The anwers is very simple. .
Balances are used to measure the mass of objects. One of the most commonly used types in the laboratory is the pan balance, which allows finding the unknown mass of a body by comparing it with a known mass, consisting of a certain number of weights.
It consists of a support on which a bar is supported from which two saucers hang. At the midpoint of the bar is a needle.
The object to be weighed is placed on one of the pans and weights of known mass are placed on the other pan until the needle indicate that the balance is balanced.