In our exhibition Time and Navigation visitors can set their watches by a working cesium frequency standard, commonly known as an "atomic clock," on loan from the National Museum of American History. The exhibit allows visitors to see different methods of measuring time, including mechanical and electrical clocks. A digital display on the atomic clock shows the global reference known as the Coordinated Universal Time or UTC. A separate display connected to the clock shows local time, which visitors can use to set their watches. While the device is not connected to outside time sources, it will keep accurate time within a tiny fraction of a second over the foreseeable future. We jokingly called it our "Box of Time."

What is an Atomic Clock?
Atomic clocks maintain very stable time references at specialized laboratories such as the U.S. Naval Observatory and the National Institute of Standards and Technology. The time is distributed all over the world by satellites, radio signals, fiber optic connections, and computer networks. These time standards are essential for synchronizing data connections, communications, transportation, and countless other aspects of modern society.

Atomic "clocks" can be more precisely called frequency standards. They maintain stable frequencies by measuring changes in the energy state of heavy elements such as cesium. These devices know exactly the length of each second with a precision of a billionth of a second. The unit sends out pulses exactly one second apart. By itself, the frequency standard doesn't actually know the time of day. Keeping track of that requires a second piece of equipment: A time code generator. This device takes the pulses from the frequency standard to keep track of hours, minutes, and seconds. (Jump to minute 17:30 of this STEM in 30 episode to see the U.S. Naval Observatory and learn more about how an atomic clock works.)

Setting Up the Clock in the Museum
Our atomic clock was the last thing to be installed in our exhibition in 2014. Before it could be installed, we needed the frequency standard to be calibrated because each atomic clock can run at a slightly different rate. To determine how ours was working, we wanted to compare its operation to the national reference time. Fortunately for us, this originates right in Washington, DC at the U.S. Naval Observatory. The staff there agreed to let us bring our clock in for calibration. It was a tricky procedure. We had to calibrate both the frequency standard and the time code generator and then bring them back to the Museum. During all this, electrical power to the clock had to be maintained. We brought along a battery power unit used for computers. Along with the internal battery backup in the frequency standard, we hoped this would give us about 90 minutes of power. To be safe, we planned to plug in the whole system to the vehicle's power supply.

With a plan in place, we picked up the clock from the National Museum of American History, along with its curator Roger Sherman. The unit had a helpful note on top that said, "Roger's Atomic Clock." The battery backup worked flawlessly as we made our way up Massachusetts Avenue to the Naval Observatory.

Once at the Observatory’s time service building, we plugged in the necessary cables to compare our clock with the U.S. master clock. The initial comparison showed that our clock was running about 24 nanoseconds (billionths of a second) slow. After a couple hours, this offset had changed to less than a nanosecond. This told us the frequency standard was running well. Over the next 10 years it will drift out of sync with the national time reference by only less than 1/10,000th of a second. That sounded good enough for museum visitors to set their watches. While there we also set the time of day on the time code generator.

We packed up the frequency standard, the time code generator, the battery backup, and began the drive back to the Museum. I was behind the wheel with the power supply plugged into the dashboard. Roger was in the back seat with the equipment. At one point, driving down Independence Avenue, something began to emit ear-splitting cries. Roger and I tried to determine which piece of equipment was complaining. It turned out to be the overloaded power supply. I pulled the plug out of the dashboard port, which was so hot it almost burned my fingers. Then the UPS on the floor started beeping loudly because it wasn't getting power. Everything was confusion in the vehicle as we shouted above the noisy equipment while checking all the units and cables. But after that brief moment of excitement, we had enough juice in the battery backup to make it the rest of the way to the Museum. After some careful coordination with all the cables, we got it mounted in its display case where it continues to display the time.

Andrew Johnston (left) and Roger Sherman (right) deliver the atomic clock to the Time and Navigation exhibition, which is under construction in the background. The atomic clock, sitting on the cart along with the time code generator and the battery power supply, is reading 16:35:13 Universal time (2:35 pm local time). Image Courtesy: Harold Dorwin

This wouldn’t be the last time we needed to adjust our atomic clock. In June 2015 we had to account for a leap second. In my next post I’ll explain what a leap second is and how we updated our atomic clock.

 A special thanks to everyone at the U.S. Naval Observatory, the people at Symetricomm (now Microsemi) who manufactured the clock, and Roger Sherman at the National Museum of American History.