The use of master clocks reached its peak sometime in the mid-20th century. During this time, large offices and factory buildings needed to keep workers on the exact same time schedule. This was achieved by using one master clock that was wired to various smaller clocks — called “slave clocks.” The master would keep time and synchronize the slaves via electronic signal. Thousands of these clocks were installed, in schools, offices, railway networks, telephone exchanges and factories the world over.
Up until the mid eighties, the master was almost always a pendulum clock housed in an ornate wooded case with a glass front door and a face (dial) mounted at the top, similar to a grandfather clock but with a less ornate case. They were usually quite beautiful and expensive, placed in high traffic areas where visitors could admire them. Some systems used magnets to keep the pendulums swinging, while others relied on mechanical mechanisms.
Today many modern clocks are synchronized (either through the Internet or by radio signals) to a worldwide time standard called Coordinated Universal Time (UTC). A modern, atomic version of a master clock can be found at the U.S. Naval Observatory.