The pressure actuated type thermostat uses a bulb containing gas which expands or contracts with variation in room temperature. This expands or contracts a bellow which, in turn, opens or closes an electrical contact and the compressor operation is automatically controlled to maintain a constant temperature. Thus, if the room air returned to the unit becomes cooler than the thermostat setting, the gas in the bulb contracts, causing the bellows to contract. The electrical circuit opens and the compressor stops. When the room air temperature goes above the thermostat setting, the gas in the bulb expands, the bellow expands, the electrical contracts close and the compressor starts. In this way, the room air-conditioner maintains the temperature desired. The room occupants need adjust the thermostat control to a new position when they wish to change the desired, temperature.
A second type of room thermostat is a bi-metal temperature running device in place of the gas filled sensing tube. If two metals are joined together in a bimetallic strip and then heated, the varying rates of expansion of the metals cause them to bend. This bending in response to heat or cold is used to close or open an electrical contact. This contact is wired in series with compressor relay and its controls compressor operation in the same manner as does the gas-bulb thermostat described above.