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- From: A. Wilmer Duff. A Text-Book of Physics
5th ed. P. Blakiston's Son & Co. Philadelphia (1921) pp.
207-208.
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- Copyright © 1998 Richard A. Paselk
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- 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.
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- 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.
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- 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.
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- 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.
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- 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.
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- © R. Paselk
- Last modified 22 July 2000
