In the figure let us assume the voltmeter to be ideal (internal resistance Rin=infinite).
So current Iv=0A
Ammeter is not ideal here.
V/I= RA+ Runknown [V/I= Measured value of resistance]
So the measured value of resistance is greater than the unknown value of resistance exceed by the value of RA.
e= Rknown-Rm/ Rknown
or e=RA/ Rknown
If R ≅0, then Rm= Rknown (e ≈0)
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