Why Did Clocks Suddenly Get Accurate? A Swing That Takes the Same Time
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Clocks before 1656 lost minutes a day and clocks after it lost seconds, because a pendulum swings at a rate set almost entirely by its length. That single change made several sciences possible.
The property that matters
A pendulum swinging through a small arc takes essentially the same time for each swing regardless of how far it travels, which means a pendulum losing energy to friction continues to keep time as its swing decays. The period depends on the length and on gravity and almost not at all on the mass or on the amplitude, which is what makes it useful as a regulator. Galileo observed the property early in the seventeenth century and proposed applying it to timekeeping without building a working clock. Christiaan Huygens built one in 1656 and published the theory, and the improvement was immediate and enormous, taking daily error from around fifteen minutes to well under a minute and eventually to seconds.
What a clock needs besides the pendulum
The pendulum regulates and several other parts are required:
- •A power source, a falling weight or a spring, supplying energy against friction
- •A gear train reducing the motion to the rates the hands require
- •An escapement, releasing the train tooth by tooth and giving the pendulum a push each swing
- •A suspension holding the pendulum with minimal friction, generally a thin spring rather than a pivot
- •A means of adjusting the effective length precisely, since that sets the rate
- •A case, which keeps the mechanism clean, still and at a steadier temperature
The errors that had to be removed
Refinement over two centuries addressed identifiable sources of error. The period is not quite independent of amplitude for larger swings, which was addressed by escapements delivering a small consistent impulse so the arc stays small and steady. Temperature changes the length of the rod, which changes the rate measurably, and the remedy was compensated pendulums combining metals that expand differently so the centre of mass stays put, with several ingenious designs produced. Air pressure affects both buoyancy and drag, addressed eventually by sealing the clock in a partially evacuated case. The mounting must be rigid, since energy leaking into a moving support is lost. The best regulators achieved errors under a second a year before quartz displaced them.
What replaced it
The pendulum held the accuracy record for nearly three centuries and was displaced twice within a hundred years. Quartz oscillators, developed from the 1920s and made cheap from the 1960s, use the mechanical vibration of a cut crystal driven electrically, which is far higher in frequency and far less affected by the disturbances that trouble a pendulum, and an ordinary quartz watch outperforms the best pendulum regulator ever built. Atomic standards, operating from the 1950s, count transitions within atoms and are so much better again that the second was redefined in terms of one in 1967, abandoning astronomical definitions entirely. Optical clocks now under development are better still by orders of magnitude, accurate enough that they detect the change in the rate of time with a height difference of centimetres.
What accurate time made possible
The consequences ran well beyond knowing the hour. Astronomy requires precise timing to determine positions, and the pendulum clock made systematic positional astronomy practical, which in turn improved navigation and the understanding of the solar system. Determining longitude at sea requires carrying accurate time, which a pendulum cannot do on a moving ship, and that problem drove the development of the marine chronometer using a balance spring instead, solved by John Harrison over decades in the eighteenth century. Measuring the local strength of gravity became possible by timing a pendulum of known length, which mapped variations across the earth. And scheduling, first of astronomy and then of railways, factories and public life, depended on clocks agreeing.
The takeaway
A pendulum's period depends on its length and barely on how far it swings, so it keeps time as friction decays the arc, which took daily error from minutes to seconds after 1656. An escapement supplies energy each swing and releases the gear train. Temperature changing the rod's length was the main remaining error, removed by combining metals that expand differently.