Conversion to Ammeter and Voltmeter

Introduction

Galvanometer, voltmeter, and ammeter are all measuring devices in an electric circuit. A galvanometer can be made to work as a voltmeter and an ammeter by making a few changes. Read this article further to know the difference between an ammeter and a voltmeter. It will also explain the conversion process of a galvanometer into an ammeter and a voltmeter.

What is a Galvanometer?

A Galvanometer is a device that is used to find the magnitude and the direction of the current. It is used to detect only small or feeble currents. A galvanometer is known to have minimal resistance and is always connected in series in a circuit. 

The galvanometer has a coil of moderate resistance (not more than 100 ohms). This coil is suspended between the two poles of a strong horseshoe magnet that is laminated. It has a small current carrying capacity of 1 mA. When this current passes through the suspended coil, the needle on the galvanometer deflects. This deflection is due to the action of the magnetic force on the current-carrying coil. The needle deflection is proportional to the current passing through the suspended coil.

Since a galvanometer can measure only a small current, it needs to be converted into an ammeter to measure large currents.

Proportional Difference Between Ammeter and Voltmeter

An ammeter and a voltmeter are electrical instruments used to measure two distinct quantities. Besides that, there are several other differences between the ammeter and voltmeter, as mentioned below.

Ammeter

• An ammeter is a measuring device used to measure large currents that flow through a conductor.
• It is a low resistance galvanometer.
• It is always connected in a series in a circuit to allow all the electrons of the measured current to pass through it.
• An ideal ammeter has zero resistance. However, it does have a small internal resistance. This internal resistance influences the power loss in a circuit. The measuring range of the ammeter depends on this resistance.  

Voltmeter 

• An electrical instrument used to measure the difference between two points in a circuit is a voltmeter.
• It is a high-resistance galvanometer.
• Similar to how the objects in parallel experience the same potential difference, a voltmeter is always connected in parallel in the circuit.

• An ideal voltmeter must have very high resistance.
• A voltmeter is of two types: analogue and digital. An analogue voltmeter shows its measurement with the help of a deflecting needle, while a digital voltmeter displays digits for its reading.
• The working principle of an analogue voltmeter is based on torque. The current passing generates torque through the device. This torque helps deflect the needle. The deflection is directly proportional to the voltage measured.

How is a Galvanometer Converted into an Ammeter?

An ammeter measures the amount of current passing through a circuit. The conversion of a galvanometer into an ammeter is done by attaching a low resistance parallel to it. This is called “shunt” or “S.” The range of this newly-formed ammeter depends upon the shunt resistance values or S. 

Consider a circuit in which a resistance R and shunt S are connected in parallel combination with a voltage source.  Suppose ‘I’ current is passing through the circuit. When it reaches a junction, it gets divided into two parts. The first part goes along the galvanometer of resistance Rg and the current is  Ig. The other part goes through the shunt resistance. It is referred to as I – Ig.  The resistance of S is kept such that there is full deflection in the galvanometer.

Now, since the galvanometer and shunt resistances are connected in parallel to each other, the potential difference across their ends is the same.

Vg = Vs.

IgRg = ( I – Ig) S   (using ohm’s law)

S = IgRg  / ( I – Ig)

Ig = S I/ S + Rg. 

This gives us the relation between Ig and I

Ig ∝ I.

This states that the deflection in the galvanometer is directly proportional to the current passing through it.

Ɵ =    Ig/ G

Ɵ ∝ Ig

Ɵ ∝ I

Since the shunt resistance is connected in parallel to the galvanometer, it can be computed with the help of effective resistance.

1/ Reff  = 1/ Rg + 1/ S

 Reff  = Rg S/( Rg + S) 

Since the shunt resistance is small and the ratio of S and Rg is also small, we can conclude that Rg is also small. This means that the resistance offered by the ammeter is also low. Hence, the reading in the ammeter is always lower than the actual current in the circuit. 

Suppose Iideal is the current passed through an ideal ammeter, and Iactual is the current passing through our circuit. In that case, the percentage error will be given by:

% error = ∆I/I×100 % 

% error = ( Iideal – Iactual)/ Iactual × 100%

Key point:

To increase the range of ammeter n times, the value of shunt resistance should be:

 S = G/ (n-1)

How is a Galvanometer Converted to a Voltmeter?

The conversion of a galvanometer to a voltmeter can also be done in the same way. The only difference here is that we connect a very high resistance series to the galvanometer.  

The potential difference in such a case will be the sum of the potential difference across the galvanometer and high resistance.

The scale is measured in volt. The range of the voltmeter will be determined by the value of high resistance connected in series. 

Let

• The resistance of galvanometer = G
• The current required for full-scale deflection in galvanometer = Ig
• Range of voltmeter= V
• Resistance of voltmeter = R

Therefore,

V = Ig(R + G)

Or,  IgR = V –  IgG

Or,  R = (V / Ig) – G

The effective resistance of the voltmeter is given by G + R. This effective resistance is very high. Therefore, the voltmeter is connected in parallel in a circuit to allow it to draw the least current from the circuit.

Conclusion 

A galvanometer is a sensitive device that is specially designed to measure the feeble current flowing through a circuit. In case you want to measure the large current passing through a circuit, you have to connect a shunt resistance in parallel to the galvanometer. This arrangement will give you an ammeter. It is offered in low resistance so that it won’t change the current passing through. And if you connect a high resistance shunt in series with a galvanometer, it will result in the formation of a voltmeter. It is used to measure the potential difference across two points in an electrical circuit.