Current balance

 I clamped an aluminium bar over a chemical balance. There was a strong magnet on the chemical balance. I zeroed the balance. I chose aluminium so that it was not attracted to the magnet.

 Then I ran a large electric current through the aluminium rod. Suddenly we got a reading on the balance. This is because the current has its own magnetic field that repels the magnet. Thus the magnet pushes down on the balance.

I swapped the electrical connections over so that current flowed the other way. The reading on the balance became negative. This time the current's magnetic field was attracting the rod. If you use a potentiometer, you can vary the current through the rod. Plotting a graph of current against force as measured by the balance will show that the two are proportional. This is why it is called a current balance: every force reading can be converted into a current.