Substitution Method for Medium Resistance Measurement

Substitution Method for Medium Resistance Measurement 

Resistance is classified into basically three categories for the Measurement. Different categories of Resistance is measured by different technique. So we are classified as:-

Low Resistance:- The Resistance value having 1Ω or below the 1Ω is under this category. (R<1Ω) 

Medium Resistance:- In this category  Resistance from 1Ω to 100 KΩ.  (1Ω<R< 100 KΩ)

High Resistance:- Resistance of the order of 100 KΩ and above is classified as High resistance. (R >100 KΩ)

List of different methods for Medium resistance Measurement's are :-

  • Wheat Stone Bridge                                                                                                                               
  • Carey Foster Meter Bridge                                                                                                                       
  • V-I method                                                                                                                                               
  • Substitution method                                                                                                                               
  • Ohm method

Substitution method:-    

The diagram for substitution method is shown. S is the standard variable resistance, Rt is the unknown resistance to be measured, ‘Rh’ is the regulating resistance and ‘G’ is an galvanometer. There is a switch for putting S and Rh in a circuit.
Substitution Method

Firstly when switch is at position 1 and ‘Rt’ is connected in a circuit. The regulating resistance ‘Rh’ is adjusted till galvanometer deflection pointer is at chosen scale. Now we switch is thrown to position ‘2’ and now ‘S’ is connected in a circuit. The value of standard variable resistance ‘S’ is varied till the same deflection is obtained as in the ‘Rt’ obtained. 

When the same deflection obtained it means same amount of current flow in both the resistances (‘S’ & ‘Rt’). It means that resistances must be equal. So that we can measure the value of unknown resistance ‘Rt’ by substituting another standard variable resistance ‘S’. Therefore this method is called Substitution method.

This is more accurate method than other method i.e. ammeter-voltmeter method. The accuracy of this method is may greatly affected if the emf of the battery changes during the time of readings. Thus in order to avoid errors on this account, New batteries operate (should operate) at a capacity of 100 percent should be used so that the emf remains constant.


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