Conversion of Galvanometer to Ammeter & Voltmeter

1. What is a Galvanometer?

A galvanometer is a deflection-type meter used to measure current. The needle in the Galvanometer deflects when a current passes through it, and the deflection is proportional to the current.

There are two types of Galvanometers : 

• Uni-Directional
• Bi-Directional

• Unidirectional: In this case, the markings on the dial start at 0 and extend to the maximum range. It has a red terminal, which indicates it must be connected to a high-potential source, and the other is a black terminal for a lower-potential source.

• Bi-Directional: The dial’s zero is in the center, and the maximum is on either side. So, from the direction of deflection, we get to know the direction of current in the conductor, and the amount of deflection gives us the magnitude. 

So, the basic difference between the two is that Unidirectional can give information only about magnitude, while Bi-directional can tell direction as well as magnitude.

The inner setup of a galvanometer has something known as Coil Resistance ‘G,‘ and at maximum deflection, the safe current which flows through the galvanometer is ‘i_g‘. The symbol for a galvanometer is :

Before proceeding, you can cover Series and Parallel Combination

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2. Conversion to Ammeter

The purpose of an Ammeter is also to measure current, but the range for current measurement is much higher. 

• How to Convert? – Just add a resistor with very small resistance (Shunt ‘S’) in parallel to the Galvanometer

What happens because of this?

• Now suppose, ‘I‘ (I > i_g) is the current flowing in the conductor. Since S and G are connected in parallel, ‘I‘ will be divided into ‘i_g‘ and ‘I-i_g‘. 
• The shunt resistance S, being very small in magnitude, will attract a lot of current (since current always prefers the least resistance path). The shunt resistance S is the reason why we are able to supply a larger current than ig.
• This helps us to measure a larger current, resulting in an increase in the range of the galvanometer

How to calculate this ‘I‘?

G and S are in parallel combination

$(I – i_g)\times S = i_g\times G$

From this, find the value of ‘I’. This will give us the maximum safe current allowed into this designed Ammeter

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3. Conversion to Voltmeter

The Voltmeter is used specifically to measure the potential difference across the given terminals.

• How to Convert? – Add a very high ‘Load’ resistance R in series to the Galvanometer

What happens because of this?

Before adding R in series,

$\Delta V = (i_g \times G)$

After adding R in series,

$\Delta V = (i_g \times G) + (i_g \times R)$

The term $(i_g \times R)$ is responsible for the increase in the range of the voltmeter. This enables us to measure larger values. And hence, we need ‘R’ as large as possible.

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4. Examples

Question 1: What shunt resistance is required to make a 1.00 mA, 20 Ω galvanometer into an ammeter with a range of 0 to 50.0 mA?

Solution :

Given:

• Maximum current (I) = 50 mA
• i_g = 1 mA
• G = 20

Solution:

$(I – i_g)\times S = i_g \times G$

By substituting, we get:

$S = 0.408\,\Omega$

Question 2: How can we make a galvanometer with $G = 20\,\Omega$ and $i_g = 1.0\,\text{mA}$ into a voltmeter with a maximum range of 10 V?

Solution :

Given:

• maximum voltage to be measured $\Delta V = 10\,\text{V}$
• $i_g = 1\,\text{mA}$
• $G = 20\,\Omega$

Solution:

For converting a galvanometer to a Voltmeter,

$\Delta V = (i_g \times G) + (i_g \times R)$

By substituting, we get:

$R = 9980\,\Omega$

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Conclusion:

So, we have learnt about the Galvanometer and how we can use it as an Ammeter and a Voltmeter.

• This topic is important not only from a practical point of view but also theory exam point of view
• And other than marks, it’s always good to know about our electrical instruments!

All the Best