These dials are not, in themselves, clocks. They are simple mechanisms connected by electrical wires to a central ‘master clock’, which is the system timekeeper. Every 'electric impulse dial' on the electrical circuit is advanced automatically at the same instant by the master clock, usually every thirty seconds. This was far cheaper than having good quality clocks in every room of a large office building or factory. This example, with an early plastic case, is by one of the leading makers, the Synchronome company.