To weigh something with a balance, you must first place it in one of the pans. You will notice that when you do, the lever tilts to one side like a seesaw with a heavy person at one end and a light person at the other. The reason for this is that one pan has something in it while the other is empty. In order to balance the lever you must put the same amount of weight in the empty pan. There are special weights for this purpose. These weights have the number of ounces or pounds they weigh printed on them.
You pick out a weight that you think will balance the lever. Then you put it in the empty pan. You watch to see if the lever is still unbalanced. If it is, you must put more weight in the pan. Sometimes the lever will tilt to the other side. This means that you have put too much weight in the pan. To balance the lever, you must take off some of the weights. When both pans are at the same level, you can count the weights. Whatever they weigh, that is the weight of the thing you are weighing. You can check to see if the balance is accurate. Switch the weights and put them in opposite pans. If the lever is still balanced, the balance is accurate. If it is unbalanced, something is wrong with the balance and must be corrected.
The kind of balance we have just described is called a lever, or equal-arm, balance. This is because the arms of the lever from which the pans hang are of equal length. Such balances were the first scales. They were used by the Egyptians, thousands of years ago, to weigh things like wheat. The wheat would be placed in one large pan or basket and a stone in the other. The stone usually weighed about fourteen pounds. When the lever was balanced, it meant that there were fourteen pounds When the lever was balanced, it meant that there were fourteen pounds of wheat in the basket. In England today you can still hear people say that a person weighs “ten stone”-that is, ten times fourteen pounds, or 140 pounds.
Balances are used in drug stores and laboratories to weigh chemicals. Some balances can be made to weigh things as light as one-thousandth of an ounce. Other balances can weigh very heavy ‘ things like people or trucks. In such a balance the arms are not equal. There is a weight on the long arm of the lever. This weight can be moved back and forth on the lever. The short arm of the lever is connected to a large platform. For trucks, this platform may be as large as the floor of a large room.
A weight on the long arm of a lever will balance a much heavier weight on the short arm. The farther out you place the weight on the long arm, the heavier the weight it will balance on the short arm. The weight on the lever is adjusted until the lever is perfectly level or horizontal. The lever looks something like a ruler. It is marked to tell you how much weight there is on the platform. This kind of balance is often called a beam balance. You may often see one in a drugstore. People can weigh themselves on it. A pound weight on the long arm of the lever may balance two hundred pounds or more on the platform.
