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AimTo convert the given galvanometer (of known resistance and figure of merit) into a voltmeter of desired range and to verify the same.
Apparatus requiredGalvanometer
Shunt resistance (low resistance)
Multiplier resistance (high resistance)
Connecting wires
Battery
Resistors of different values
Ammeter
Voltmeter
TheoryConverting a galvanometer into an ammeter and voltmeter:
A galvanometer is an instrument used to detect small electric currents. To measure larger currents and voltages, a galvanometer can be converted into an ammeter or a voltmeter by adding a shunt resistor or a series resistor, respectively.
To convert a galvanometer into an ammeter:
Select a shunt resistor of appropriate value (Rsh) that will allow the galvanometer to measure the desired current range. The shunt resistor should be connected in parallel to the galvanometer.
The total resistance of the ammeter (Ra) can be calculated using the formula: Ra = G * Rsh / (G – 1), where G is the sensitivity of the galvanometer (in divisions per unit current).
Connect the shunt resistor in parallel with the galvanometer and connect the ammeter in series with the shunt resistor.
Verify the ammeter reading for different values of current.
To convert a galvanometer into a voltmeter:
Select a series resistor of appropriate value (Rs) that will allow the galvanometer to measure the desired voltage range. The series resistor should be connected in series with the galvanometer.
The total resistance of the voltmeter (Rv) can be calculated using the formula: Rv = G * Rs – Rg, where Rg is the resistance of the galvanometer and G is the sensitivity of the galvanometer (in divisions per unit current).
Connect the series resistor in series with the galvanometer and connect the voltmeter in parallel with the combination of the series resistor and the galvanometer.
Verify the voltmeter reading for different values of voltage.
Verification of the conversion:
The conversion can be verified by comparing the readings of the converted ammeter or voltmeter with a standard ammeter or voltmeter for different values of current or voltage, respectively. The percentage error can be calculated using the formula:
Percentage error = |(Measured value – Standard value) / Standard value| * 100%
If the percentage error is within the acceptable range, the conversion can be considered successful.
ProcedureConnect the resistance box in series with the galvanometer and take out the plugs of resistance R.
A and B are the fixed terminals and C is the variable terminal of the rheostat.
The galvanometer is now connected as a voltmeter of range V Volts.
Take out the plugs of calculated resistance R from the resistance box.
By using a key, adjust the movable contact of the rheostat such that the deflection of the galvanometer becomes maximum.
Note both the readings of the galvanometer and voltmeter.
Convert the readings of the galvanometer into volts.
Find the difference in the readings. This difference between voltmeter reading and galvanometer reading gives the error.
By moving the variable contact of the rheostat, take 5 readings covering the range of voltmeters from 0-3 Volts.
Observation and ResultCalculation:
For ammeter: Shunt resistance (S) = (G/Ra) – 1, where G is the galvanometer resistance and Ra is the desired range of the ammeter.
For voltmeter: Multiplier resistance (M) = (Ra/G) – 1, where Ra is the desired range of the voltmeter and G is the galvanometer resistance.
Result: After converting the galvanometer into an ammeter and voltmeter of desired range, the values of current and voltage can be measured in the circuit. The accuracy of the readings depends on the accuracy of the shunt and multiplier resistances used.
Observation: The readings obtained from the ammeter and voltmeter can be compared with the readings obtained from a standard ammeter and voltmeter to check the accuracy of the conversion. The readings should be within the range of error specified for the instrument.