Floating Crust:

Two centuries ago, scientists noticed a curious thing about the behavior of a pendulum clock. When such a clock was taken towards the equator it ran slower.

When it was taken away from the equator it ran faster. Why?

Gravity and the Pendulum:

The Earth has a bugle around the equator. Points near the equator are farther from the Earth’s centre than points elsewhere.

The added distance lessens the force of gravity. A pendulum swings because gravity pulls upon it.

The weaker the pull, the slower the swing; the greater the pull, the faster the swing. The pendulum clock ran slower or faster depending on the force of gravity tugging on the pendulum.

As a pendulum goes on swinging, friction gradually breaks the action. The swings become slower, but they also become narrower, so each swing still takes the same amount of time.

This time is called the period of the pendulum. It is a good indicator of the force of gravity. Where gravity is greater, the period is shorter.

Where gravity is less, the period is longer. Gravity differences are slight, and cause only slight differences in the period of the pendulum.

Therefore, the period must be measured very accurately. It is not enough to time one swing.

Instead the pendulum is allowed to make thousands of swings, which are counted automatically. Then the time is divided by the number of swigs. This methods gives the period to within one ten millionth of a second.

Gravity and the Spring Scale:

The pendulum has drawbacks as a gravity to take a measurement. In addition, the instrument is too heavy to be carried by hand.

So scientists also use a simpler tool. This is the gravimeter, which is just a sensitive tool. This is the gravimeter, which is just a sensitive spring scale.

The spring is a fine, hair like coil, from which a small mass hangs. The forces of gravity pulling on the mass are shown by the stretching of the spring.

With this light, easily-carried instrument, a reading can be taken in a few minutes. Scientists begin a gravity survey by taking a measurement with the pendulum.

This gives them a figure which they use in setting the scale of the gravimeter. Then readings from the scale stand for small variations in gravity.

Floating Crust