Elinvar Hairsprings

A cut compensating balance on an
Illinois Watch Co "Bunn Special"
Railroad watch.
Click on the image for a larger view.

One of the big problems in making a watch run too time is the effect of temperature on the steel hairspring, the biggest problem is not with respect to expansion or contraction (although that is an issue) but with the change in it's springiness (it's modulus of elasticity). On more expensive watches and clocks this was for many years addressed by the use of the cut compensating (or compensation) balance which reduces in size with increased temperature compensating for the weakening of the balance spring.

This was achieved by making the balance with two bimetallic rim sections with steel on the inside and brass on the outside. Moving or adjusting the screws let into the rim would vary the amount of compensation.

In the late 1890's a new alloy known as "Elinvar" was devised by Charles Édouard Guillaume (who got the 1920 Nobel prize for physics for the development of this and Invar a slightly less effective alloy), its key property was that unlike steel its modulus of elasticity did not change with temperature. When used as a hairspring it removed the need for a compensating balance.

A screwed balance on a Swiss watch
by Tavannes for J.W. Benson.

Many makers, particularly in Switzerland, quickly  implemented the Elinvar hairspring with a screwed balance replacing the expensive compensating balance.

The screws were retained to allow the balance to be balanced fairly quickly(as a car wheel with a new tyre) and the weight and number of screws could be used to adjust the moment of inertia to match the hairspring to make the watch run too time, for fine tuning and, when adjusted asymmetrically,to make small corrections to remove positional timing errors, for example when the watch was pendant up compared to face up.

Modern mechanical watches use the same approach although with different alloys.