A vibration galvanometer is a type of
mirror galvanometer
A mirror galvanometer is an ammeter that indicates it has sensed an electric current by deflecting a light beam with a mirror. The beam of light projected on a scale acts as a long massless pointer. In 1826, Johann Christian Poggendorff devel ...
, usually with a coil suspended in the gap of a magnet or with a permanent magnet suspended in the field of an
electromagnet
An electromagnet is a type of magnet in which the magnetic field is produced by an electric current. Electromagnets usually consist of wire wound into a coil. A current through the wire creates a magnetic field which is concentrated in the ...
. The natural
oscillation frequency
Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
of the moving parts is carefully tuned to a specific
frequency
Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
; commonly 50 or 60 Hz. Higher frequencies up to 1 kHz are possible. Since the frequency depends on the mass of the moving elements, high frequency vibration galvanometers are very small with light coils and mirrors. The tuning of the vibration galvanometer is done by adjusting the tension of the suspension spring.
The vibration galvanometer is used for detecting alternating currents in the frequency of its natural resonance. Most common application is as a null indicating instrument in AC
bridge circuit
A bridge circuit is a topology of electrical circuitry in which two circuit branches (usually in parallel with each other) are "bridged" by a third branch connected between the first two branches at some intermediate point along them. The bridge ...
s and current comparators.
The sharp
resonance
Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillatin ...
of the vibration galvanometer makes it very sensitive to changes in the measured current frequency and it can be used as an accurate tuning device.
[W. J. M. Moore, W. J. M. Moore P. N. Miljanic '' The Current Comparator'' IET, 1988 pp. 41-42]
Frequency display
The frequency-sensitive behaviour of the galvanometer allows their use as a crude frequency meter, commonly used for adjusting the speed of AC
generator sets. The galvanometer is constructed as a number of moving-iron galvanometers, sharing the same excitation coil. As each is tuned to a slightly different frequency, one of them will resonate at a time, according to the input frequency. The magnets are conveniently constructed as a single iron 'comb' of individual reeds, each of different length. Their range is typically from around 45-55 Hz (for a 50 Hz base frequency), with around 2 Hz resolution between each. As it is the identity of the vibrating reed (and thus frequency) that is of interest, rather than its amplitude, these are not calibrated. They are often viewed end-on, as the clearest viewpoint for the whole comb.
See also
*
Reed receiver
A reed receiver or tuned reed receiver (US) was a form of multi-channel signal decoder used for early radio control systems. It uses a simple electromechanical device or '' 'resonant reed' '' to demodulate the signal, in effect a receive-only mod ...
References
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Galvanometers