The wheel of Barlow is a tool purely didactic, but is an example of an electric motor, in fact it transforms electrical energy into mechanical energy. The metal wheel with a horizontal axis has the edge outcropping in the mercury contained in a tank. An intense magnetic field is generated by a magnet. To the two clamps set on wooden board it connects a battery or a power supply of 12 volts DC. The circuit goes from the clamp at the center of the loop down along the wheel and get into the tank where is located the mercury below which there is a wire connected to the other clamp. As soon as you close the circuit the wheel starts spinning.
The explanation lies in the Lorentz force acting on the electrons which run radially when it closes the circuit.