Choice of liquid for thermometers
Whether mercury or alcohol is used in a thermometer depends on the range over which temperature is to be measured. Mercury freezes at – 39°C and boils at 357°C, while alcohol freezes at – l l S “C. It is therefore essential to use alcohol thermometers in places such as northern Canada and Russia, where winter temperatures of – 40°C are not uncommon. Alcohol also possesses the advantage of having an expansive of about six times that of mercury. Apart from these advantages, mercury is to be preferred to alcohol as a thermometric liquid for the following reasons:
(I) It does not wet glass. Alcohol tends to cling to the wall of the tube, and this leads to low readings when the thread is falling.
(2) It does not, like alcohol, vaporize and distill on to the upper part of the bore.
(3) It is opaque and easily seen, whereas alcohol has to be colored.
(4) It is a better conductor of heat than alcohol, and therefore responds more rapidly to changes of temperature.
For extra low-temperature work (down to – 200°C) repentant is used instead of alcohol. Water is unsuitable for use in thermometers, not only because it freezes at o °C but also because of its irregular expansion (see page 166).