The connection diagram for substitution method is shown in fig. below. R is the unknown resistance to be measured, S is the standard variable resistance, ‘r’ is the regulating resistance and ‘A’ is an ammeter. There is a switch for putting S and R in a circuit.
substitution method for measurement of resistance
Firstly switch is at position 1 and R is connected in a circuit . The regulating resistance ‘r’ is adjusted till ammeter pointer is at chosen scale. Now switch is thrown to position ’2′ and now ‘S’ is in a circuit . The value of standard variable resistance ‘S’ is varied till the same deflection as was obtained with R in the circuit is obtained. When the same deflection obtained it means same current flow for both the resistances . It means resistances must be equal . Thus we can measure the value of unknown resistance ‘R’ by substituting another standard variable resistance ‘S’. Therefore this method is called Substitution method.
This is more accurate method than ammeter-voltmeter method. The accuracy of this method is greatly affected if the emf of the battery is changes during the time of readings. thus in order to avoid errors on this account, a battery of ample capacity should be used so that the emf remains constant.
