Potentiometer

[eil2001]

I have a hw question about a potentiometer. There is a resistor between a and b that is a uniform wire of length l, with a sliding contact c at a distance x from b. An unknown potential difference V is measured by sliding the contact until the galvonometer G reads zero. I need to show that under this condition the unknown potential difference is given by V=(x/l)E. Any ideas???

[npgreat]

What is meant by E?

[eil2001]

What is meant by E?

E is the emf.

[clive]

The sliding contact divides the resistor in two other resistors R=R_1+R_2 where R_1 and R_2 are proportional with corresponding lengths (x and l-x). The current is given by
I=\frac{E}{R}
and the potential difference between the ends of R_1
V=I\cdot R_1
and so on....