John Arnold - Arnold's First "Watch Machines"

Arnold's First "Watch Machines"

Arnold's approach to precision timekeeping was completely different to that of Harrison whose technical ethos was rooted in seventeenth and early eighteenth century theory and practise. Arnold knew that as the balance and balance spring control the timekeeping in a portable watch, he only needed to find a way of giving the balance a consistent impulse with minimal interference from the wheel work, together with an effective temperature compensation. After making some experimental machines, he produced what could be regarded as a production model to the Board of Longitude in March 1771.

This machine was completely different to Harrison's watch. It was a mahogany box of approximately 6 x 6 x 3 inches that housed a movement that, though relatively simple, was close to the same size as Harrison's, with a balance of a similar diameter. The radical difference, however, was a newly designed escapement that featured a horizontally placed pivoted detent that allowed the balance to vibrate freely, except when impulsed by the escape wheel. 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. Arnold proposed manufacture of these timekeepers at 60 guineas each.

Three of these timekeepers traveled with the explorers James Cook and Captain Furneaux during their second voyage to the southern Pacific Ocean in 1772-1775. Captain Cook also had Kendall's first timekeeper on board as well as one of Arnold's. Whereas Kendall's performed very well and kept excellent time during the voyage, only one of Arnold's was still running on their return to England in 1775.

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

This surviving watch watch dates from around 1769-1770, and is signed Arnold No. 1 Invenit et Fecit. The movement, which indicates centre seconds, has a steel balance with a bimetallic temperature compensation strip that acts on the flat balance spring. 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.

Around 1772, Arnold modified this escapement so that it now was pivoted vertically and acted on by a spring. This was a much more successful arrangement, and it is known that in 1772 at least two pocket timekeepers with this escapement were supplied to Joseph Banks at a cost of £100 and also Banks' fellow Etonian Captain Constantine John Phipps, 2nd Baron Mulgrave.

In 1773, Captain Phipps made a voyage to the North Pole taking with him not only his Arnold pocket timekeeper and an Arnold box timekeeper in gimbals, but also Kendall's "K2" timekeeper. From Phipps's account, it appears that the pocket watch performed very well indeed, and was a convenient instrument for ascertaining the longitude.

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. However, the latter appears to have proved ineffective, which seems to have substantially slowed the rate of production.

Even though he produced a number of pocket timekeepers, from around 1772-1778, Arnold was still experimenting with different types of compensation balance and methods of balance spring adjustment. The most difficult problem to surmount was the problem of making an effective and continuously adjustable temperature compensation device. For technical reasons, the temperature compensation for the balance spring had somehow to be incorporated into the balance itself and not act on the balance spring directly as had been done previously by Arnold and others.