Steelyard ammeter, late 19th century.
© Science Museum / Science & Society Picture Library
This device measures the current of electricity flowing in laboratory experiments. The more current, the further along the scale the weight has to be moved to balance the magnetic force produced in the coil. The current is a measure of the rate of flow electricity and the voltage is a measure of its energy. The total power, or amount of electrical energy supplied every second, is simply equal to the current multiplied by the voltage.