Static Balance

Those bi-metallic balance wheels which were, and are, so pretty but require a steady hand and the patience of several saints to set up properly. As the wheel expands with a rise in temperature the free ends curve inwards, so that the moment of inertia remains constant. If only it were that simple! For a start, the wheel must be statically balanced for in a pocket watch, which may run in a variety of positions, a heavy point would be affected by gravity and act as a pendulum. The wheel is placed with its pivots on two horizontal knife-edges, and when rolled should stop in any position, without preference. (A wheel on an upside-down bicycle will often oscillate before stopping, with the valve at the bottom.) My first, home-made, tool had two razor blades and sat on a sheet of glass, but later I found a real one second-hand, with agate edges.

A real compensation balance has more holes than screws around its circumference. Although these screws have slots they are impossible to move with a screwdriver, no matter how fine, without ruining them - we use a tiny 4-jawed hand chuck which closes by sliding a collar forwards, holding the outside of the screw. It would also be almost impossible to replace the screws if they were not held in this way ; in a lady's 6"' watch they are seriously small...

Soooo... having a statically balaced wheel we mount the hairspring and collet, taking into account the dynamic balance of the whole assembly, for the split collet is not symmetrical. Having "counted" the oscillations against a standard balance and pinned the outer stud in place, mounted the whole in the movement and regulated the watch (in the 5 standard positions, winder-down being ignored for some reason), we can begin to check the temperature compensation. Once the watch is running well at 20ºC it must be placed alternately in an ice-box at 0º, then in the étuve at 35º, and an error curve plotted, sometimes over several weeks.
The nearer the compensation screws are to the moving ends of the wheel-rim, the greater their influence. All changes must be made symmetrically. For finer adjustment, the screws have extremely thin "washers" under their heads, so fine that they cannot be drilled, but are punched from 2/100mm brass shim on a piece of lead sheet.... a labour of Love...

An "officially certified chronometer" must keep time to a minute a month, in the five positions and at from 0 to 35ºC. Ha! Now the nastiest, crappiest dime-store quartz is within a few seconds a month! Tompion and Breguet would turn in their graves....