Resistance is classified into three categories as low, medium and high resistance. Medium Resistance ranges from 1Ω to 0.1 MΩ.
The different methods used for the measurement of Medium resistance are as follows:
- Ammeter Voltmeter method
- Substitution Method
- Wheatstone Bridge Method
- Ohmmeter Method
In this section, we will learn about Measurement of Medium Resistance by Substitution Method.
In Substitution Method, the Resistance whose value is to be measured is compared with the Standard Resistance. The connection diagram for Substitution Method is shown in fig.1.
R is the unknown Resistance,
S the Standard variable Resistance,
A is Ammeter
r is Regulating Resistance.
When we put the Switch at position 2 then R is connected to the circuit. The Regulating Resistance r is adjusted till the reading of Ammeter is at a chosen scale mark. Now the Switch is thrown to position 1 putting the Standard variable Resistance S in the circuit. Now the variable Resistor S is adjusted till the reading of Ammeter is same as when R was in the circuit. The setting of the dial of S is read. Since the substitution of one resistance for another has left current unaltered, and provided that EMF of battery and position of Regulating Resistance r remain unaltered, the two Resistance R and S must be equal. Thus the value of unknown Resistance R is equal to the dial setting of Standard Resistance S.
Advantages of Substitution method
This method of measurement is more accurate as compared to the Ammeter Voltmeter Method as in this method measurement is not affected by the accuracy of Ammeter.
1. However, the accuracy of this method is greatly affected if there is any change in the Battery EMF during the time when the reading in two settings is taken. Therefore to avoid the error because of change of EMF of Battery, a Battery of enough capacity is used so that it remains constant during the entire period of testing.
2. The accuracy of this method also depends on the resistance of circuit excluding R and S, upon the sensitivity of the instrument and upon the accuracy with which the Standard Resistance S is known.
3. This method is not widely used for simple Resistance measurement and is used in modified form for the measurement of High Resistance. The Substitution Method is however very important as it finds its use in the application of bridge method and in high accuracy A.C. measurement.