This type of thermometer is probably the most widely used and consists of a liquid in a thin-walled glass bulb to which is fixed a thin capillary tube (Figure 1). The property that varies with temperature is the volume of the liquid, which is measured by reading the length of the liquid thread, assuming the bore of the capillary tube to be uniform.
| Mercury in glass | -39 oC to +357 oC |
| Pressurised mercury in glass | -39 oC to +500 oC |
| Pressurised mercury in quartz | -39 oC to +800 oC |
| Alcohol in glass | -120 oC to +60 oC |
| Pentane in glass | -200 oC to +30 oC |
The mercury-in-glass thermometer has the
advantages that it is direct-reading and that mercury is opaque, does not wet glass, is easily
purified and does not distil at low temperatures. The relative expansion of mercury, though
uniform, is fairly small, however, and the movement of the liquid thread is sometimes jerky.
This thermometer is subject to various errors, of which the largest is due to the expansion of
the glass bulb with rising temperature, which may be up to 10% of the mercury expansion,
while with falling temperatures the glass contracts relatively slowly. There is also a small
long-period expansion of the glass, known as the secular change (around 0.01
oC per year). Further errors may arise if the bore of the capillary tube is not
uniform, in marking the fixed points and from internal and external pressure on the bulb.
Finally, a correction for the exposed mercury column may be required: this
correction is equal to 0.000 16n(t – tm), where n is the number of exposed Celsius
degrees, t is the indicated temperature and tm is the mean temperature of the
stem. The error may be as much as 10 oC at 400 oC.