With the aid of electric currents, one clock can be made to control other clocks, so as to make them keep accurate time.

By means of this method one high-class clock, usually in an astronomical observatory, compels a number of other clocks at considerable distances to keep time with it.

The clocks thus controlled ought to be so regulated that if left to themselves they Would always gain a little, but not more than a few minutes per day.

The pendulum of the controlling clock, in swinging to either side, makes a brief contact, which completes the circuit of a galvanic battery, and thus sends a current to the controlled clock. The currents pass through a coil in the bob of the pendulum of the controlled clock, and the action between these currents and a pair of fixed magnets urges the pendulum to one side and to the other alternately. The effect is that, though the controlled clock may permanently continue to be a fraction of a second in advance of the controlling clock, it can never be so much as half a second in advance.

An electrically controlled clock usually contains a small magnetic needle, which shows from which direction the currents are coming. The arrangements are usually such that at every sixtieth second no current is sent, and the needle stands still. Any small error is thus at once detected.