Slave clock

UTC is based on a network of atomic clocks in many countries.

For scientific purposes, precision clocks can be synchronized to within nanoseconds by dedicated satellite channels.

Many slave clocks of these types were in operation, most commonly in schools, offices, military bases, hospitals, railway networks, telephone exchanges and factories the world over.

In schools, the master clock was in the principal's office, with slave units in classrooms which were in other buildings on campus.

In factories, a system with a bell or horn could signal the end of a shift, lunchtime or break time.

Diagram of electric time system used around 1910 to keep time in factories, schools, and other large institutions. The master clock (bottom center) , controlled by a temperature-compensated mercury pendulum, is wired to slave clocks throughout the building. In addition to wall clocks, it also controls time stamps that are used to stamp documents with the time, and a turret clock used in a clock tower. The "program clock" is a timer that can be programmed with punched paper tape to ring bells or turn machines on and off at preprogrammed times.