What instrument measures changes in temperature?

Temperatures are measured by instruments called thermometers.

The instrument usually used to measure air temperature is the maximum-minimum thermometer. It consists of a U shaped tube with bulbs at each end and Mercury at the bottom. Air heats or cools the bulbs causing the air inside to expand or contract, therefore pushing or pulling the mercury upwards or downwards.

There are 3 types of thermometer: minimum thermometer, maximum thermometer,and the "Six's thermometer" -- these record the temperatures over a period of time.
it says in my book that a thermometer measures air temperature