His practical designs facilitated the mass production of quantities of shipboard marine chronometers from around 1782 and the design remained essentially unchanged until the late 20^th century when the mechanisms were replaced with digital equipment. One of his innovations, however,  remains in most mechanical watches to this day.

The son of a clockmaker, Arnold spent time in The Hague as a watchmaker returning to Cornwall in 1757.  In 1762, Arnold set up shop on the Strand and presented several watches to King George III including one, in 1768, which displayed minutes and seconds, a highly significant innovation of its era. 

These gifts brought Arnold to the attention of the Astronomer Royal, Nevil Maskelyne, who was seeking a skilled watchmaker to reproduce John Harrison’s marine timekeeper for use on all ships. Maskelyne had also employed another Cornishman, Malachy Hitchins (May 18th) to verify the calculations for the annual Nautical Almanack.

Whilst other watchmakers had produced copies of Harrison’s model they were simply too expensive for widespread use.  Arnold’s version was simpler, smaller and cheaper and  Captain Cook used it on his second voyage. Arnold’s timepiece was still running on Cook’s return whilst a pocket watch was also used, successfully, on Phipps’ voyage towards the North Pole.