The diagram depicts a potentiometer circuit. The contact T is positioned on the wire and then slid along it until the galvanometer gives a zero reading. The length $XT$ is then recorded. To work out the potential difference per unit length of the wire $XY$, what other value has to be known?
- Athe e.m.f. of the cell $E_1$
- Bthe e.m.f. of the cell $E_2$
- Cthe resistance of resistor $R$
- Dthe resistance of the wire $XY$