A gs thermometer essentially consists of an amount of air enclosed inside a glass bulb. The bulb is attached to some large equipment needed for changing the temperature of the air and measuring its temperature and pressure.

The bulb is in a bath of glycol whose temperature can be changed from about – 40°C to above 100°C. Inside the bath are a couple of thermometers to measure the temperature of the glycol. If the changes are made slowly, we can assume the temperature of the air inside the bulb to be equal to the temperature of the bath.

The pressure of the air inside the bulb is measured with the help of an mercury manometer that also keeps the volume of air in the bulb constant. (Not shown is a device for measuring the ambient pressure.)