"Classic" Timekeeping, Part II

(Part I)

The state-of-the-art timekeeping technology a century ago was comprised of pendulum clocks. Refinements were made in the areas of obvious problems, such as the mechanical escapement which robs the system of energy, the vulnerability to changes in length from temperature and humidity, and vibrations. The culmination of this was the clock of W. H. Shortt, which had two pendulums, a master and a slave. The master oscillator was a free pendulum, and as it did no work to drive any mechanism, it was able to keep very precise time. The pendulum was made of invar, a material that had a very low thermal coefficient of expansion, and was encased in a chamber that was evacuated to several millitorr of pressure. The chamber was bolted to a wall that typically rested on a massive platform of the type used for telescopes, which minimized effects from vibrations. The pendulum was given an occasional boost to keep its amplitude roughly constant. The slave pendulum, which did the mechanical work of the system, received periodic electronic impulses from the master clock to correct its motion. This type of clock could keep time to better than a millisecond a day. A shortcoming (as it were) was in the measurement of the time; as Loomis notes

This remarkable result is accomplished through the possibility of averaging a large number of observations. A single impulse from a master Shortt clock has an uncertainty of 1 or 2 milli-seeonds. The master pendulum carries a small wheel. The impulse arm rests on this wheel, and as the pendulum swings out the pallet on this arm travels down the edge of the wheel, finally falling clear . It then trips an arm which falls, making the electric contact . If the small wheel is not exactly circular the arm will fall at slightly different times as the wheel is given a small turn with each fall. These variations are entirely smoothed out when a series of sparks are averaged.

So while the clock is precise in the long-term, the system of measuring it (described below) is limited at shorter durations.

By the late 1920’s true electronic timing had begun with quartz oscillators. Quartz is piezoelectric, meaning that an external pressure will induce a voltage in the crystal, and likewise, an applied voltage will cause a stress or strain in the material, and the size and shape of the material will dictate its resonance frequency. The crystal used in these measurements oscillated at 100,000 Hz, which allowed for much more precise comparisons between these types of clocks. This was necessary, as crystal oscillators tend to drift, and pair-wise measurement of at least three clocks is required to begin to properly characterize any single device.

What is then needed is a way to compare clocks and record the timing differences, and this was the Loomis chronograph. It was basically a spark chart recorder, whose motion was tied in to the master crystal oscillator located at Bell Telephone Labs in New York City; the frequency was divided down to 1 kHz (division can be done cleanly, without the addition of noise, while multiplying always adds noise) and sent over a dedicated phone line. This was fed into a mechanical device that further reduced the frequency through gears to turn a rotor at 10 revolutions per second. Also connected was a ring (which rotated with the device) with 100 equally-spaced steel phonograph needles, which were each wired to a corresponding needle above the paper of the chart recorder, like a giant comb. This made each needle correspond to a time difference of a millisecond (100 needles at 0.1 second from the rotation), and this translated directly into a measurable distance on the paper. Impulses from external clocks are fed in and triggered a 240 V discharge, which was sufficient to leave a visible mark on the paper. If the external clock was advancing at the same rate as the crystal oscillator, there would be a straight line of spark marks, but if they were at slightly different frequencies, the line would have a slope which could be measured. Multiple clocks could be recorded and compared not only to the crystal, but to each other; since the crystal was common to both measurements, if they were differenced, the crystal’s performance would drop out of the data, leaving only the comparison of the two clocks.

Three of the Shortt clocks were installed at Tuxedo Park:

The three clocks at Tuxedo are mounted on independent massive masonry piers. The piers are arranged in a triangle so that the planes of the three pendulums form an equilateral triangle. The piers are built directly on the solid rock which makes up the mountain on which the laboratory is situated. The neighbourhood is remarkably free from vibration, as the nearest railway is more than two miles distant and there is no heavy road traffic within a mile. The vault in which the piers are located is kept at a constant temperature of 21°’00 ± ’02 centigrade.

Loomis ends the paper with a few tidbits about operating the clocks, noting that there’s an optimum pressure of 15-25 millitorr; higher pressures degrade the performance from air resistance, but the lower pressures add errors from larger oscillations of the pendulum. The system works best when pumped out as low as possible and backfilled with nitrogen, since the presence of oxygen the sparks that occur at the electrical contacts caused oxidation and the loss of oxygen reduced the pressure in the chamber, affecting the clock’s rate. If a pendulum were to stop for some reason, it could be restarted by briefly opening one of the valves to let in a puff of air, which would disturb the pendulum. By repeating this several times at multiples of two seconds*, the impulses will be in phase and the amplitude can be built up, at which point the chamber can be pumped down again to its operating value. Finally, a note to avoid using all four mounting bolts, since that overconstrains the chassis (there being only three rotational degrees of freedom) and this applies stress that can cause a vacuum leak.

*Two seconds is the period of a pendulum approximately one meter in length; the square root of g is almost exactly equal to pi (they agree to better than half a percent). I managed to go completely through school (undergrad and grad) before noticing that, though I’m convinced that had I been born a few years earlier and had been required to do my physics calculations with a slide rule rather than an electronic calculator, I would have picked up on this shortcut.

It had once been proposed that the meter be defined this way, with a two-second period defining a meter, but there would have been considerable difficulty in realizing that standard due to variations in g. That quadrant of the earth representing 10,000 km was chosen instead.

Part III will discuss results using these clocks and measurement system