Conversion of Galvanometer into a Voltmeter of Required Range

A galvanometer is a device generally used to measure the magnitude and direction of current in a circuit. It is a very sensitive device that can measure only small currents. It may get damaged if we pass a current through a galvanometer more than the specified amount. The maximum current that can be passed through a galvanometer is the full-scale deflection current. 

To measure the potential difference in an appliance, we need a device known as a voltmeter. This device is always connected in parallel with the appliance.

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 a voltmeter of desired range and to verify the same.

Apparatus Required

• A galvanometer

• Key and battery

• Sandpaper

• A rheostat and connecting wires

• A voltmeter of 0-3V range

• Resistance box

Theory

Galvanometer can be converted into a voltmeter by connecting suitable high resistance in series with it, so that very small current flows through the galvanometer. A voltmeter should have high resistance so that current flowing through it is negligible. 

Now, we will calculate the required series resistance to convert galvanometer into voltmeter.

According to Ohm’s law,

$V = {I_g}\left( {G + R} \right)$         … … …(i) (From the figure given below)

Conversion of galvanometer to a voltmeter

From this equation, we can say that potential difference is directly proportional to the current which is flowing through the galvanometer.

From equation (i), we can write,

$\dfrac{V}{{{I_g}}} = G + R$

So, $R = \dfrac{V}{{{I_g}}} - G$ 

This is the formula which we can use to find the value of high resistance.     

Where, 

$R$ is high resistance which is connected in series with the galvanometer

$V$ is the maximum voltage to be measured

$G$ is the resistance of the given galvanometer

${I_g}$ is the full scale deflection current and this is the maximum current that can flow through the galvanometer.

${I_g} = nk$ Where, $k$ is the figure of merit and $n$ is the number of divisions in the galvanometer scale.

Procedure

• First of all, we need to connect the resistance box in series combination with the galvanometer and take out the plugs of resistance R. 

Conversion Of Galvanometer Into Voltmeter

• Now, the given galvanometer can be used as a voltmeter of range V volts. 

• Now, we should take out the plugs of calculated resistance R from the resistance box. Then, insert the key K and adjust the movable contact of the rheostat such that the deflection of the galvanometer becomes maximum. Now, note the readings of the galvanometer and voltmeter. 

• We need to convert the galvanometer reading into volts. Carefully observe the difference in the readings. This difference between voltmeter reading and galvanometer reading will give us the error. 

• By moving the variable contact of rheostat, take at least 5 readings covering the whole range of voltmeter from 0-3 Volts and record your observations.

Observations

• Least count of the galvanometer converted into voltmeter is L.C. = \[\dfrac{V}{n}\] = _____

• Resistance of the given galvanometer $G$ = _____

• Figure of merit $k$ = _____

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

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

• Range of conversion $V$ = _____

• Resistance to be placed in series with the galvanometer $R = \dfrac{V}{{{I_g}}} - G$ = _____

Result

From this experiment, we have found that,

• The calculated value of series resistance which we calculated using the formula and by performing this experiment, we found that the observed value of series resistance is equal to the calculated value of series resistance. 

• The potential difference's actual and measured value is very small. So, conversion is verified.

Precautions

• We need to calculate resistance accurately.

• In this experiment, the rheostat should be used as a potential divider.

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

Lab Manual Questions 

1. What is a voltmeter?

Ans: Voltmeter is a device generally used to measure the potential difference between two points of the electric circuit. 

2. How can we change the range of the voltmeter?

Ans: We can increase the range of the voltmeter by increasing its resistance. This can be done by connecting an additional resistance in series with it.

3. How can we change the voltmeter into a millivoltmeter?

Ans: By decreasing its resistance, we can change the voltmeter into a millivoltmeter because the resistance of the millivoltmeter is much less than that of the voltmeter.

Viva Questions

1. What should be the resistance of a voltmeter in this experiment?

Ans: In this experiment, the resistance of a voltmeter should be high.

2. How can we change an ammeter into a voltmeter?

Ans: By putting a suitable high resistance in series with an ammeter, we can change an ammeter into a voltmeter.

3. How can we convert a voltmeter into an ammeter?

Ans: By shunting the voltmeter with low resistance, we can convert voltmeter the into an ammeter.

4. What is the resistance of an ideal voltmeter?

Ans: The ideal resistance of an ideal voltmeter is infinity.

5. What is the use of a shunt?

Ans: Shunt protects galvanometer from damage from high voltage and current.

6. Can we use a galvanometer to measure heavy currents?

Ans: No, we can not use a galvanometer to measure heavy currents.

7. On which principle does a galvanometer work?

Ans: Galvanometer works on the principle of converting electrical energy into mechanical energy.

8. How can we convert a galvanometer into a voltmeter?

Ans: After deciding the range of the voltmeter to be designed, we find the value of high resistance to being connected in series with galvanometer resistance. This combination will then act as a voltmeter.

Practical-Based Questions

1. Which instrument can be used to measure electric current?

1. Galvanometer

2. Voltameter

3. Tube tester

4. Voltmeter

Ans: Option A - Galvanometer

2. A device which measures voltage, current and resistance is called a:

1. Resistor

2. Voltmeter

3. Screwdriver

4. Multimeter

Ans: Option D - Multimeter

3. Current is measured in 

1. Amperes

2. Voltage

3. Ohms

4. Watts

Ans: Option A - Amperes

4. In an ammeter, the shunt resistance is usually _____ the metre resistance.

1. Equal to

2. Greater than

3. Less than

4. Greater than or equal to

Ans: Option C - Less than

5. What physical quantity does the voltmeter measure?

1. Voltage

2. Magnetic field

3. Volume

4. Temperature

Ans: Option A - Voltage

6. We can use a  milliammeter as 

1. A fuse

2. An ammeter

3. An electric field

4. None of the above

Ans: Option B - An ammeter

7. _____ voltmeter has the least power consumption.

1. Induction type

2. Moving coil type

3. Hot wire type

4. Electrostatic type

Ans: Option D - Electrostatic type

8. The internal resistance of a voltmeter must be high so that

1. Maximum current can pass through it

2. Minimum current passes through the meter

3. Accuracy becomes high

4. It can measure magnetic field

Ans: Option B - Minimum current passes through the meter

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

1. Increase the specific resistance of the ammeter

2. Increase the resistance of the ammeter

3. Bypass the current

4. Increase the sensitivity of the ammeter

Ans: Option C - Bypass the current

10. An ammeter is a 

1. Integrating instrument

2. Absolute instrument

3. Secondary instrument

4. Recording instrument

Ans: Option C - Secondary instrument

Conclusion

From the above experiment,

• We have found that the value of calculated series resistance which we calculated using the formula $R = \dfrac{V}{{{I_g}}} - G$ is equal to the value of resistance which we found by performing this experiment. So, the calculated value and observed value are the same. 

• We can also conclude that the difference in actual value and the measured value of the potential difference is very small and hence conversion is perfect. 

So, by performing this experiment, we converted the galvanometer into a voltmeter of desired range by connecting a very high resistance in series with the galvanometer.