A technician has five rigid conducting loops. Each loop has one marked, straight section. The lengths of these sections that will be inside the field are shown in the table.
| Loop | Length of marked section in the field / m\text{m}m |
|---|---|
| P | 0.020 |
| Q | 0.030 |
| R | 0.040 |
| S | 0.050 |
| T | 0.060 |
The marked section is the only part of a loop that will be between the poles of one magnet. A loop can be suspended from a digital force meter, which gives a force reading in newtons.
The technician also has a d.c. power supply, an ammeter, a variable resistor and a switch.
Devise an investigation the technician could carry out to determine whether the magnetic force on the marked section is directly proportional to its length.
Your answer should include how the technician would use the measurements to reach a conclusion.