Milk is a watery fluid. It gets its body and its colors from tiny white and cream particles suspended in the fluids Leave a pan of fresh milk in a cool place for a day or so. The cream will float to the top and you can take it off with a skimmer. This is because the cream is made of the lighter particles in the milk. Given time the heavier particles will sink in the fluid, just as a stone sinks through soft mud. The job is done by gravity, the force which hugs all things to the earth, The heavier substances with greater weight tend to sink through the lighter substances.
Hundreds of years ago* people discovered how to speed up this sorting work done by gravity. You have swung a ball on a string around and around your head. You felt the ball try to pull away from you with the whirling, speed. ‑Maybe you have„ dared to swing a pail of water over and over, if you swung fast enough, the water stayed in the pails even as it turned upside down. The water, like the ball, was trying to pull away from the center of a whirling circle..
These are simple examples of centrifugal force. This pulling power is gathered by any object whizzing around in a circle. A heavy substance tends to pull harder and get farther away from the center of the circle. A lighter substance pulls less and whizzes around in a smaller circle. Centrifugal force acts like speeded‑up gravity.
The faster the spin, the greater the centrifugal force. An object spinning at 600 times a minute in a circle just under 8 inches wide has centrifugal force equal to 41 times the pull of gravity. Twice as fast, it has four times this gravity, ten times as fast it has a hundred times this force. In the laboratory, centrifugal force has been built up to equal five million times the pull of earth's gravity.
This is the kind of force that does the work in the cream separator, In the past 60 years, the wonderful machine has taken over the job done by slow gravity. It can separate the cream from the skimmed milk in a few minutes. What’s more, it can do a better job. By the old method, maybe 20% of the butter fat was left in the skimmed milk. A modern separator leaves only one blob of fat in a thousand.
The separating is done in a cylinder or drum whirled around by an electric motor. The fluid milk enters the merry‑go‑round from below. Centrifugal force tosses it away from the center of the spinning drum. The force acts on the fluid and on each tiny particle of fat or solid suspended in it.
The heavier particles feel more of the centrifugal force. They are thrown farthest from the center and gravitate to the wall of the drum. The lighter particles of fat feel less of the force and come to the surface of the spinning fluid. The separator revolves from 5000 to 9000 times a minute. The job is soon done and the light and heavy particles are separated into layers. The fluid with the light particles is the cream. The fluid with the heavier particles is the skimmed milk. Each fluid is drawn off through its own exit.