Voltmeter-Ammeter method

In the figure let us assume the voltmeter to be ideal (internal resistance Rin=infinite).

So current Iv=0A

I= IA+IV

I=IA       [IV=0A]

V=VA+VK

V=I×RA+I×Runknown

Ammeter is  not ideal here.

V=I(RA+ Runknown)

V/I= RA+ Runknown            [V/I= Measured value of resistance]

RM=RA+ Runknown

So the measured value of resistance is greater than the unknown value of resistance exceed by the value of RA.

Error calculation-

e= Rknown-Rm/ Rknown

or        e=RA/ Rknown

If R ≅0, then Rm= Rknown (e ≈0)

Voltmeter-Ammeter method

I=IV+IK

I=V/RV+V/RK

I=V(1/RV+1/RK)

I/V=(1/RV+1/RK)

I/RM=I/RV+I/RK

I/RM=RV+ Rknown/RV Rknown

RM= RV Rknown/ RV+ Rknown

If the voltmeter is not ideal then measured value is Rknown.

 

Also Read-Wheatstone bridge

Also Read- Method of substitution


Sachidananda Dhar

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