John Arnold - A Revolution in Precision Timekeeping

A Revolution in Precision Timekeeping

Around 1777 Arnold redesigned his chronometer to make it larger, probably to accommodate the new balance that worked with his pivoted detent escapement and patented helical spring. The first chronometer of this pattern was signed "Invenit et Fecit" and given the fractional number 1 over 36, as it was the first of this new design.

It is generally known as Arnold 36 and was, in fact, the first watch that Arnold called a chronometer, a term that subsequently came into general use and still means any highly accurate watch. The Royal Observatory, Greenwich tested Arnold 36 for thirteen months, from 1 February 1779 to 6 July 1780. The testers placed it in several positions during the trial, and even wore it and carried it around. The watch exceeded all expectations, and demonstrated extremely accuracy. The timekeeping error was 2 minutes 32.2 seconds, but the error in the last nine months amounted to just one minute. The greatest error in any 24 hours was only four seconds, or one mile of Longitude.

Subsequently, Arnold produced a pamphlet that detailed the trial and results, with attestations of veracity from all those concerned with the tests. Maskelyne's assistant, the Rev. John Hellins, was in charge of the pamphlet. The astonishing performance of this watch caused controversy, because many thought the result was either a fluke or a "fix" of some kind, particularly as Maskelyne was, effectively, one of Arnold's patrons.

From a technical point of view, however, the design was entirely sound and highly accurate over long periods. Arnold evidently learned the lessons that Harrison had learned before him—using a large, quickly oscillating balance (18,000 beats per hour) with small pivots. Arnold's detent escapement provided minimal interference with the controlling helical balance spring, since the temperature compensation was in the balance itself. Harrison had suggested this as a prerequisite, though he never developed the idea. Arnold's pivoted detent escapement did not need oil on acting surfaces, with the advantage that the rate of action did not deteriorate, and remained stable for long periods. At the time, only vegetable oil was available, which degraded quickly compared to modern lubricants.

This chronometer, 60mm in diameter, is housed in a gold case, and miraculously has survived in perfect and original condition. It can be seen in the collections of the National Maritime Museum, Greenwich, London, having been saved for the nation in 1993.

In Britain prior to Harrison's marine watch, generally thought was that a small, or very small watch (Such as Arnold's ring watch) was the ultimate test of watch making skill—especially with regard to complex and accurate watches. Both Harrison and Arnold, however, demonstrated that an accurate watch had to be of large diameter—so, by the end of the 18th century, large diameter was considered characteristic of a superior watch.