What is primarily found in the bellows of a remote bulb thermostat?

In a remote bulb thermostat, the bellows are primarily filled with a volatile liquid. This liquid is key to the thermostat's operation because it has the ability to expand and contract significantly with temperature changes. When the temperature rises, the liquid inside the bulb heats up and expands, causing the bellows to move. This movement can trigger a switch to control heating or cooling systems based on the temperature setting of the thermostat.

The choice of a volatile liquid is important because it allows for sensitive and rapid response to temperature variations, ensuring effective regulation of the environment being controlled. The properties of these liquids ensure that they can vaporize and condense at relatively low temperatures, which makes them ideal for the functioning of the thermostat. In contrast, solids, gas mixtures, and air do not provide the same responsiveness or effectiveness in temperature-sensing applications as the volatile liquid used in this context.