Mixed bonanza

Inside an old pocket watch

A look inside to see how the hands turn and the watch ticks:

Once people used an hourglass to tell the time. Every hour they had to turn the hourglass over. But a watch will go for a much longer time before it needs to be wound up. When you wind up a watch you are winding up a spring.

As the watch ticks away, the spring is allowed to unwind very slowly.

This main spring (coloured yellow in the picture) turns a whole system of cog-wheels. These cog-wheels are all 'geared up'.

This means that each cog-wheel turns the next one at a faster rate. Here is the way the system works. Follow it on the picture.

1. The yellow-and-red pair of wheels drives the red-and-green pair.

2. The red-and-green pair drives the green-and-mauve pair.

3. The green-and-mauve pair drives the mauve-and-white pair.

How the hands are turned

The whole system is geared to make the green-and-mauve pair of wheels turn 60 times faster than the yellow-and-red pair.

Why 60 times? Because the green-and-mauve pair turns the second hand, the yellow-and-red pair turns the minute hand and there are 60 seconds in a minute.

The hour hand, however, has to be 'geared down' to turn 12 times more slowly than the minute hand. So there are four extra cog-wheels (pale blue) which reduce the speed of the hour hand.

Keeping the speed constant

At the end of the system of gears you will see a white cog-wheel. This is called the escapement wheel. This wheel is allowed to unwind one tooth at a time by the pallet.

The pallet rocks to and fro and holds each tooth of the escapement wheel for a moment before letting it go. It rocks because it is joined at the top to the heavy balance wheel. This bounces from side to side on the hair spring. This movement is what makes the watch tick.

Courtesy: Finding Out