Bimetallics - an explosive solution

Harrison's earliest compensations (1750 ?) acted on the effective length of the hairspring, first with a miniature version of his "Harrison's Grid", which is quite common in pendulum clocks, and later with bi-metallic strips such as we would recognise in thermostats today. Pierre Le Roy made the first split bi-metallic balances, at around the same time, and these were used right through the 19th century, until Elinvar was developed in the 1920's by Guillaume.

Bi-metallics can have surprising force. We were once asked to build a ventilation system for greenhouses which were far from power lines, and so had to be autonomous. After a few experiments with fluids, plagued by sealing problems, we tried making stout bi-metals, about two feet long, from steel and brass riveted together. They easily provided enough force to open and close the vents, but after a hundred cycles they started to deform, bulging between the evenly-spaced rivets. Evidently brazing the whole surface would be a better solution, but of course as the first experiment cooled from red-hot it curled itself round into a half-circle! Hmmm...

We found a low-tech solution by riveting the two bars together at one end, with the brass bar slanting upwards at some 30º, coating the upper surface of the brass with lead azide (on top of cellotape, for it reacts with copper) and detonating it from the fixed end. The steel bar rested on a fat piece of boiler plate cast into a concrete pad. The shock-wave at some 18,000 ft/sec surface welded the metals together before they had time to get hot, and impressed the neighbours no end. Kept well greased, they may last forever.

Don't Try This at Home, Kids ! And don't grease your neighbours either...