Half the fun of getting vintage test equipment is getting to open it up. Maybe that’s even more than half of the fun. [CuriousMarc] got an HP 5061A Cesium clock, a somewhat famous instrument as the model that attempted to prove the theory of relativity. The reason? The clock was really the first that could easily be moved around, including being put on an airplane. You can watch the video below.

According to the video, you can simplify special relativity to saying that time slows down if you go fast — that is known as time dilation. General relativity indicates that time slows down with increasing gravity. Therefore, using airborne Cesium clocks, you could fly a clock in circles high up or fly at high speeds and check Einstein’s predictions.

We really like the line from the device’s manual: “The Model 5061A Cesium Beam Frequency Standard is itself capable of defining frequency and therefore needs no calibration.” The definition of the hyperfine transitions of cesium was — by international agreement — 9,192,631,770 Hz.

A 5 MHz crystal oscillator generates a signal that is modulated by a 137 Hz signal. The modulated signal gets multiplied by 18 and then, using harmonics, by 102. This provides a signal of roughly the correct value which excites the cesium beam tube. The tube’s output is used in a phase-locked loop to discipline the 5 MHz clock to be right on. This works because the tube will put out twice the modulated frequency if the carrier frequency is exactly the same as the cesium’s hyperfine transition frequency. If there is any variation, the output will include the fundamental frequency (137 Hz) which is absent if the loop is in lock.

The tube eventually runs out of cesium because the instrument literally boils away a few grams of cesium. [Marc’s] appeared to have some life still left in it. You can tell the instrument has been serviced because the tube is marked as Agilent, so it isn’t the original tube.

The video has a good explanation of cesium and exactly what hyperfine transition is. The defined frequency is in the absence of magnetic fields, but since the Earth has a magnetic field you have to have a strategy to deal with that. In the tube, there are strong magnets that overrule the Earth’s field. This cause an error, but a known error that the instrument can correct.

Ever wonder how clock standards were before the nuclear age? Most of us still have to settle for rubidium or GPS discipline.