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.

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.

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.

A similar repeating watch by Arnold has survived; it is of interest that the basic movement is Swiss in origin but finished in London.

In addition, Arnold fitted bi-metallic temperature compensation, and not only was every pivot hole jewelled but the escapement also had a stone cylinder made of ruby or sapphire.

[3] Other early productions by Arnold from 1768 to 1770 display both originality and ingenuity; this includes a centre seconds watch wound up by depressing the pendant once a day.

Arnold's facility and ingenuity, coupled with his undoubted charm brought him to the attention of the Astronomer Royal Nevil 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.

[6] 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.

In 1769, Arnold modified Maskelyne's centre seconds watch by John Ellicott, changing the cylinder escapement from steel to one made of sapphire.

He lent this watch to the Astronomer William Wales for use in assessing the practicability of Maskelyne's Lunar distance method for finding the ship's longitude during the voyage of the Transit of Venus expedition to the West Indies in 1769.

Arnold's approach to precision timekeeping was completely different from that of Harrison, whose technical ethos was rooted in seventeenth- and early eighteenth-century theory and practice.

The spiral balance spring also had a temperature compensation device similar to those in Arnold's watches, and based on Harrison's bimetallic strip of brass and steel.

[8] Three of these timekeepers travelled with the explorers James Cook and Captain Furneaux during their second voyage to the southern Pacific Ocean in 1772–1775.

During this period, Arnold also made at least one precision pocket watch, a miniature version of the larger marine timekeepers.

Though now altered, the original escapement was Arnold's horizontal pivoted detent as fitted to the larger timekeepers, which was, it seems, not entirely successful and needed improvements.

[13] By 1772, Arnold had finalized the design of his pocket timekeepers and started series production with a standardized movement caliper, this being around 50 mm in diameter, larger than a conventional watch of the period, and showing seconds with a pivoted detent escapement and spiral compensation curb.

This spiral actuated two weighted arms, making them move in and out from the centre, changing the radius of gyration, and thus the period of oscillation.

"[15] The fact that Arnold had recognized the technical advantages of a balance spring of this form clearly demonstrates a high degree of insight.

A spiral bimetallic curb acting on this spring was intended to provide the temperature compensation, but this system evidently did not work, as every watch was subsequently altered and improved by Arnold shortly afterwards.

Around 1777, Arnold redesigned his chronometer, making it larger in order to accommodate the new "T" balance that worked with his pivoted detent escapement and patented helical spring.

[19] 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.

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.

[20] The fact that Arnold had gained great success by modifying the technology of the timekeeper by means of simple yet effective mechanical techniques also meant that other watchmakers could copy these methods and use them without permission.

In recent years, research has established that Arnold's success was not due to the form of detent escapement, but to his original methods of adjusting the balance spring for positional errors by manipulating the overcoil terminal curve.

For obvious reasons, Arnold tried to keep these methods secret; certainly it is recorded that he clearly expressed his concerns about possible plagiarism to Earnshaw, warning him in no uncertain terms not to use his Helical balance spring.

[25] Nevertheless, a year later, in 1783, Earnshaw—through another watchmaker, Thomas Wright—took out a patent that included Earnshaw's pattern of integral compensation balance and spring detent escapement in the multiple specification.

The bimetallic compensation balance and the spring detent escapement in the forms designed by Earnshaw have been used essentially universally in marine chronometers since then.