The going work and the control mechanism are driven by a single weight using a differential gear. This is discussed further under secondary clocks. The contacts may consist of a conventional contact block or a mercury switch. As this category of electric pendulum clocks are weight or spring driven, the problem of a power reserve for running the master clock is solved - during a power failure the clock is driven by the power reserve provided by the main spring or the drive weight. However, the secondary clocks cannot be driven because the electrical power is removed.
More ingenious designs store the time at which a power failure occured by means of cam disks.
When the power is restored these cams are driven by motors to supply the missing pulses to the secondary clocks, so that they are automatically resynchronised with the master clock. The problem posed by the loss of drive pulses during a power failure is overcome in the case of master clocks that use an inductor. The master clock therefore generates the electrical energy required by the secondary clocks itself. Mains power is only used in this case for winding the drive weights. In the event of a power failure such clocks can be wound manually, so that the entire clock system continues to run correctly.