The Princeton union. [volume] (Princeton, Minn.) 1876-1976, 1881-02-24
Thermometers.

The word thermometer means heat
measure, hence any instrument en-
ployed to measure heat should be called
thermometer. When very high tenn
peratures are to be measured, the in
struments employed are called pyrome-
ters, or measures Of fire. Thermome-
ters do not, OF course, measure the
quantity Of heat in q body, but only tell
us the relative temperature. There are
several forms of thermometers, all based
upon the principle that cheat expands,
while cold contracts. Some substances
expand unequally for equal increments
OF temperature, others expand so slightly
that they fail to indicate small changes
OF temperature; both are untitled for
thermometers. It is believed that air
expands equally for equal changes of
temperature, and, as this expansion is
quite considerable -273d part for each
degree centigrade-and as it does not
become either liquid or solid under or
binary pressure, at any temperature
which we can produce, it is the SUBJ
stance employed for the most accurate
measurements of temperature. ANY of
the difficulty -condensible gases, oxygen,
hydrogen, marsh-gas, might unemployed
Instead of air, but with no advantage
and with much inconvenience in their
manufacture.
Next to air, the best material we have
IS mercury, which expands very evenly,
does not freeze readily, and boils at a
comparatively high temperature. For
temperatures below 40 deg. alcohol is
generally employed, although it is
claimed that glycerine could be used.
For temperatures above 300 deg. G. air
Thermometers alone are admissible
and, for very high temperatures, where
glass begins to soften, they are made of
platinum.
The mercury thermometer being the
one usually employed in the arts, in
meteorology, in medicine, and in other
sciences, a few words in regard to the
manner OF making one may be Of inter
est. A glass tube with very fine bore
has a suitable bulb, of any desired form.
blown upon one end. At the other end
may be bulb of larger size, blown
merely for convenience in filing. Meth
er bulb can be blown with the mouth
but with a bellows, containing pure, dry
air. A small capsule is filed with pure
mercury, heated to boiling to expel both
air and moisture. While still hot the
second or temporary bulb is warmed to
expel portion Of the air therein the
open end is placed in the mercury, which
ascends into the bulb because the air
contracts on cooling. When sufficient
quantity Of the hot mercury has been
introduced into this bulb the tube and
! the other bulb are heated to expel part
Of the air, and some of the mercury,
which must be always kept hot to pre-
vent it's chilling and thus breaking the
hot glass, enters the real bulb. BY re-
peating the operation the bulb and stem
are completely filed with mercury,
| which is then boiled to expel every trace
Of air. The tube is now drawn out close
beneath the auxiliary bulb to fire
thread and cut off; the thermometer is
| placed in bath heated a few degrees
higher than the highest temperature
which the thermometer is to show the
excess OF mercury flows out, and the
point is closed with a fine blowpipe
flame. AS the mercury contracts on
cooling it leaves perfect vacuum above
| it.
The graduation is effected by putting
it into ice or snow, then in the steam
from boiling water, marking each OF
these points, dividing the space between
into 100 parts il it is to have Celsius
or centigrade scale, into 80 if Reau.
! muir or $50 if Fahrenheit. This grad
uation is carried on in each direction ia
the end of the stem. On the Fahren-
heit scale the freezing point is marked
80, on each of the other scales it is
marked zero Scientific american.
Trees anu Moisture.
/ A report sent us by an undoubted au-