# Why is mercury used in thermometers?

I know it expands when hot and grows compact when cool (that's about all I know), but I suppose many other substances do, too.

Also, and excuses for my ignorance, what the term used to describe the function of mercury in a thermometer? I mean, would you call it an "indicator" (insofar as it indicates the temperature), or is there some technical term?

Thanks!

In addition to the answer and the comment given, the use of mercury in thermometers had another advantage frequently used in the lab:

Mercury, as a metal, has a high electrical conductivity. Imagine two thin wires inside the capillary of your thermometer. One of them is fixed. The other wire is can move along a threaded rod. Typically, this was realized via magnetical coupling to a freely rotating knob on top of the (hermetically sealed) thermometer. Upon heating, the mercury expands and finally reaches the upper wire previously adjusted to a particular working temperature. At this point, an electrical current may flow through the thermometer.

These contact thermometers, nowadays replaced by mercury-free electronic thermoswitches, once were the working horses in the lab when it came to controlled heating of oil baths or monitoring cooling circuits for uv lamps.

• "In addition to the answer and the comment given..." -- There is no other answer or comment for me. Too bad, curious what this answer was made in addition to. Thanks! – Teusz Sep 5 '14 at 14:55
• Unfortunately, the comment by Michael D. M. Dryden was attached to an answer that was deleted by the owner. So the comment went with it. In the comment, it was pointed out that mercury can't be used for temperatures below -30 °C; kerosene and isoamyl acetate were mentioned as for low-temperature thermometers. Furthermore, it was mentioned that water would a possible but limited alternative for a narrow range (> 0 - < 100 °C. – Klaus-Dieter Warzecha Sep 5 '14 at 17:24

Reasons:

• It expands linearly (i.e a 1 kelvin temperature increase almost always produces the same change in volume over a reasonable range of temperatures). You might think that this is the norm - but not always. For example, water does not expand linearly. It has a maximum density at $4~\mathrm{^\circ C}$.

• It can measure over a reasonable range of temperatures, from $-39~\mathrm{^\circ C}$ up to its boiling point.

• It is quite dense in the first place. This is so that it can be used in configurations such as the Six's Thermometer — a type of thermometer that records the maximum and minimum temperatures of an area (I won't go into schematics now, although it is quite simple)