> Using modern optical sensors which detected the precise time of passage of the pendulum without disturbing it, he compared its rate to an atomic clock for a m

> Using modern optical sensors which detected the precise time of passage of the pendulum without disturbing it, he compared its rate to an atomic clock for a month. He found that it was stable to 200 microseconds per day (2.31 ppb), equivalent to an error rate of one second in 12 years, far more accurate than the 1 second per year that was previously measured. This is incredible. Its performance matched or exceeded quartz oscillators in modern electronic systems [0]. It's the ultimate (electro-)mechanical clock [1]. A good general-purpose quartz crystal oscillator is only stable to 20,000 ppb, a good temperature-compensated oscillator is stable to 1000 ppb, and a crystal oven (a quartz crystal in a sealed, temperature-controller oven, the best quartz oscillator you can ever made) is stable to 1 ppb. And this ultimate mechanical clock is stable to 2.31 ppb [0], I'd happy to use it as the time base for my SDR receiver or spectrum analyzer if I was given the chance... It's only measured for a month, so long-term stability can be worse (and crystal oven's short-term stability can be better), but you get the idea. > Shortt clocks kept time with two pendulums, a master pendulum swinging in a vacuum tank and a slave pendulum in a separate clock, which was synchronized to the master by an electric circuit and electromagnets. The slave pendulum was attached to the timekeeping mechanisms of the clock, leaving the master pendulum virtually free of external disturbances. The basic principle of accurate timekeeping - using a free-running primary oscillator as a reference, and lock a less-accurate secondary oscillator to the primary, is vital in electronic systems. Atomic clocks, radio/GPS time signal receivers, or NTP, all use this principle. This clock basically implemented a rudimentary form of PLL mechanically. [0] https://en.wikipedia.org/wiki/Crystal_oven#Comparison_with_other_frequency_standards https://en.wikipedia.org/wiki/Crystal_oven#Comparison_with_o... [1] Now electro-mechanical clocks are making the comeback, in the form of MEMS oscillators in consumer electronics. But MEMS oscillators are often slightly worse than its equivalent crystal oscillators, and their main advantage is size (i.e. chip-scale integration), not accuracy.