Clock types · Time systems and signals

Master and Slave Clocks

Before networked time, one carefully kept pendulum clock could send electric pulses down a wire and move the hands of dozens, even hundreds, of dials at once.

By David Ray · clocks.fyi

What a master and slave system is

A master and slave system splits a clock in two. The master is a single, well-made pendulum clock that does all the timekeeping. The slaves are plain dials, often spread across a building or several buildings, that have no timekeeping ability of their own and simply move when the master tells them to.

The link between them is a wire carrying short electric pulses. Britannica notes that one master clock could control many dials across several buildings, along with other equipment such as time recorders and sirens. Because every dial answers to the same master, they all show the same time, which was the whole point in offices, colleges and other large buildings.

Different organizations use different words for the same idea. The Institution of Mechanical Engineers, for instance, describes its own system in terms of a transmitter clock and receiver clocks, while Britannica and Royal Museums Greenwich speak of master and slave clocks.

See also: how one reference clock sets the time on all your devices on OnlineClock.app.

How the impulse works

Britannica describes the arrangement used in later master clocks. The pendulum turns a count wheel one tooth for every double swing, and every half minute the wheel releases a lever. An electromagnet then resets that lever, and the same burst of current travels out to the slave dials and advances each of them by half a minute.

The pendulum also gets its push from the system. In Frank Hope-Jones's Synchronome remontoire, the Institution of Mechanical Engineers explains, a gravity arm impulsed the pendulum every 30 seconds and was then reset by an electromagnet. Britannica points out that an impulse delivered symmetrically, as the pendulum passes the middle of its swing, disturbs it very little, which is one reason these clocks kept good time.

The result is a clock that is wound by electricity rather than by hand, and a set of dials whose minute hands step forward in small jumps instead of gliding.

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Early electric clocks

Britannica dates the first battery electric clock to 1840: a spring-and-pendulum clock that used an electric impulse to work several dials. The paperwork followed quickly. The Science Museum Group holds the patent granted on 11 January 1841 to the chronometer maker John Barwise and the inventor Alexander Bain for improvements in applying driving power to clocks and timepieces, which the museum describes as the first patent for an electric clock.

That patent came early in a long run of experiments. Over the following decades many inventors tried their hand at electric clocks that could run other dials, and by the end of the century the most successful British answer was coming from Frank Hope-Jones.

The Synchronome

One widely used British system grew from Hope-Jones's Synchronome remontoire, which the Institution of Mechanical Engineers dates to about 1898. Hope-Jones realized that the same 30-second impulse that kept the pendulum going could also drive the hands of the master clock and of any number of receiver dials. The Institution's own headquarters at One Birdcage Walk in London had a Synchronome system when the building opened in 1899, and the Institution notes that these systems could often be recognized by an audible click every half minute.

The University of Queensland Physics Museum dates Synchronome's Brisbane operation from 1903 to 1991 and holds a Synchronome movement of about 1907. Its records show how far the system spread: a master clock was installed at the building housing the new Queensland University on 4 February 1911, and on 31 December 1914 the Brisbane agent installed a master at the Central Technical College that ran 112 dials. Hope-Jones also wrote a book, Electric Clocks and How to Make Them, aimed at people who wanted to build their own.

The Shortt clock: master and slave at their most precise

The idea reached its peak in the Shortt free-pendulum clock, which Britannica calls the most accurate mechanical timekeeper. National Museums Scotland credits its design to William Hamilton Shortt in 1921, with the Synchronome Company as maker, and its own example went into service at the Royal Observatory, Edinburgh, in 1922.

The Shortt pairs two clocks. Its timekeeping pendulum swings almost untouched in a sealed box where the air is kept at a constant low pressure, receiving a gentle impulse every half minute. A separate slave clock releases those impulses, and a synchronizing signal sent back to it after each one keeps every impulse exactly half a minute after the last. In observatories, Britannica says, Shortt clocks kept time to within a few thousandths of a second a day.

Royal Museums Greenwich holds Shortt system No. 40, whose slave clock was made by the Synchronome Company. It was commissioned by H.R. Fry of Barnet, an amateur astronomer, and given to the Royal Greenwich Observatory; the museum record gives the year as 1930, while a 1977 report it quotes says 1936, and the record does not settle which is right. The clock was moved to the emergency time station at Abinger in 1942 and later to Herstmonceux Castle.

See also: the atomic clocks that set official time now on OnlineClock.app.

How to recognize a slave dial

Because a slave dial is moved by electric pulses rather than by a spring or weight, it needs no winding, so look for a dial with no winding holes. On a half-minute system the minute hand jumps forward in visible steps, and you may hear the click that the Institution of Mechanical Engineers describes. A wire running out of the case to a distant master is the clearest giveaway.

The master itself is a pendulum clock. The Queensland museum notes that some early Synchronome controllers had a small slave dial inside the case, though its own example leaves it out so the mechanism stays visible.

Questions people ask

Can a slave clock keep time on its own?

No. A slave dial keeps no time of its own. It only moves when it receives a pulse from the master clock, so if the master stops or the wire is cut, the slave stops too.

Why does the minute hand on an old school clock jump?

Many master clock systems sent a pulse every half minute, and each pulse moved the slave dials forward by half a minute at a time. That produces the familiar stepping minute hand.

How accurate were master clocks?

Ordinary office systems were only as good as their master pendulum. At the top end, Britannica says observatory Shortt clocks, which pair a free pendulum with a slave clock, kept time to within a few thousandths of a second a day.

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