A Rare Rotherham and a bit about hairspings & balances.

A Size 12 Rotherham for Hoefler &
Co of Devenport. 19 Jewels, 1901.

This is at first sight a normal high end size 12 Rotherham with 19 jewels but two things make it unusual, first is still has an original watch paper in the back as described in my last post, but more important is the very rare balance assembly.

Normally these movements have a cut compensating balance with a normal steel Breguet hairspring, however in the late 1890's a new alloy known as "Elenvar" was devised by Charles Édouard Guillaume (who got the 1920 Nobel prize for physics for the development), its key property was that unlike steel its flexibility (modulus of elasticity) did not change with temperature. When used as a hairspring it removed the need for a compensating balance.

A very rare solid balance with an
Elenvar Breguet hairspring.

Most companies implemented the Elenvar hairspring with a screwed balance which allowed 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. 
 
With this movement Rotherham tried a different route and used a traditional 3 armed solid balance which is probably gold. Balancing and tuning was done by removing slight amounts of material from the balance as can be seen from the picture to the right..

This is the only watch I have seen with this arrangement and it is likely that Rotherham only made a few, probably because the amount of skilled work required to set up the balance would more than likely have made it more expensive than the normally sprung cut compensating balance.