Maker's Name: 
Where made: 
31.5 × 16 × 145 cm

This electrically powered pendulum clock was made in Sydney, probably in the 1950s by the firm ACElec, to a design developed by Prouds Electric Clocks and Scientific Instruments thirty years before. The firm Scientific Clocks referred to on the dial was formed by Cecil Gross and Lawrence Taprel in 1947 to install and maintain tower clocks and master/slave clock systems.

A pendulum clock uses a swinging weight, the pendulum, to keep time. If the amplitude is very small, the time taken for the two swings of the pendulum, one cycle, is equal to 2*pi* sqrt(L/g). L is length and g is local gravity. The length of the pendulum in this clock is adjusted so that each cycle takes 'exactly' two seconds.

Each time the pendulum swings to the left, one tooth of a 15 tooth wheel is gathered. One larger tooth on the wheel closes a pair of contacts once each revolution, sending a current pulse to the slave dials each half minute. Note that the dial in the door is electrical and not connected mechanically to the pendulum.

Ideally a pendulum would continue doing this forever, however friction and air resistance will slowly remove energy from the system. The energy can be restored from a source such as a spring, a falling weight, or an electromagnet. This clock has a very interesting mechanism that prevents it from stopping. In the top left of the glass window,to the left of the pendulum, is a pair of electrical contacts, surmounted by a small metal cone with a groove at its top. The pendulum has a metal flag, a 'Hipp toggle', that brushes over the top of the cone as the pendulum swings. When the pendulum's swing becomes sufficiently small, the flag will catch, and as the pendulum swings back, it will push the cone down and the contact point will complete the circuit. A current flows through the electromagnet on the right, causing a roller to roll down an incline on a bracket attached to the pendulum, giving it an impulse and replacing the energy that has been lost over time. This happens about once every thirty swings, and happens very quickly, so you need to watch closely.

While pendulums are generally very accurate at keeping time, their motion is dependent on their length and the local gravity. As the metal of the pendulum heats or cools it can expand or contract, thus changing the length, so the pendulum rod in this clock is made of Invar, a low expansion alloy.  At different points on earth the gravity can be slightly different. Both these things will affect the amount of time each pendulum cycle takes and so will affect the accuracy of the clock. Pendulum clocks also cannot be moved as any motion or acceleration will interfere with its motion, making them unsuitable for portable timepieces.

This item is part of the UQ Physics Museum Electric Clocks Tour
< Previous Item | Return to Tour Menu | Next Item >


  • door open 
  • dial movement