arnold : biography
- This article is about the watchmaker and inventor, and his son. For others of the same name, see John Arnold (disambiguation).
John Arnold (born 1736 probably in Bodmin, Cornwall – died 1799 in London) 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, with a few modifications unchanged until the late twentieth century. With regard to his legacy one can say that both he and Abraham-Louis Breguet largely invented the modern mechanical watch. Certainly one of his most important inventions, the Overcoil balance spring is still to be found in most mechanical wrist watches to this day.
From around 1770, Arnold continued to develop portable precision timekeepers, almost from the point where John Harrison ended his work on precision timekeeping. Compared to Harrison's watch, Arnold's basic design was simple whilst consistently accurate and mechanically reliable. Importantly, the relatively conventional movement design facilitated its production in quantity at a reasonable price while facilitating easier maintenance.
Three elements were essential to 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
Arnold's second patent of 1782
In 1782, Arnold took out another patent to protect the latest and most important inventions, which were potentially lucrative. Several other watch makers, most notably Thomas Earnshaw, had started to copy Arnold's work. Around 1780, Earnshaw modifyied his detent escapement by mounting the detent on a spring to create the spring detent escapement.
During the same period, between 1779 and 1782, Arnold finalized the form of his chronometer watches. Through continuous experimentation, he worked out a way to make an effective but simple form of compensation balance. At the same time, he discovered a simple modification to his helical balance spring that let develop concentrically and, also, confer the property of isochronism on the oscillating balance. Not only this, but adjustments to the compensation balance and the balance spring could be carried out in a simple, calculated way. These were the main subjects of the patent, which he took out in 1782.
The balance consisted of a circular steel balance wheel with two bimetallic strips attached diametrically. Each bimetallic strip terminated with a screw thread mounted with a weight or balance nut. The further along the strip this nut was screwed, the greater the compensating effect. Another part of the patent concerned an addition to the form of the balance spring—a coil of smaller radius at each end of the helical spring, which offered increasing resistance to the rotating balance as it turned towards the end of each vibration. This was an important invention, as it largely eliminated the problem of the positional adjustment of balance controlled watches. This device, known as the Overcoil balance spring is still used today in most precision mechanical watches.
Another part of the patent concerned the escapement—a modification of Arnold's pivoted detent escapement that essentially mounted the detent on a spring. The specification only shows the part of this escapement that is the method of impulse on the impulse roller.
The technical challenge
Arnold's facility and ingenuity, coupled with his undoubted charm brought him to the attention of the Astronomer Royal Neville Maskelyne, who at this time was seeking a watchmaker skilled enough to make a copy of John Harrison's successful marine timekeeper. A full and detailed description of this watch was published by the Board of Longitude in 1767, entitled "The Principles of Mr. Harrison's Timekeeper", the intention clearly being for it to act as a blueprint for future quantity production. In fact it was a highly complex and technically very advanced piece of micro engineering, and capable of being reproduced by less than a handful of watchmakers. However, the challenge was taken up by Larcum Kendall, who spent two years making a near identical copy (now known as "K1") that cost £450, a huge sum at the time. Although successful as a precision timekeeper, the Admiralty for obvious reasons wanted a timekeeper on every major ship, and Kendall's was too expensive and took too long to make. Kendall made a simplified version (K2) in 1771, leaving out the complicated remontoir system. But the result was still too costly, and moreover not as accurate as the original.Rupert T. Gould London 1976 Ed. "The Marine Chronometer" pp. 71-74.
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