What Actually Makes a Clock Tick? A Gate That Opens Once Per Swing

A weight or spring drives a train of gears that would otherwise unwind in seconds, and a timekeeper such as a pendulum or a balance wheel oscillates at a steady rate but has no power of its own and would soon stop. The escapement connects the two, doing two things at once. It lets the gear train advance by exactly one step for each oscillation, which converts continuous power into counted intervals and drives the hands. And it takes a small amount of energy from the train and delivers it to the oscillator at the right moment, replacing what friction and air resistance remove, which keeps the oscillation going indefinitely.

Delivering energy to the oscillator interferes with the thing it is trying to preserve:

• •A pendulum swinging freely keeps a rate set almost entirely by its length
• •Any push applied off-centre changes that rate slightly
• •The size of the push varies as the driving force varies through the winding
• •Friction in the contact varies with lubrication, temperature and wear
• •Better designs deliver the push near the bottom of the swing, where it disturbs least
• •The best deliver it briefly and let the oscillator swing free the rest of the time

The history is a sequence of attempts to reduce that interference. The verge, used from the fourteenth century, engages the oscillator continuously and is accordingly poor, losing minutes a day. The anchor, from the 1670s, allows a much smaller pendulum swing and made long pendulum clocks practical. The deadbeat, developed shortly afterwards, eliminates a recoil that the anchor produces and was the standard for precision clocks for two centuries. The detached lever, from the 1750s, freed the balance wheel of a watch for most of its cycle and remains in every mechanical watch made today. The grasshopper, devised by John Harrison, has almost no friction and was used in his sea clocks.

A portable timepiece cannot use a pendulum, since a pendulum depends on gravity and on being held still, so watches use a wheel oscillating against a hairspring instead. That spiral spring pulls the wheel back towards its rest position, and the wheel overshoots and swings the other way, producing an oscillation whose rate depends on the stiffness of the spring and the inertia of the wheel rather than on gravity. The escapement then does the same two jobs, releasing the train and pushing the wheel. Temperature is the enemy here, since it changes both the spring and the wheel, and compensating for it occupied watchmakers for two centuries until alloys were developed whose properties barely change with temperature.

The limit of the approach was reached in 1921 with a design by William Hamilton Shortt, in which two clocks work together. A master pendulum swings in a sealed vacuum chamber and does nothing except keep time, receiving a push once every thirty seconds delivered by a falling lever that is reset by the second clock. A slave clock drives the hands, takes all the load, and is corrected by the master. The master pendulum is therefore almost entirely free of the interference that limits every other design. The result kept time to within about a second a year, served as the primary standard in observatories worldwide until quartz replaced it, and was accurate enough to detect small variations in the rotation of the Earth.

The mechanism releases the gear train one step per oscillation and pushes the oscillator to replace lost energy, which is the source of the ticking. The push disturbs the rate it is trying to preserve, so the history of the field is a sequence of designs delivering it later, briefer and with less friction. Shortt's arrangement of 1921 kept a master pendulum in a vacuum doing almost nothing else and reached a second a year.