Precision electromechanical pendulum clock (master/slave system) · England
Shortt-Synchronome Free Pendulum Clock
VerifiedRegarded as the most accurate mechanical/electromechanical pendulum clock system ever put into regular service, used as a time standard by observatories including Greenwich before quartz clocks superseded it.
Technical facts
- Accuracy
- Documented to within roughly a few hundredths of a second per day under observatory conditions
- Configuration
- Master pendulum in an evacuated tank, electrically synchronised with a slave clock every half-minute
Historical significance
- Regarded as the most accurate mechanical/electromechanical pendulum clock system ever put into regular service, used as a time standard by observatories including Greenwich before quartz clocks superseded it.
- Separates a free-swinging 'master' pendulum in near-vacuum from a synchronised 'slave' clock that performs all the work of driving hands and impulsing, so the master pendulum itself does almost no work and swings with exceptional regularity.
Background
The Shortt–Synchronome free pendulum clock was invented in 1921 by British railway engineer William Hamilton Shortt in collaboration with horologist Frank Hope-Jones. It was manufactured by the Synchronome Company, Ltd., and functioned as a precision electromechanical system consisting of a primary pendulum in a copper vacuum tank and a secondary precision pendulum clock. To prevent coupling, the units were placed far apart or oriented with their planes of swing at ninety degrees to one another.
The primary pendulum, made of invar to reduce thermal expansion, swung in a vacuum to prevent atmospheric pressure changes and reduce drag. These clocks served as the highest standard for timekeeping globally from the 1920s to the 1940s, aiding in scientific research and astronomical observations. The clock was highly accurate, serving as the first timekeeper more precise than the rotation of the Earth itself.
Written from the encyclopedia entries listed under Sources; each statement was checked against the cited text.
Construction and mechanism
- The primary pendulum rod and its 14-pound weight are constructed from invar to minimize the effects of temperature variations.
- The clock's vacuum tank is maintained at a pressure of 30 mmHg to improve the pendulum's Q factor.
- The secondary clock displays the time of both pendulums and features terminals to output a 1 Hz timing signal.
- An impulse is delivered to the primary pendulum once every thirty seconds by a gravity lever.
Later history
- About 100 of these clocks were produced between 1922 and 1956.
Photographs
Photographs are illustrative examples of the type, not necessarily the exact object described. Verify details against the source file page.
Sources & references
- Shortt–Synchronome clock, Wikipedia (English), retrieved 2026-10-28 (text CC BY-SA 4.0)
- Royal Museums Greenwich, published catalogue notes on the Shortt-Synchronome clock
- Science Museum Group collection records
Where sources disagree, the disagreement is stated rather than resolved silently.