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The galvanometer is used to detect small values of current in a circuit. The voltmeter is used to calculate the potential difference across a circuit. However, it is possible to convert a galvanometer into a voltmeter. 

The voltmeter can be defined as an instrument used to measure the electrical potential difference across any two points in an electrical circuit. Usually, voltmeters are connected with high resistance so that minimal current passes through them and the measurement of potential differences becomes easier. The potential difference between the two points is the difference in electrical potential between them. Volts (V) is the unit of potential difference.

Voltmeter

A voltmeter can measure both direct and alternating currents. It generally gives value in volts, millivolts or kilovolts. 

  • 1 millivolts = 0.001 volts
  • 1 kilovolts = 1000 volts

The working principle of the voltmeter

The voltmeter is connected to a parallel circuit in which we want to measure the potential difference. It is done so because voltmeters are built with very high resistance. If they are connected in series, no current will flow, and the circuit will become open. A parallel connection makes the current break and passes through both of them. 

Working mechanism

We always have to connect the voltmeter in parallel circuits. In the DC voltmeter, positive and negative indicators are given. We must connect the positive end to higher potential and the negative end to lower potential. However, in the AC voltmeter there are no such indications and can be connected anyways. The working formula is given by  V = I(full scale)* R(series) + v(full scale).  

  • V= Voltage
  • I= current in full scale
  • R= resistance connected in series
  • v= voltage in full scale

The sensitivity of the voltmeter is the reciprocal of current in full scale. The smaller the current value, the more accurate and sensitive the voltmeter is. 

Types of voltmeter

  • Permanent Magnet Moving Coil Voltmeter (PMMC)
  • Moving Iron (MI) voltmeter
  • Electro Dynamometer Type voltmeter
  • Induction Type voltmeter
  • Electrostatic type voltmeter
  • Rectifier Type voltmeter
  • Digital Voltmeter (DVM)

Conversion of Galvanometer to Voltmeter

If we connect a high resistance in series with the galvanometer, it can be successfully converted into a voltmeter. A galvanometer can act as a voltmeter as long as we pass a very minimum voltage like 1mv. This is because:

  • A galvanometer has a specific resistance RG 
  • When we pass some current I, the pointer in the galvanometer will deflect
  • Now, voltage V can be calculated by the formula: V= IRG

But the problem arises when we are to pass a large voltage through the galvanometer; naturally, it breaks. Therefore we connect a high resistance in series with it so that the voltage that enters the galvanometer drops, and only a small part of that voltage enters. This high resistance is often called a multiplier. The formula of the high resistance is

R =(V – IGRG)/ IG.

Here, 

  • V= voltage provided
  • R= the resistance of the multiplier
  •  IG= the current of the galvanometer
  • RG= the resistance of the galvanometer
  • IGRG= the maximum voltage the galvanometer can handle

Advantages and disadvantages 

There are several benefits and drawbacks of using a moving coil galvanometer. Studying those will enlighten us about the benefits and drawbacks of converting a galvanometer to a voltmeter.

Advantages

  • It is highly accurate and reliable
  • It has high torque to weight ratio
  • Sensitivity can be increased
  • It is not easily affected by nearby magnetic fields
  • It has a uniform scale

Disadvantages

  • It can only measure direct currents and not alternating currents.
  • It becomes faulty as it becomes old. Nearby permanent magnets have an adverse effect, and the suspension might get damaged due to mechanical stress.
  • The counter torque changes when the surrounding temperature changes.
  • The counter torque can not be changed at our will.
  • Any overload in the system will result in a complete breakdown.

Conclusion

We have read how helpful a single moving coil galvanometer is. Besides a voltmeter, it can also be converted into an ammeter and an ohmmeter. The ammeter measures current, and the ohmmeter measures resistance.

To summarise, we always have to connect the voltmeter in parallel circuits. Connecting it in a series will result in an open circuit. We have to use high resistance while converting the galvanometer to a voltmeter. The resistance should be connected in a series circuit.