It is made according to the lever principle. There are two small plates at both ends of the lever, one for putting heavy objects and the other for weighing objects. The center of the lever is equipped with a pointer. When the two ends are balanced, the masses (weights) of the two ends are equal. It is a common weighing instrument. The accuracy is not high, generally 0. 1 or 0.2g The load is100g, 200g, 500g,1000g, etc.

It consists of a tray, a cross beam, a balance nut, a scale, a pointer, a knife edge, a base, a scale, a stroke code, a weight and the like. The balance beam is supported at the center of the beam by a fulcrum (shaft), forming two arms, each of which is suspended or supported by a disk.

One disc (usually the right disc) contains an object (weight) with a known weight, and the other disc (usually the left disc) contains an object to be weighed, and the walking code slides on the scale. When the pointer fixed on the beam does not swing and points to the middle scale, or when the swing amplitude from left to right is small and equal, the sum of the weight and the traveling code position indicator indicates the weight of the weighed object.

The earliest scales were invented by ancient Egyptians, more than 7000 years ago. This ancient balance is made of limestone. The length of the beam is 8.5 cm, and the fulcrum on both sides is equal in length. Heavy objects are also made of limestone. There are seven kinds of weights.

Around 1350 BC, a wooden balance appeared in Egypt, with a beam 30 cm long and weights made of small animals and birds made of bronze. In ancient Babylon, heavy objects were shaped like lions and sleeping ducks. At that time, goldsmiths and jewelers used this kind of balance, which was mainly used to weigh placer gold, medicines and precious stones.

It is said that in the 3rd century BC, the ancient Romans also invented the balance. This strange balance has different lengths on both sides of the fulcrum. In fact, it has become a steelyard. Interestingly, the Romans made the shape of the weight into the shape of pomegranate.

In the Middle Ages, Arabs also began to use scales. These unscrupulous Arab businessmen seem to have mastered the mystery of the balance. They hollowed out the beam and injected a small amount of mercury. Such a scale will weigh things lightly and heavily, defrauding customers of their money.