William Thomson's Current Balance
Figure 1: William Thomson's Current Balance
Figure 2: William Thomson's Current Balance
Figure 3: Manufacturer's mark on William Thomson's Current Balance
A balance that measures current by using that current to make a magnetic field that deflects the lever arm, then putting weights on the arm to bring it back to its starting position. How much torque you need to apply is proportional to how much force the field applied, which is in turn proportional to how much current there is.