Electrical Engineering ⇒ Topic : Vibration Galvanometer
Vibration galvanometers are widely used as null-point detectors in a.c. bridges and a.c. potentiometers. These galvanometers are very sensitive because they utilise the principle of mechanical resonance.
The most commonly used vibration galvanometer is of moving coil type in which a coil is suspended between the poles of a permanent magnet. When alternating current is passed through the coil, alternating deflecting torque is produced which causes the coil to vibrate with a frequency equal to the frequency of curr ent passing through the coil. Due to inertia of the moving system, the amplitude of vibrations is small. However, if the natural frequency of the moving system is made equal to the frequency of the current passing through the coil, mechanical resonance occurs and the moving system vibrates with a large amplitude.
Construction. Fig. (a) shows the basic construction of a typical moving coil vibration galvanometer. The moving coil consists of a single loop of a fine bronze or platinum silver wire.This wire passes over a small pulley at the top and is pulled tight by a spring attached to the pulley as shown in Fig.(a). The moving system carries a small min-or upon which a beam of light is thrown. The natural frequency of the moving system can be changed by varying the length and/or tension of the suspension. The length of the suspension can be changed by raising or lowering the bridge piece. The tension in the suspension is changed by the spring. Tuning of a vibration galvanometer means adjusting the natural frequency of the moving system so that it is equal to the frequency of current passing through the coil of the galvanometer.
Working. The vibration galvanometer is *tuned and a beam of light is thrown on the minor.When current through the coil of the galvanometer is zero, a single spot of light is observed on the scale. However, when a small alternating current flows through the coil, the amplitude of vibration of the moving system is very large due to mechanical resonance. As a result, a large band of light is produced on the scale. The size of this band of light decreases as the current through the coil decreases and vice-versa. The bridge or potentiometer circuit is adjusted until a single spot of light is observed on the scale. Under this condition, the current in the coil of the galvanometer is zero and null-point is obtained. Some practice is necessary in observing when this condition has been obtained. It is usually best to switch the galvanometer in and out of the circuit and to note if there is any observable difference in the size of the spot in the two cases.
Note. The galvanometer should be well shunted to protect it from excessive current when the circuit is out of balance. When the balance is almost obtained, the shunt should be removed in order to achieve maximum sensitivity.
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