27.9: Galvanometer
Common devices, including car instrument panels, battery chargers, and inexpensive electrical instruments, measure potential difference (voltage), current, or resistance using a d'Arsonval galvanometer. This electromechanical instrument is also known as a moving coil galvanometer.
The galvanometer consists of two concave-shaped permanent magnets, providing a uniform radial magnetic field in the annular region. In the center, a pivoted coil of fine copper wire is placed in the uniform magnetic field. Attached to the coil is a spring, similar to the hairspring on the balance wheel of a watch. A pointer needle is attached to the coil via the spring. There is a calibrated scale provided below the pointer for measurement.
In the equilibrium position, when there is no current in the coil, the needle points to zero. When there is a current flow in the coil, the magnetic field exerts a torque on the coil, which is proportional to the current. As a result, the coil turns, and the spring exerts a restoring torque proportional to the angular displacement. Thus, the angular deflection of the coil and pointer is directly proportional to the coil current, and the device can be calibrated to measure current.
The maximum deflection is called the full-scale deflection. The essential electrical characteristics of the meter are the current required for full-scale deflection and the coil's resistance. The meter deflection is proportional to the current in the coil. If the coil obeys Ohm's law, the current is proportional to the potential difference between the terminals of the coil, and the deflection is also proportional to this potential difference.
