Voltmeter Ammeter Method for Measurement of Resistance

Voltmeter Ammeter Method for Measurement of Resistance

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

Ammeter Voltmeter Method:-

We have the connection for the measurement of Medium Resistance using  the Ammeter Voltmeter Method :-

  Voltmeter Ammeter Method 


In the above diagram the reading of  Voltmeter and Ammeter is taken. Let take a example if the Voltmeter reading is V and Ammeter reading is I then the measured Resistance will be:-
                             
                                                           Rm = V/I

So using this we can measured test Resistance Rt. But we know for measuring the true value of the Resistance (Rt) Ammeter internal Resistance should be zero and that of Voltmeter internal Resistance should  be infinite. But this is not possible practically so to achieve zero resistance Ammeter and infinite Resistance. so that's why measured value of resistance Rm will deviate from the true value R .

Voltmeter is used for measuring the Voltage drop across the resistor as well as . So V =   Vr + Va
Let current measured by Ammeter in circuit = I

So, Resistance measured Rm = V/I

So, Rm = (Va+Vr) / I =(IRa+IR) / I = Ra+R

So that, the measured Resistance is the sum of Resistance of true Resistance and Ammeter. So the measured value will only and only represent true value if the Ammeter Resistance Ra is Zero.

True value of the Resistance R = Rm –Ra

                                                  = Rm(1-Ra/Rm)

Relative Error = (Rm-R)/R = Ra/R

 This method is used when measuring  Resistance near to the 100 KΩ but it should be under Medium Resistance category.And in this method  the Relative Error will be less if the true value of Resistance to be measured is high as compared to the internal Resistance of Ammeter.


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