Last night while getting to sleep, I couldn鈥檛 help but notice the sound of the second hand on my wall clock. I would have thought that the mechanism would produce an identical noise at each movement, but there is a discernible difference, much like the 鈥渢ick-tock鈥� used to describe the sound. So is there really a difference or am I just conditioned to hear one?
鈥� The questioner doesn鈥檛 say if the clock is electric or mechanical, old or modern, but I鈥檓 supposing it is a pendulum clock powered by a falling weight or a spring. It will have an escapement that converts the unidirectional force of the weight or spring into the oscillating impulses needed to keep the pendulum swinging (see diagram) 鈥� or the balance wheel for a compact clock or watch.
There are hundreds of escapement designs and they all make a noise each swing. But the left and right swings will sound different because the escapement is made asymmetric to ensure the mechanism always turns in one direction. There will be two ticks for each full swing of the pendulum: left-right, tick-tock.
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You might notice a longer cycle of sounds, especially if there is a second-hand. If it isn鈥檛 perfectly balanced, it will give a once-per-minute addition to the tick-tock pattern. I look after . It uses what鈥檚 called a 鈥渄ouble three-legged gravity escapement鈥�, which has an especially loud tock every six ticks.
Using the ticks for accurate timing isn鈥檛 easy. I have fitted an optic sensor next to the Trinity pendulum, so I can time the accuracy of the clock to the nearest millisecond. It was more fun doing tick timing with a microphone but it was inaccurate, and hopeless amid the cacophony of the clock striking 12.
Some modern quartz clocks have an asymmetric drive mechanism, but most produce a sterile little tick once per second, and no tock. If I鈥檓 going to be kept awake by a clock, I would rather it played around a bit.
Hugh Hunt, Cambridge, UK