Electric drive

A mainspring is a poor power source. It has to be wound, and it delivers falling torque as it unwinds; two centuries of horological ingenuity went into hiding that fact. By the early 1950s the miniature battery made an alternative conceivable: a cell that delivers constant voltage for a year and needs no winding at all.

The first attempts kept everything else. Hamilton’s calibre 500 of 1957 retained a balance wheel and a gear train, but the balance carried a coil, and current from a button cell produced a magnetic field that pushed against permanent magnets in the plate to keep it swinging. There were no transistors: switching was done by mechanical contacts, and those contacts were the movement’s weakness. Bulova’s Accutron of 1960 went further and removed the balance altogether, replacing it with a tuning fork vibrating 360 times a second, driven by coils and switched by a transistor. A fork’s frequency is set by its own geometry, so it is far less sensitive to position and shock than a balance.

Neither survived long. The contacts in the Hamilton wore and arced; the Accutron was displaced within a decade by quartz, which uses the same idea of a solid-state resonator driven by a transistor, at a frequency a hundred times higher. Both are best understood as the bridge between the mainspring and the quartz crystal.