# To Convert The Given Galvanometer (of Known Resistance and Figure of Merit) Into a Voltmeter of Desired Range and To Verify the Same

## Aim/Aim of Experiment

To convert the given Galvanometer of known resistance and figure of merit into a Voltmeter of desired range and to verify the same.

## Apparatus/Material Required

• A Weston Type Galvanometer
• A Voltmeter (0-3 Volt)
• A Battery (Two Cells)
• Two Resistance boxes (10,000 & 200 ohm)
• A Rheostat
• Two One-way Keys
• Connecting wires
• A piece of Sand paper.

## Theory

A Galvanometer is an ideal device which is usually used to detecting the weakest or little amount of electric currents in an electric circuit. The galvanometer only shows the deflection when an electric current is passed through the coil, while a Voltmeter is an important instrument used for determining/estimating the electrical potential difference (V) between two points in an electric circuit.

The relationship between deflection and the passing current is directly proportional to each other.

So, the Series resistance required for conversion,

R=V/Ig-G,

where V is the range of conversion.

## Procedure

1. Calculate the value of Series resistance R to be connected in series with the galvanometer for the given range V (say 3 volts).
2. Now connect the resistance box in series combination with the galvanometer and have to take the plugs of the resistance (R). Now the given galvanometer is ready for use as a voltmeter of range V volts.
3. Make the connections as shown in the circuit diagram.
4. In Circuit diagram AB is a rheostat being used as a potential divider, here C is the variable terminal and A and B are the fixed terminals of the rheostat.
5. Take out the plugs of calculated resistance R from the resistance box.
6. Insert the key k and adjust the movable contact of the rheostat so that deflection in the galvanometer becomes maximum.
7. Note down both the reading of voltmeter and galvanometer and converts the reading of the galvanometer into volts. It also helps to find the difference between these two meters.
8. Find the difference, if any, between the readings of voltmeter and galvanometer. This difference gives the error.
9. By moving the variable contact of rheostat, take 5 readings covering the whole range of voltmeters from 0-3 Volts.

## Calculations

• The resistance of the given Galvanometer (G) = 80 ohm.
• Figure of Merit (k) = 1.9×10-5 Amp/Div.
• Number of divisions (Galvanometer scale) (n) = 30.
• Current (Full scale deflection) (Ig) = nk = 57×10-5 = 0.00057A.
• Range of conversion (V) = 0-3 Volt.
• Resistance to be placed in series with Galvanometer (Rs) = R=V/Ig-G =3/0.00057-80 = 5183.15 ohm.

## Verification

Least count of the Galvanometer converted into Voltmeter = V/n = 3/30 = 0.1V.

## Observation

Observation Table for Verification:

## Result

The result of this conversion is as the difference (Error) in actual value and measured value of potential difference (as recorded, in column 4) is very small or about zero, so here the conversion is perfect.

## Precutions

• Make sure all the connections should be neat, clean and tight.
• The Voltmeter used for verification should preferably be of the same range, As the range of conversion.
• The diameter of the wire to be used for shunt resistance, should be measured accurately.
• Use correct length shunt wire, Length of shunt wire should be neither too small nor too large.
• The resistance box should be a high resistance one.
• The value of required series resistance (Rs) should be calculated accurately.

## Sources of Error

• The connection may be loose.
• The e.m.f. of the cell or battery may not be constant (variable).
• Length of shunt wire may be either too small or too large.

## Viva Voice Questions with Answers

1. What is a galvanometer?

Answer: It is a device (instrument) used for detecting feeble electric currents in circuits.

2. What is a voltmeter?

Answer: A voltmeter is a device (instrument) for measuring electric potential difference between two points in a circuit.

3. What do you mean by resistance of a galvanometer?

Answer: The resistance of the coil of a galvanometer, is called the resistance of the galvanometer. It is represented by the symbol G.

4. How is a galvanometer converted into a voltmeter?

Answer: A galvanometer is converted into a voltmeter by connecting a high resistance in series with the galvanometer coil.

5. Why is a galvanometer not suitable to work as voltmeter?

Answer: A galvanometer has less resistance and more current-carrying capacity from those required by a voltmeter.

6. How the high resistance in series gives required properties to the galvanometer?

Answer: The series high resistance increases the overall resistance of the voltmeter (converted galvanometer) and decreases its current-carrying capacity.

7. What is the order of resistance of a voltmeter?

Answer: The voltmeter resistance is of the order of series high resistance (R is in ten thousands, G is in hundreds).

8. What do you understand by the range of a voltmeter?

Answer: It is the maximum value of the potential difference which the voltmeter can measure.

9. Can we increase/decrease the range of a voltmeter?

Answer: Yes. The range of voltmeter can be increased by connecting a suitable high resistance in series and can be decreased a suitable resistance in parallel.

10. What happens when a voltmeter connected in series in a circuit?

Answer: The voltmeter cannot measure the actual potential difference in the circuit because overall resistance of circuit increases.

11. What is shunt? State its S.l. unit.

Answer: A small resistance connected in parallel with a galvanometer is called shunt. Its S.l. unit is Olim.

12. What do you mean by the resistance of a galvanometer?

Answer: The resistance offered by the coil of galvanometer to the flow of current through it is known as resistance of the galvanometer (G).

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