(ii) As the shunt and galvanometer are connected in parallel: 1
The total resistance of the ammeter is 0.05 .Æ 1
Rtotal
1 Rshunt
1 Rgalvanometer
.0503 1
10 19.98QRtotal 0.05
Conversion of a Galvanometer
to a Voltmeter A moving-coil galvanometer can be converted to a voltmeter by connecting a large resistance in series with it.This resistance is called a multiplier.
Based on Ohm’s law (V IR), the voltage across the resistor plus galvanometer is propor- tional to the current. It can be calculated accurately if the total resistance is known, therefore one can set a scale on the galvanometer to give a reading of voltage.
SAMPLE PROBLEM 24B Æ
SAMPLE ANSWER 24B
Ohm’s law states that V IR. The maximum current flowing through the galvanometer is 10 mA; this occurs when the voltage across it is 5 V. , where Rtotal is the resistance across the multiplier plus galvanometer.
5 (10 103) Rtotal Rtotal 500 Æ
As the multiplier and galvanometer are connected in series: Rtotal Rmultiplier Rgalvanometer 500 Rmultiplier 4 Rmultiplier 496
The resistance of the multiplier is 496 .Æ Conversion of a Galvanometer
to an Ohmmeter A moving-coil galvanometer can be converted to an ohmmeter by connecting a variable resistance and a battery in series with it.
The resistance is set so that the galvanometer gives a full-scale deflection when terminals A and B are connected together. The angle of deflection is inversely propor- tional to the resistance between A and B. Therefore one can set a scale on the galvanometer to give a reading of resistance.
R
Fig 24.10: Using a galvanometer as an ohmmeter
APPLIED ELECTRICITY (OPTION 2) 451
A B
High resistance
Fig 24.9: Using a galvanometer as a voltmeter
A galvanometer has a resistance of 4 and gives a full-scale deflection with a current of 10 mA. Find the resistance of the multiplier (resistor) that must be connected in series to enable the galvanometer to measure a voltage of up to 5 V.
