Queen & Company Makers Electrodynamic Ammeter

This is an ammeter, an instrument used to measure electric current. Specifically, this is an electrodynamic ammeter, a variant which functions by passing a current through both a fixed coil and a movable coil which is free to rotate around the fixed coil. When a current is run through the coils, the resulting magnetic field will attempt to align the moving coil with the fixed coil with a force proportional to the strength of the current.  By turning the knob at the top, one can twist the spring which the movable coil hangs from and exert an opposing force on the coils. By seeing at what strength this opposing force cancels out the magnetic force, one can determine the strength of the magnetic force and from that calculate the strength of the current.