BIMETALLIC

The adjective bimetallic describes a component formed by the assembly of two Metals with different thermal Expansion coefficients, joined together. Under the effect of a variation in Temperature, each Metal expands differently, causing a controlled curvature or deformation of the part. In watchmaking, the most famous application is the compensating bimetallic Balance, whose split felloe, made of Brass on the outside and steel on the inside, deforms with temperature to correct variations in rate caused by the expansion and change in elasticity of the Hairspring (balance spring). This ingenious device dominated precision chronometry before being supplanted by low-expansion alloys such as Invar and Elinvar, which made compensation unnecessary. The bimetallic principle is also found in thermostats and various thermal sensors. It illustrates the subtle use of the properties of Materials to compensate for the disruptive effects of the environment on the running of a timepiece over Time.

Useful link:

– The Balance