Q15

An investigation was performed with the aim of measuring the value of the magnetic permeability constant, $\mu_0$.

Two wires, both of length $70.0\ \mathrm{cm}$, were arranged horizontally, with one wire above the other, as shown in the diagram below. The bottom wire is resting on an electronic balance.

When a current of $25.5\ \mathrm{A}$ flows through both wires, the reading on the electronic balance changes by $0.500\ \mathrm{g}$.

Which measurement made in the above investigation has the greatest relative error?
(A) length of the wires
(B) separation of the wires
(C) current in wires
(D) change in the reading of the electronic balance