Like So many countries, the British Post Office is also England's telephone company. Phone messages are billed by the minute, and so the accuracy of the timing of the length of calls is important. The GPO type 36, a seconds beating timepiece has circuits that provide precise electrical timing impulses every second, every six seconds and at half minute periods for measuring telephone circuit usage. The half-minute impulses are distributed to secondary or "slave" dials within a telephone office facility.

The GPO 36 is not a clock in the strictest sense as the dial is located remotely and for that reason, some refer to the device as a "time transmitter." A smaller version, the GPO 64, with similar functions was also used by the postal authority to control and manage telephone circuit usage and billing.

The clock or time transmitters, which were installed in every telephone exchange or central office in the British Empire, including India, where some remain in present use, where made by several manufacturers to Post Office specifications. They employ the Hipp toggle principle and are very accurate owing to the massive pendulum bob on the pendulum rod, which is sometimes made of "invar" for additional timekeeping precision. Invar metal, a French invention, is an alloy of iron and nickel. It does not change dimensions appreciably with changes in temperature. The length of a pendulum is critical to timekeeping accuracy, even slight day-to-day changes in temperature affect the accuracy of nearly all timepieces.