Figure shows a potentiometer circuit for comparison of two resistances. The balance point with a standard resistance 
is found to be 58.3 cm , while that with the unknown resistance 
is 68.5 cm . Determine the value of 
. What might you do if you failed to find a balance point with the given cell 
.
Text Solution
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Ans. In first case, resistance 
is in parallel with cell 
, so p.d. across 
i.e., 
….(i)
In second case, 
is in parallel with cell so p.d. across 
i.e., 
XI….(ii)
Let k be the potential gradient of potentiometer wire. If l 1 and l 2 are the balancing length corresponding to resistance respectively, then
…(iii)
…(iv)
From (i) and (iii) 
(v)
From (ii) and (iv) 
(vi)
Dividing (vi) by (v), we get
Here, 
If we fail to find the balance point with the given cell 
, then we shall take the driver battery (B1) of higher emf than given emf ( 
).
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