The number of revolutions per minute of the driving motor of an electric clock driven from the public electricity supply is directly related to the frequency of the supply. A motor controlled in this way is said to be 'synchronized', and the addition of a simple train of gears to drive the hands makes a cheap, reliable, time indicator possible. A synchronous clock of this type is not a timekeeper, but merely a slave controlled by the frequency of the electricity supply, which is regulated within close limits. In periods of heavy electricity demand, load shedding causes synchronous clocks to lose, but this loss is regained automatically during the following night.