Quartz

A mechanical watch is limited by its balance and hairspring. The balance swings a few times a second and its rate changes with temperature, with position, with the state of wind and with the ageing of its oils. For most of history that was enough. Radio, long-distance telephony and precise navigation changed it, and physicists had known since the Curies’ work of 1880 that a quartz plate cut to shape will ring at a sharply defined frequency when driven electrically, and hold it far more stably than any mechanical resonator.

The first quartz clock, built at Bell Telephone Laboratories in 1927, filled a room. Shrinking it to the wrist took the transistor and the integrated circuit, and forty years. What made it small was the choice of frequency. A chain of fifteen flip-flops, each halving the one before, turns 32,768 Hz into exactly one pulse a second with no arithmetic and almost no power; the frequency is also above the range of human hearing, so the watch does not whistle, and low enough that the fork can be tiny and the drain small. The pulse drives a Lavet stepping motor, whose asymmetric geometry advances the rotor exactly half a turn per pulse, in one direction.

The compromises are different in kind, not absent. Quartz frequency varies with temperature, falling away from a turnover point set at around 25 degrees, so a watch on the wrist sits near its optimum while one left in a cold drawer runs slow. Crystals age. The battery is a consumable and will destroy the movement it powers if left to leak. And circuit boards cannot be repaired, only replaced, so a quartz watch of the 1980s is often beyond economic repair in a way that a mechanical watch of the 1880s is not.