The diagram illustrates a section of a circuit that uses a potentiometer wire to measure a potential difference (p.d.) in an external circuit. The potentiometer wire $XZ$ has length $L_1$. It is connected to a cell with known electromotive force (e.m.f.) $E$ and negligible internal resistance. The terminals $P$ and $Q$ are linked to the external p.d. that is to be measured. The length of the potentiometer wire from $X$ to $Y$ is $L_2$. The ratio of the lengths $L_1$ and $L_2$ is used to find the p.d. between $P$ and $Q$ in terms of $E$. What condition has to be satisfied to determine this p.d.?
- AThe current $I_1$ must be zero.
- BThe current $I_2$ must be zero.
- CThe p.d. across $YZ$ must be zero.
- DThe resistance of the external circuit must be zero.