The clock consists of two units that are electrically connected: the master pendulum and the slave unit. The master pendulum is contained within a partially evacuated tank. The tank consists of a painted flanged copper tube, which is capped to the underside by a thick bevelled glass and surmounted by a heavy glass dome enabling inspection of the electrical impulse system, thermometer and manometers. The slave unit consists of a mahogany cased standard Synchronome master clock, which is fitted with a ‘hit and miss’ pendulum synchroniser and also a further contact that sends impulse to the master pendulum every thirty seconds. The slave unit is fitted with two silvered dials: the fist is a nominal seven-inch dial signed ‘Synchronome Electric’ and the second, a four-inch silvered dial engraved ‘Free Pendulum’.

The pendulum within the slave unit (the mahogany cased synchronome clock) is regulated to run slower than the master pendulum and the pendulum is kept in phase with the master by the ‘hit and miss’ synchroniser. The slave clock receives impulse to the pendulum in the conventional manner from a gravity arm that is dropped every thirty seconds. This also activates impulse to the master pendulum: a small semi-circular sectioned jewel, mounted on the end of the gravity arm drops onto a light wheel, mounted to the pendulum rod. As the master pendulum swings away the jewel passes over the edge of the wheel giving impulse at the central point of swing. When this gravity arm passes beyond the wheel it is re-cocked and in doing so an electrical pulse is sent to the slave dial and also to the ‘hit and miss’ synchroniser. The synchroniser is simply a small armature that is pulled into the path of a buffer spring, fixed to the pendulum. If the pendulum is in phase with the master it will be behind the armature at the moment of electrical contact but if the pendulum is ahead of the master then the armature will connect with the buffer spring, causing an acceleration of the pendulum. The slave pendulum is normally rated to lose approximately six seconds per day compared to the master, and when it is regulated thus, the synchroniser and buffer spring will normally engage every alternate swing of the pendulum. In this manner the master pendulum is sure to receive regular and equal impulse allowing for superior timekeeping.

Shortt No. 3 replaced the Cottingham Riefler as the Sidereal Standard Clock on 1 January 1925. The clock was used in this manner until 1938, when the use of a ‘standard’ clock was abandoned in favour of using a mean of many clocks, including some from outside the Royal Observatory, to provide sidereal time.