What every clock needs

A clock does not measure time the way a ruler measures length. There is nothing to hold up against "a second". What a clock really does is count something that repeats at a steady pace, and show you the count.

That "something that repeats" can be a swinging weight, a spinning wheel, a vibrating crystal or a vibrating atom. Whatever it is, every clock, ancient or modern, is built from the same four parts:

• An oscillator: the thing that repeats. It makes the same motion over and over, and it is the part that sets the pace.
• A counter: something that adds up the repeats (gears with teeth, or electronic circuits).
• A power source: a falling weight, a coiled spring or a battery, because every real oscillator loses a little energy each time it swings and would stop without a top-up.
• A display: hands on a dial, or digits on a screen, turning the count into hours, minutes and seconds.

The size of one repeat is the period, written T, measured in seconds. How many repeats happen each second is the frequency, written f, measured in hertz (Hz). They are two ways of saying the same thing:

f = 1 / T and T = 1 / f

A swing that takes 0.5 s has a frequency of 1 / 0.5 = 2 Hz: two swings every second. A slow swing of 2 s has a frequency of 0.5 Hz. Remember these two numbers: a pendulum with a 2 s period is the famous "seconds pendulum", because it ticks once each second (once on the way out, once on the way back).

The history of clocks is mostly the history of better oscillators. Here is the family you will meet in this course:

Higher frequency alone does not make a better clock, but it gives you finer slices of time to count, and the best oscillators are also very good at staying at their pace.

One word for that last quality: Q, the quality factor. A high-Q oscillator keeps ringing for a long time before it fades, like a good bell compared with a dull thud. It loses very little energy per swing, so small pushes and small disturbances change its pace very little. Moving from a pendulum to a balance wheel, then to quartz, then to atoms, mostly means moving to higher and higher Q. You will not calculate Q yet; just keep the picture of a bell that rings on and on.

An oscillator is also what separates a clock from a sundial. A sundial shows the time using the Sun's motion, so it works only in daylight and clear skies, and it shows solar time, which differs a little from "clock time" (the Sun, Earth & Moon lab explains why). A pendulum or a crystal keeps going at night, in a cave, on a ship, on the Moon. That independence from weather and sky is what made mechanical clocks so useful.