English: Diagram of a typical master clock system used around 1900 to keep clocks synchronized in factories, schools, and other large institutions. It consists of a precision elecrically-powered master clock with a temperature-compensated mercury pendulum, which is wired to slave clocks all over the building. In addition to wall clocks, it also controls time stamps which are used to stamp documents, and a tower clock which can be used in a clock tower. The "program clock" is a timer which can be programmed with a paper tape punched with holes to ring bells or turn machines off or on at preprogrammed times; up to 8 machines can be controlled.