Since the prohibition of the use of mercury, bar thermometers work by non contaminated and tinted liquids, basically alcohol.
The measurement is performed when visualising the dilatation of the liquid inside the glass tube. As a precision element for laboratory measurement, the thermometer has only a bulb where the dilatation of the volume is produced, and a granulated tube where one can observe the point reached by the liquid in the scale.
For installation in field, the glass is protected by a body in brass, bronze or stainless steel.
This type of thermometers can have multiple shapes and dimensions. Depending on the placement there has been built some standards compatible within the major part of manufacturers.