A student has a magnet on a top-pan balance, a stiff wire held in clamps above the magnet so that part of the wire is between the poles, a d.c. power supply, an ammeter and a variable resistor.

The student wants to find out whether the force on the wire is directly proportional to the current in the wire.

Devise an investigation the student could carry out.

Your answer should include how the student would use the measurements to reach a conclusion.