Copyright © 1998 Richard A. Paselk
266. Mercury Thermometers.--For
ordinary purposes thermometers depending on the expansion of mercury confined
in a bulb and tube of glass or other transparent 
substance are
most convenient and universally used. Two standard forms are shown in Fig.
173, the mercury being confined in a thin-walled glass bulb attached to
an extremely fine capillary tube. For use at ordinary temperatures the
upper part of the capillary contains only mercury vapor. Since mercury
expands somewhat less than 1/5000 part of its volume at 0°C. for a
degree rise in temperature (compare with air above), it is necessary to
have a very fine capillary in order to obtain an easily observable motion
of the column for a degree change in temperature. All such thermometers
should, for precise work, be calibrated or standardized by comparison with
the hydrogen standard.
Fig. 173 shows the two standard ways of marking the "scale"
on the thermometer. In one the scale is marked directly on the stem of
the thermometer- this is the most accurate and permanent way used in all
standard scientific thermometers and clinical thermometers: in the other
the scale is on paper or white glass and enclosed in an outer glass tube
back of the capillary stem- this usually gives more legible scales but
they are somewhat likely to become loose and shift with respect to the
capillary. A third method is used for cheap "household" thermometers:
in this the thermometer is simply mounted on a support which carries the
scale.
The glass used for the thermometer (especially the bulb) is of the
greatest importance, and in recent years great improvements have been made
in the qualities of glass used for this purpose. A bulb made of ordinary
glass has the fault of slowly changing its volume with time, and of permanently
and quickly increasing its volume whenever it is heated; say to 100°C.
or higher. Such changes, of course, alter the reading for a given temperature.
Some of these effects gradually disappear after the bulb has been made,
so that thermometer bulbs should be kept for some time, or else artificially
"aged" by heating and cooling, before being graduated.
Through the development of special glasses having high melting-points
it has become possible to construct mercury-in-glass thermometers reading
to 550°C. or even higher. In such high-range thermometers the space
above the mercury column must be filled with a gas (usually carbon dioxide
or nitrogen) at a final pressure of about 19 atmospheres, in order to keep
the mercury from boiling. For such thermometers the properties of the glass
are of the greatest importance, and the glass known as "Jena 59III"
is the best one to use. Even with this glass if the thermometer is kept
at 550°C. for an hour or more a permanent expansion of the bulb will
result; this will permanently lower the freezing-point reading, but if
this change is applied as a correction (added) to subsequent readings of
the thermometer, fairly correct results can be obtained. Thermometers of
mercury in clear fused quartz have also recently been satisfactorily constructed
for use up to about 700°C.
In using thermometers it is well always to avoid too
sudden heating or cooling; and in measurements above 100° (or in all
cases where extreme accuracy is required) it must be remembered that thermometers
are usually graduated to read correctly when bulb and stem are all at the
temperature to be measured. If the stem is cooler than the bulb the thermometer
will read too low and this error may amount to as much as 40° at 550°C.
In careful work thermometers should always be compared with a standard
or standardized at known temperatures or sent to the Bureau of Standards
for comparison.