A potentiometer wire of length 100 cm has aresistance 10 ohm. It is connected in series with a tesistance and an accumulator of emf 20V and of negligible resistance. A source of emf 10 mV is balanced against a length of 40 cm of the potentiometer wire.What is the value of the external resistance.

Let ext. resistance be R ohma

let current be I

so 20 = I(10+R)

also I*4 = .01 V

so, I=2.5*10^-3

on substitution, we get R = 7990 ohms

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