Experimental watch by Ferdinand Berthoud

Ferdinand Berthoud was one of the leading clock, watch and chronometer makers in Paris during the second half of the eighteenth century. The effect of temperature change on the balance and balance spring of precision timekeepers was a constant problem and it is no surprise that Berthoud should work on a watch with a form of temperature compensation. Englishman John Harrison pioneered such devices: one of his long-lasting inventions, still in use today, was the gridiron compensator.

This experimental watch, signed around the edge of the dial plate, 'Ferdinand Berthoud Inv et fecit 1763' and on the back plate, 'Ferdinand Berthoud à Paris No 417', is housed in a gold case and has a white enamel dial typical of French watches of the period. The main interest, however, lies in the movement and Berthoud's use of a gridiron compensator. The movement is conventional, with a fusee and verge escapement controlled by a gilded-brass three-arm balance and spiral balance spring. To compensate for the effects of temperature change, Berthoud used a simple gridiron of six steel rods and six brass rods mounted in a frame. To compensate for changes in the elasticity of the balance spring, caused by changes in temperature, the gridiron acts, via a lever, on the spring's outer end, altering its effective length.