Voltmeter is an instrument that measures the voltage (difference in potential) between two points of an electric current. Most commercial voltmeters are galvanometers connected in series with a high resistance. They have scales that read in volts. A typical direct-current voltmeter has a magnet shaped like a horseshoe. To each pole (end) of the magnet is attached a semicircular piece of soft iron that also becomes magnetized. These pieces of soft iron direct the magnetic field toward a small iron cylinder placed between the poles of the magnet. Since soft iron becomes highly magnetized, this cylinder concentrates the magnetic field.
Surrounding the cylinder is a coil of thin copper wire wound on a light, rectangular frame. This coil is the movable coil that carries the electric current. Each end of the wire coil is connected to a small spiral spring. As the coil moves, a needle attached to the coil also moves. This needle moves across a dial and indicates the reading in volts. A resistance of up to several thousand ohms is connected in series with the movable coil.
When the voltmeter is not in use, the frame does not move and the needle reads “zero.” When a current passes through the movable coil, the magnetic field of the horseshoe magnet acts on the current-carrying wires of the coil to produce a force on the coil. This force causes the coil to turn. The springs oppose the motion of the coil and are adjusted so that the position of the needle indicates the correct voltage. A voltmeter is always placed across the part of the circuit that is to be measured.
