John Arnold - Arnold's Second Patent of 1782

Arnold's Second Patent of 1782

In 1782, Arnold took out another patent to protect the latest and most important inventions, which were potentially lucrative. Several other watch makers, most notably Thomas Earnshaw, had started to copy Arnold's work. Around 1780, Earnshaw modifyied his detent escapement by mounting the detent on a spring to create the spring detent escapement.

During the same period, between 1779 and 1782, Arnold finalized the form of his chronometer watches. Through continuous experimentation, he worked out a way to make an effective but simple form of compensation balance. At the same time, he discovered a simple modification to his helical balance spring that let develop concentrically and, also, confer the property of isochronism on the oscillating balance. Not only this, but adjustments to the compensation balance and the balance spring could be carried out in a simple, calculated way. These were the main subjects of the patent, which he took out in 1782.

The balance consisted of a circular steel balance wheel with two bimetallic strips attached diametrically. Each bimetallic strip terminated with a screw thread mounted with a weight or balance nut. The further along the strip this nut was screwed, the greater the compensating effect. Another part of the patent concerned an addition to the form of the balance spring—a coil of smaller radius at each end of the helical spring, which offered increasing resistance to the rotating balance as it turned towards the end of each vibration. This was an important invention, as it largely eliminated the problem of the positional adjustment of balance controlled watches. This device, known as the overcoil balance spring is still used today in most precision mechanical watches.

Another part of the patent concerned the escapement—a modification of Arnold's pivoted detent escapement that essentially mounted the detent on a spring. The specification only shows the part of this escapement that is the method of impulse on the impulse roller.