Change of state - Matter changes state when it switches from one physical form to another. A change of state occurs when the atoms that make up the matter either gain or lose enough heat energy to alter their physical form.

Air in the bulb - Referred to as the thermometric medium, i.e. the medium whose property changes with temperature.

Mercury - The element mercury is liquid when it is in the temperature range of -38.9°C to 356.7°C. As a liquid, mercury expands as it gets warmer. Its expansion rate is linear and can be accurately calibrated.

Wheatstone Bridge Circuit - Invented by Charles Wheatstone for measuring resistances.