Experiment to investigate how the temperature and pressure of the air in a ....

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chamber affects the minimum voltage needed to generate a spark across a gap.

There are three variables: pressure, temperature and potential difference

Keep pressure constant: vary temperature with voltage
Keep temperature constant: vary pressure with voltage

Use an electrical heater to change the temperature of the air inside the chamber
Use a thermocouple to measure the temperature of the chamber
Connect a voltmeter

Precautions:
Use goggles to view the spark.
Because of dangers from UV radiation, safety screens should be placed around the chamber in case the chamber exploded under high pressure.
Calibrate the thermocouple before proceeding with the experiment.