Page 323 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

V V
R m 2 = =
I I + I
R V
V
R m 2 = V V .....Eqn.( 4)
R + R V
By multiplyin g the denominato r and numerator
R
by , Eqn.(4) becomes
V
R
R m 2 = .....Eqn.( 4)
1 + R
R V
From equation 4, it is clear that the true value of resistance is equal to the measured value only if
• the resistance of the voltmeter RV is infinite

• the resistance to be measured ‘R’ is very small when compared to the resistance of the voltmeter.
Relative error

By elimination process, we get
...Eqn.(5)
The value of R is approximately equal to R.
m2
Therefore er .....Eqn.(6)

Conclusion: From equation (6), it is clear that the error in measurement would be small if the value of resistance
under measurement is very small as compared to the resistance of the voltmeter. Hence the circuit shown in
Fig 1(b) should be used when measuring resistances of a lower value.

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CITS : Power - Electrician & Wireman - Lesson 50-53