27.9:
Galvanometer
A galvanometer is an electrical device that detects electric current, ranging from micro to milli amperes.
It consists of two concave-shaped permanent magnets, providing a uniform radial magnetic field in the center.
A coil of fine copper wire wound on a soft iron core is placed between the permanent magnets. A needle and spring assembly is attached to the axis of this coil.
If no current flows through the coil, the needle points at the center of the scale.
When there is a current flow, the magnetic field exerts a torque on the coil; as a result, the coil turns.
The magnitude of torque is proportional to the current passed in the coil. When there is no current, the spring exerts a restoring torque.
Under equilibrium conditions, the angular deflection of the coil is obtained which is directly proportional to the coil current.
The full-scale deflection is typically on the order of milli amperes.
The potential difference required for full deflection can be obtained using Ohm's law, where the coil resistance is typically ten to thousand ohms.
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.
Suggested Reading
- Young, H.D. and Freedman, R.A. (2012). University Physics with Modern Physics. San Francisco, CA: Pearson. pp.860