In comparing the resistance of two cells `P and Q` with a sides wire bridge, a balance point is obtained when the sliding contact is `30 cm` from the zero end of the wire. The resistances `P and Q` are interchanged and the balance is obtained at `120 cm` from the same emf. Find the ratio of the resistance `P and Q` and the length of the bridge wire

Answer

`150 cm`

Explanation

Let `1` be the total length of bright wire
AS per question `(P)/(Q) = (30)/(l - 30)`…..(i)
and `(Q)/(P) = (120)/(l - 120) or (P)/(Q)= (l - 120)/(120)`……(ii)
Solving (i) and (ii) we get
`(P)/(Q) = (1)/(4) and l = 150 cm`