John Arnold - Arnold's First Patent of 1775

Arnold's First Patent of 1775

In 1775, Arnold took out a patent for a new form of Compensation balance with a bimetallic spiral at the centre. 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.

In the same patent, he included a new helical balance spring. This shape reduced lateral thrust on the balance pivots as they rotated, and reduced random errors from the "point of attachment" effect, which any balance with a flat spring suffers from. As Arnold stated rather succinctly in a 1782 letter to the Board of Longitude, "...the power in all parts of the spring is uniform."

The fact that Arnold had recognized the technical advantages of a balance spring of this form clearly demonstrates a high degree of insight. The balance that was the subject of the patent appears to have been an unsuccessful design. Certainly some marine chronometers used this balance, but none have survived. Pearson records a balance of this kind in his possession that was 2.4 inches in diameter.

From 1772 to 1775, Arnold also made about thirty five pocket timekeepers. Not many, about ten of these, survive and none in their original form, as Arnold was constantly upgrading their specification. They appear originally to have had a pivoted detent escapement, with a steel balance and a helical balance spring. 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. Surviving chronometers from this series include Numbers 3,29 and 28.

Further experimentation and invention by Arnold led to a breakthrough in the late 1770s. He redesigned the compensation balance and developed two designs that showed promise. Known as the "T" and "S" balances, and marked as such in Arnold's 1782 patent (probably because of their appearance), both employed bimetallic strips of brass and steel with weights attached, which changed the radius of gyration with change in temperature. Although these probably needed a lot of adjustment, they appear to have worked well compared to his previous attempts at a compensation balance.