The horseshoe-shaped magnet is almost visible on the left of the dial. The lower dial is the first external clock, and drives a signalling arrangement to mark the start and end of work periods, for example in a factory. This removes the burden of signalling from the master clock. The 24V accumulator for the power reserve has been removed. Now for further discussion of secondary clocks.
Secondary clocks consist only of a step control mechanism for advancing the hands each minute.
This control mechanism receives a phase-changed signal from the master clock each minute lasting approximately half a second. For the hands to advance the polarity must be continuously changed in this way. When compared to unipolar systems this principle has the advantage of immunity to poor impulse quality. If contact bounce at the master clock causes several pulses to arrive instead of only one incremented by several minutes is impossible. Here is a typical contact arrangement for generating the alternating trigger pulses.