Conversion of Galvanometer into Ammeter of Desired Range

A galvanometer is a sensitive electrical instrument generally used to measure the magnitude and direction of current in a circuit. Due to its sensitivity, a galvanometer measures only small currents.

To measure large currents, a galvanometer can be converted into an ammeter. This conversion of a galvanometer into an ammeter can be done by connecting a shunt resistance parallel to the galvanometer. In this article, we will study the conversion of a galvanometer into an ammeter in detail. 

Table of Contents:

• Aim

• Apparatus Required

• Theory

• Procedure

• Observations

• Result

• Precautions

• Lab Manual Questions

• Viva Questions

• Practical Based Questions

Aim

To convert the given galvanometer (of known resistance and figure of merit) into an ammeter of desired range and to verify the same.

Apparatus Required

• A galvanometer

• Key and battery

• Sandpaper

• A rheostat and connecting wires

• An ammeter of 0-3 A range

• Resistance box 

• Piece of sandpaper

Theory

An ammeter is a device which we can use to measure the strength of electric current through a circuit, and it is a modified form of the galvanometer. We should always connect this device in series in a circuit. To convert the galvanometer into the ammeter, we should connect a low resistance, also known as a shunt resistance, parallel to the galvanometer. The value of the shunt is so chosen that most of the current pass through the shunt. This prevents large currents from flowing through the galvanometer, which may damage it.

Conversion Of Galvanometer Into Ammeter

If ${I_g}$ is the current required to produce full-scale deflection in the galvanometer, then the remaining current will flow through the shunt resistance $S$, which is shown in the above figure. 

Now, we know that the potential difference across $G$ and $S$ would be the same as they are parallel.

So, the potential difference across $G$ = Potential difference across shunt resistance $S$

${I_g}G = \left( {I - {I_g}} \right)S$ (From Ohm’s law $V = IR$)

So, the value for shunt resistance $S$ is, 

$S = \dfrac{{{I_g}G}}{{I - {I_g}}}$

Where, 

$S$ is the shunt resistance (low resistance)

$G$ is the resistance of the given galvanometer

${I_g}$ is the full-scale deflection current

$I$ is the range of the converted galvanometer

$I - {I_g}$ is the current flowing in shunt

The above equation gives the value of shunt resistance to be connected in parallel with the galvanometer to convert it into an ammeter of the desired range.

Procedure

• First of all, we should count the total number of divisions on either side of zero of the galvanometer scale. Let it be $n$. Then, we need to calculate the current ${I_g}$ for full-scale deflection by using the formula ${I_g} = nk$

• Now, use the formula,

$S = \dfrac{{{I_g}G}}{{I - {I_g}}}$ and calculate the value of shunt resistance for conversion into an ammeter.

The value of shunt resistance is minimal, such that the range is not available in the resistance box. Wires of copper, manganin etc., are used with suitable lengths and diameters to obtain the value of this small resistance.

• Now, cut a length of the wire 2 cm more than the calculated value of $I$ such that there is 1 cm extra available at each end. Now, mark points on each end of the wire and connect them to the two terminals of the galvanometer. The wire should be such that the points are on the outside of the terminal screws. This galvanometer with shunt wire will now work as an ammeter in the range of $I$

• Now, we need to make the electrical connections as shown in the above diagram. 

• Now, insert the key and adjust the rheostat to observe maximum and minimum deflection in the galvanometer.

• We need to note the readings from the galvanometer scale and the corresponding ammeter reading and record the observations.

Observations

• The resistance of the galvanometer is $G$ = _____

• The figure of merit is $k$ = _____

• Number of divisions in the galvanometer scale is $n$ =_____

• Current for full scale deflection is ${I_g} = nk$

• Range of conversion is $I$ =_____

• Shunt resistance is $S = \dfrac{{{I_g}G}}{{I - {I_g}}}$

Least count of the galvanometer converted into an ammeter is $\dfrac{I}{n}$ = _____

Result

From this experiment, we have found that the actual value and measured value of currents are minimal. So, conversion is verified. 

Precautions

• We should calculate resistance accurately.

• In this experiment, the length of the shunt wire must be correct.

• For verification, we should use the same range conversion ammeter.

Lab Manual Questions 

1. What is an ammeter?

Ans: It is an instrument designed to read electrical circuit currents. It is generally placed in series with the circuit.

2. What will happen if an ammeter is connected in parallel to the circuit?

Ans: If we connect an ammeter in parallel to the circuit, then it will measure only a part of the current flowing through it and not the total current.

3. Why should we connect an ammeter in series in this experiment?

Ans: An ammeter is connected in series in this experiment so that the whole current to be measured in the circuit can pass through the ammeter.

Viva Questions

1. State various types of galvanometers.

Ans: Following are the various types of galvanometer:

• Tangent galvanometer,

• Moving coil galvanometer,

• Ballistic galvanometer,

• Astatic galvanometer

2. On which principle does an ammeter work?

Ans: It works on the principle of the magnetic effect of electric current. 

3. State two advantages of a moving coil galvanometer.

Ans: The moving coil galvanometer provides high sensitivity, and we get more accurate readings due to its high sensitivity. The second advantage is that strong magnetic fields do not easily affect it.

4. On which factors does the sensitivity of the galvanometer depend?

Ans: The sensitivity of the galvanometer depends on the area of the coil, the number of turns in the coil, the strength of the magnetic field and the magnitude of a couple per unit twist.

5. What should we measure with the help of a Clamp-on ammeter?

Ans: We can measure large alternating currents with the help of a Clamp-on ammeter. 

6. Write the SI unit of electric current.

Ans: The SI unit of electric current is Ampere and it is denoted by the symbol A.

7. State different types of the ammeter.

Ans: Different ammeters are rectifier-type ammeters, moving iron ammeters, electrodynamometer ammeters and hot-wire ammeters.

8. State a few disadvantages of the galvanometer.

Ans: Following are some disadvantages of the galvanometer:

• It measures only direct current

• It is not suitable for high current values

• Restoring torque can not be changed easily.

Practical-Based Questions

1. Which instrument can be used to measure resistance?

1. Voltmeter

2. Voltameter

3. Ammeter

4. Ohmmeter

Ans: Option D - Ohmmeter 

2. What should we connect in parallel in an electric circuit?

1. Voltmeter

2. Volatmeter

3. Fuse

4. Ammeter

Ans: Option A - Voltmeter

3. The main function of a shunt in an ammeter is to 

1. Bypass the current

2. Increase the resistance of the ammeter

3. Increase the sensitivity of the ammeter

4. Increase the specific resistance of the ammeter

Ans: Option A - Bypass the current

4. Which type of electric current are we using when we turn on our TV at home?

1. DC

2. Conductive

3. AC

4. Both

Ans: Option C - AC

5. The symbol for voltage is 

1. W

2. VO

3. V

4. Amp

Ans: Option C - V

6. What physical quantity does the ammeter measure?

1. Voltage

2. Current

3. Power

4. Resistance

Ans: Option B - Current

7. We should connect a voltmeter in _____ across the cell.

1. Parallel

2. Series

3. Either series or parallel

4. None of the above

Ans: Option A - parallel

8. Resistance of an ideal ammeter is

1. Zero

2. Infinite

3. Very low

4. Very high

Ans: Option A - Zero

9. We should connect an ammeter in _____ across the cell.

1. Series

2. Triangular

3. Either series or triangular

4. Parallel

Ans: Option A - Series

10. Resistance of an ideal voltmeter

1. Very low

2. Zero

3. Infinity

4. Very high

Ans: Option C - Infinity

Conclusion

From the above experiment,

• We have found that the actual value and measured value of the potential differences are very small and conversion is perfect. 

• We can also conclude that the galvanometer can be converted into an ammeter of desired range by connecting a shunt resistance in parallel with the galvanometer this shunt prevents a galvanometer from being damaged due to large currents and it is also used to increase the range of ammeter.