The common balance works on the principle of equality of

HAve two arms suspended and one known weight

Explanation:

the common balance is a balance that we all have seen in examples of Justice, it was well known in the old times, but nowadays its more common seeing it as a jsutice symbol, it works in the principle of having two weights accross a balance and letting gravity work on them, while you have an object on one side of the balance of which you know the mass and you add mass to the object in the othar side of the balance to equalize the weights.

the higher the current the higher the mag field around it the higher the influence on the compass