Answer:
thermometer
Explanation:
The most important tool to use when collecting data relating to how thermal energy affects metals would be the thermometer.
The thermometer is an instrument used to measure temperatures and the temperature of a body is a measure of the amount of thermal energy present in the body.
In order to obtain data relating to how thermal energy affects metals, the most important variable to take note of would be the thermal energy of the metals and this can only be done by the thermometer.
