1. Its measurement uses null method.
2. Principle:
Potential difference between two points of a current carrying conductor (having uniform thickness) is directly proportional to the length of the wire between two points.
3. I = 600 cm, I = 350 cm
\(r=R\Big(\frac{I-I_1}{I_1}\Big)\) r = 3 \(\Big(\frac{600-350}{350}\Big)\)
= \(\frac{3\times250}{350}\) = 2.1 Ω