This is the hydrostatic balance.
It's useful to demonstrate the existence of buoyancy or upthrust, a force exerted by fluids, that is, liquids and gases.
First, balance the scale with lead balls.
Archimedes' principle says that: "Every object immersed in a fluid experiences an upward thrust equal to the weight of dislodged fluid."
The greater the volume of the object, the greater the thrust.
The forces acting on the scale are: thrust 1, thrust 2, weight 1 and weight 2.
Weight 1 minus thrust 1 equals force 1.
Weight 2 minus thrust 2 equals force 2.
Forces 1 and 2 are equal, so the balance is balanced.
Place the balance in the vacuum hood.
Put the bell.
Plugg in.
Extract the air.
As the air is removed, the thrusts disappear and the balance tips towards the heaviest side: that of the ball.
The ball has greater weight and had greater thrust.
By allowing air to enter, the balance balances again.