SOLUTION

Vibration galvanometer instrument can be used in ac bridges for less frequencies upto 200 Hz only.

• A galvanometer is an instrument that is used to detect the presence and measure magnitudes of the small currents or voltages in the measuring circuits.
• Thus galvanometer term is generally used for any current or voltage indicating instrument which is not calibrated.

A.C Galvanometers: The A.C galvanometers are mainly used as a null detector in the bridge and potentiometer circuit. These are also used for measuring an effective or r.m.s. value of the small A.C current flowing in the circuit. The a.c. galvanometers are further classified as phase-sensitive galvanometers and frequency-sensitive galvanometers.

• The phase-sensitive galvanometers are dynamometer-type galvanometers in which a moving system is suspended with a long suspension win a mirror attached to it.
• To make the instrument is highly sensitive, the moving system should be light.
• To protect the moving coil from the electrostatic field between fixed and moving coils it is necessary to screen the moving item properly.
• To increase the sensitivity of the instrument, a laminated iron core is used for the fixed coil to achieve a stronger magnetic field In this type of galvanometer, the magnetic field is setup by an a.c electromagnet.
• The frequency selective galvanometer is called a tuned detector type galvanometer.
• It is tuned at an operating frequency and it is generally used at frequencies below 250 Hz as the galvanometer is found to be more sensitive than the telephone.
• The common frequency-selective galvanometer is the vibration galvanometer. 

Vibration Galvanometers

Vibration galvanometers are the more sensitive bridge detectors and are constructed in the frequency range from 5 c/s to 1000 c/s but their principal application is below 200 cps where their sensitivity is greater than telephone receiver. At power frequency, their sensitivity closely matches with those of d.c. galvanometers. Thus the response of vibration galvanometer is limited to a very narrow range of frequency. When tuned to the fundamental frequency, it will have an extremely small response to harmonics. Vibration galvanometers are of two types:

(i) Moving-magnet-type vibration galvanometer

(ii) Moving-coil-type vibration galvanometer