In the following potentiometer circuit AB is a uniform wire of length 1 in and resistance R the potential gradient along the wire and balance length AO (= l).

Explanation

Current flowing in the potentiometer wire
`I=E/(R_"total ")=20/(15+10)=2/25A`
`:.` Potential difference across the wire `=2/25xx10=20/25=0.8A`
`:.` Potential gradient `k=(V_(AB))/(I_(AB))=0.8/1.0=0.8V//m`
Now, current flowing in the circuit containing experimental cell,
`=15/(1.2+0.3)=1A`
Potential difference across length `AO – 0.3 xx 1 = 0.3 V`
Length `AO=(0.3)/(0.8)m=0.3/0xx100cm = 37.5cm `