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importance but greater difficulty: are the indications of the
normal thermometers of Dr. Geissler conformable to the reed
temperatures? The author first tried to decide this question by a
strict comparison of the Geissler thermometers with the
air-thermometer, described in the Journal fur prakt. Chemie1
1882. The result was the following:
The mercury thermometers only agree exactly with the
air-thermometer a few degrees above and below zero, for
higher temperatures the indications of the two instruments
differ slightly, for lower temperatures the discrepancy
gradually but rapidly increases and at — 20° C is considerable.
If the indications of the air-thermometer are considered
to be the standard measure of temperature, the normal
mercury thermometers are far from being right, especially below
zero. It remained to be seen, if the Geissler thermometers
were correctly adjusted after the principle of the mercury
thermometers, i.e. if every centigrade degree on their scales
corresponded to an equal apparent dilatation of the mercury in the
bulb. I resolved to test this in the following way.
The dilatometer, see page 262 and plate 20, was filled with
mercury, which was boiled - in the reservoir. After cooling, the
instrument was placed in melting snow for several hours and
the excess of mercury allowed to escape through >b,» until the
dilatometer was filled exactly to the stopcocks and weighed.
Afterwards the dilatometer was immersed in a mercury vessel
and the dilatation of the mercury in the instrument between
+ 2°.65 C and — 17°.is C determined in the ordinary way (see
page 264) by means of a scale-tube. The Geissler
thermometers were placed immediately beside the reservoir and the
temperatures read off with a kathetometer. The dilatometer
was thus transformed into a mercury thermometer of
extraordinary size with a carefully calibrated stem (the scale-tube).
1 Journ. pr. Ch [2] Bd 25 p. 103.
2 In spite of the fragile form of the instrument, this operation is not
very hazardous. Care must be taken, that the quantities of mercury entering
through the capillary tube are heated beforehand to 200° or 300’ C.
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