State four other items of laboratory equipment the engineer requires to construct a test circuit and obtain the data needed to plot current–voltage curves (excluding the candidate components and standard connecting wires).

To ensure the constantan alloy specimens behave as ohmic conductors during testing, their temperatures must remain strictly constant.

Explain why a change in temperature would affect the electrical resistance of a metallic conductor.

Describe two practical protocols the engineer could implement in the laboratory to prevent the temperature of the constantan specimens from rising during the experiment.

Four different current–voltage ($I$–$V$) characteristics are recorded during the tests:

• Graph P: A linear plot passing through the origin with a very steep positive gradient.
• Graph Q: A linear plot passing through the origin with a shallow positive gradient.
• Graph R: A symmetrical curve through the origin whose rate of increase of current with respect to voltage decreases at higher potential differences in both directions.
• Graph S: Zero current for all reverse-bias and low forward-bias voltages, followed by a rapid, near-vertical increase in current once a specific forward threshold voltage is exceeded.

Match each of the four candidate components to its corresponding graph description.