History Lesson | Master Clocks and Slave Clocks

Today, the experts at Chelsea Clock have a fun history lesson about the master clock time system. Many modern clocks are digitally synchronized (either through the internet or by radio signals) to a worldwide time standard called Coordinated Universal Time (UTC). This is why you never have to worry about setting the clock on your computer or cell phone - and why even many present-day watches and wall clocks automatically set to the correct time.

The reality, though, is that this modern synchronization is simply a more advanced version of the master clock and slave clock. Before atomic and digital clocks, this transmission system was used on a localized level by companies and organizations that needed everyone in a large building to be operating on the same time. This overview of the master time system will help you understand the origins of how people have kept time in sync for better efficiency.

What Is the Master Clock?

A master clock is one that is electronically wired to a series of secondary clocks or "slave clocks." The master would keep time and synchronize the slave clocks via an electronic signal, ensuring they displayed the same time. The phase of the secondary clocks is adjusted to account for the time it takes for the master clock signal to arrive. Sometimes the master clock is what keeps precision time; in other cases, a "program clock" is wired to the master clock to properly regulate it. Wall clocks, tower clock motors, time stamps and much more can act as slave clocks.

The master clock system originated in the late 1800s and 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. Thousands of these clock networks were installed in schools, offices, railway networks, telephone exchanges and factories the world over.

Up until the mid-1980s, the master was almost always a pendulum clock housed in an ornate wooded case with a glass-front door and a dial mounted at the top. Its look was 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.

Present-Day Master Clocks

A modern atomic version of a master clock can be found at the U.S. Naval Observatory. Maintained by the USNO Time Service Department, it's used by a large portion of the world's communications, scientific and financial infrastructure to ensure accuracy.

Today, the master clock source for many timepieces is usually a satellite- or radio-based reference. Satellite master clock references include GPS and GLONASS, while countries often use a radio signal. For example, the National Institute of Standards and Technology operates WWVB (located near Fort Collins, Colorado) as a master time signal transmitter. Most of North America's radio-controlled clocks sync with its transmission.