Galvanometer Type Measuring Instrument

Galvanometer is a basic electrical instrument. We use it to detect current in a circuit. Sometimes for measuring their magnitude also.

PRINCIPLE OF OPERATION OF GALVANOMETER

It works on the principle of conversion of electrical energy into mechanical energy. When we place a current carrying conductor in a magnetic field. Then it experiences a force. Then, this force tends to move in a direction. Fleming’s left hand rule decides direction of force.

CLASSIFICATION OF GALVANOMETERS

  1. D’Arsonval Galvanometer is the simple and most widely used.
  2. Ballastic Galvanometer: We use it to measure the quantity of charge passes through it. The “Flux meter” is a special type of Ballistic galvanometer type instrument.
  3. Use of vibration galvanometer as tuned detector is very wide. Frequency range is 5Hz to 1KHz.
  4. The Duddels Oscillograph is basically a vibration galvanometer. It has a low period of vibrations. The natural frequency is about 12 KHz.

D’ARSONVAL GALVANOMETER

It is the most popular Galvanometer. It is for detection of currents and voltages and also for the measurement purposes.

Construction of Galvanometer

It is a moving coil permanent magnet type instrument. The coil of many turns wound on a metal former. And it is suspended, so as to move about its vertical axis. The coil moves in the air gap between the pole pieces. The air gap is about 1.5 mm.

D'ARSONVAL GALVANOMETER
D’ARSONVAL GALVANOMETER

Coil is on a non-magnetic former. So the damping torque is provided by the eddy currents induced in the former during motion of the coil. The damping can also be provided by two methods:

  • By connecting a low resistance across the galvanometer.
  • By adjusting the value of the resistance.

The coil is suspended by a phosphor bronze filament. It also acts as a lead for the coil. The other lead is a flexible spiral filament at the bottom of the coil. The coil names are “upper” and “lowersuspensions. The poles of the magnet are usually cylindrical in shape. The length of air gap reduces by having a cylindrical poles. So that the amount of flux linking with the coil is increase. This increases the sensitivity of the instrument. And the flux between the poles becomes radical. Moreover, in a radial field, the deflection of the coil is directly proportional to the current in the coil. This is the reason of using a uniform scale.

Working

When the current to be measured is made to flow through the coil. A deflecting torque proportional to the product of flux density, current and dimensions of the coil makes the coil to rotate about its vertical axis. Now the suspension filament produces controlling force. And the pointer stops when the deflecting and controlling torques equals each other. As the deflecting torque is directly proportional to the current. And the amount of deflection of the coil indicates the magnitude of current on a calibrated scale.

On few galvanometers, a mirror is attached to the moving system. A light beam is reflected from the mirror on to a glass scale. When the coil deflects. Then the light beam moves over the scale. The scale is about one meter away from Lamp the instrument. This provides high sensitivity to the galvanometer.

MIRROR ARRANGEMNT OF GALVANOMETER
MIRROR ARRANGEMNT OF GALVANOMETER

The upper and lower suspensions are very weak. Hence the instrument is not very strong mechanically. So the galvanometer needs to be handled carefully.

Derivation of deflecting torque equation

derivation OF GALVANOMETER

CONVERSION OF D’ARSONVAL GALVANOMETER INTO AMMETER

We can convert it into a ammeter. By connecting a low resistance parallel to it. And we call it shunt resistance. This resistance is called “shunt”. The value of shunt is to be designed according to the “range” of the currents under measurement. Usually, it is a low resistance.

CONVERSION OF D’ARSONVAL GALVANOMETER INTO VOLTMETER

We can also convert it to an voltmeter. By connecting a high resistance in series with it. And we call it as Multiplier. We decide the value of multiplier according to the value of voltage under measurement.

EXTENSION OF RANGE OF AMMETERS and VOLTMETERS

Ordinary ammeters and voltmeters can measure very small currents and voltages. But they requires higher values to measure in practice. Hence, it becomes necessary that the current or voltage we are measuring should be low. Current and voltage should be fetch in the range of voltmeter and ammeter.

Following are the devices we use for extension of range

  1. Shunts or Current transformer for ammeter.
  2. Multipliers or Potential transformers for voltmeters.

We use current and potential transformer to measure a high value of current and voltage. For example current and voltage of transmission lines.

Conclusion

From this article we can conclude that galvanometer is essential instrument. We can use it as voltmeter and ammeter.

Read also: Basic Measuring Instruments
           Types of Measuring Instruments
           Moving Iron Instruments
           Permanent Moving Coil Instruments
           Dynamometer Type Instruments