In the fig. 3.6 (a) the connection diagram for the substitution method for the measurement of medium resistance is shown. In this figure, R is the unknown resistance while S is the standard variable resistance “A” is an ammeter and Rh is a regulating resistance In order to connect R & S into the circuit alternatively there is a switch.
The switch is put at position 1 and resistance R is connected in the circuit The planting resistance Rh is adjusted till the promoter gives the indication of a doos scale mark. Now the switch is thrown to the position ‘2’ putting the standard variable resistance S in the circuit. The value of S is varied till the same deflection as was obtained with R in the circuit is obtained. The settings of the dials of S are read. Since we substitute one resistance for another which has left the current unaltered, and it is provided that the emf of battery and the position of Ry are unaltered, the two resistances & must be equal. Thus the value of unknown resistance R is equal to the dial settings of resistance S.
This method is more accurate than the ammeter voltmeter method because it is not subjected to the errors encountered in the latter method. However, its accuracy is greatly affected if there is any change in the battery emf during the time the readings on the two settings, are taken. Thus in order to avoid errors on this account, a battery of ample capacity should be used so that its emf remains constant.
The accuracy of the measurement naturally depends upon the constancy of being a battery and of the resistance of the circuit excluding R and S, upon the sense of the instrument, and upon the accuracy with which standard resistance S is known.
This method is not widely used for simple resistance measurements and is used in a modified form for the measurement & of high resistances. The substitution principle, however, is very important and finds many applications in bridge methods and in high-frequency ac. measurement.