A liquid thermometer works by using the thermal expansion of a liquid within a capillary tube to measure temperature. The precision of this method varies by the liquid used, but all use the fact that when most liquids are heated, they expand. Common types of liquid thermometers use mercury, toluene or less toxic biodegradable liquids.
The thermal expansion of liquids in normal temperatures is generally slight, but is fully sufficient for liquid thermometers. The liquid is normally stored in a bulb at the bottom of the thermometer, but this bulb is very wide compared to the capillary tube, which is thin enough that any expansion in volume causes a significant difference in liquid height.
Liquid thermometers must be carefully calibrated to display the correct temperature. Each liquid used has a different rate of thermal expansion. In addition, the liquid behaves differently based on level of immersion, and so any liquid thermometer is only properly calibrated for a certain level. Greater or lesser immersion in the substance being measured results in error.
The liquid thermometer is a very simple mechanism that requires only the human eye to read, and so it sees widespread use. They are more accurate than many other thermometer types in common use, although they tend to be less convenient for many applications.