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Sensitive Mercury Thermometers.--In order that it be possible to read a temperature
with great accuracy, it is obvious that the extremity of the
mercury column should move through a considerable distance along
the stem  for a small
alteration of temperature. Consequently, if such a thermometer
is to be used for measuring temperatures from 0° C. to 100°
C., either a very long stem must be provided, or some modification
of the usual form must be employed.
The most usual procedure is to furnish the upper extremity of
the thermometer tube (which is bent over as shown in Fig. 8),
with an enlargement into which part of the mercury can be driven
by heating. A sufficient amount of mercury must be left in the
bulb and stem to give readings between the required temperatures.
Fig. 8 represents a thermometer of this description. The following
points in its construction may be noted. In order to avoid the
errors due to the irregular motion of the mercury in a very fine
tube, a tube of comparatively large bore is employed. It has
already been pointed out that the sensitiveness of a thermometer
depends on the ratio, Volume of bulb: Sectional area of bore
of tube, and as the bore is made comparatively large, a very
large bulb, is required. The stem is provided with an enlargement
into which part of the mercury can be driven, for reasons explained
above. The thermometer tube is made with comparatively thin walls,
and to protect it from injury it is contained within a wider
tube, which is fused at its lower extremity on to the bulb. The
graduations are marked on a separate enamelled scale placed behind
the thermometer tube, and inclosed in the outer guard tube. In
order that this thermometer should be capable of furnishing readings
for very quick changes of temperature, the walls of the bulb
must be made very thin. This of course will expose it to considerable
errors due to variations of pressure. (See Chapter II).
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- The fixed points of such a thermometer obviously
cannot be obtained in the manner previously described. Its scale
must be calibrated by comparison with a standard thermometer.
Such a thermometer as that considered is, however, more often
used to measure small changes of temperature than to determine
actual temperatures.
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