The Cambria freeman. [volume] (Ebensburg, Pa.) 1867-1938, 1891-02-27
USING A THERMOMETER.
Things Not Generally Known OF
Temperature-Test Devices.

There are some peculiarities about
the ordinary thermometer that are not,
l think, as well understood as they
should be. Every one knows, ofcourse,
that a thermometer is an instrument
designed to measure temperature by
the expansion of a quantity of mercury
contained in bulb with fine stem.
So, we hang up our thermometer in a
convenient place, look at the fine thread
of mercury when the humor-seizes Us,
and say that the temperature at that
place is co degrees. 20 degrees or SO de-
grees, as the case may be.
The temperature Of what?
All that the thermometer really re-
cords is its own temperature: that is 10
say, the temperature of the mercury in
its bulb. Now this mercury Is inclosed
in glass, which is a very poor conductor
of heat.
II cold Thermometer Is put in pail
of hot water, although the mercury win
begin TO rise quite rapidly. its motion
viii gradually become slower until at
last the column seems 70 be at rest AT
SAY % degrees.
But the mercury Is still rising imper-
ceptibly, and later viii be seen TO stand
At 955z degrees, or, perhaps oG degrees
All this time the mercury is being
warmed by the heat flowing from the
hot water outside through the poorly
conducting glass
This flow OF heat becomes very slow,
indeed. as the mercury approaches the
temperature of the water, 50 that sev-
eral minutes may elapse before the
column Of mercury becomes quite sta
tionary. When IL does SO. and not be-
fore. the reading of the thermometer
gives the true temperature Of the water.
If the thermometer is surrounded by
air, the heating process Is very much
slower, and the mercury does not come
10 rest for 4 long time. An ordinary
house thermometer. when taken from
cold to a warm place, or rice rcrsa, viii
not attain the temperature of the air
surrounding it in its new position for
many minutes.
1t VIII not do so ~ all except under
favorable circumstances
Suppose. for example. that the air is
quiet and the sun IS shining brightly on
the thermometer. OF course. the in
strument viii be heated above the tenn
perature Of the air, Just as a stone or
piece of iron would be. Most thermom-
eters are provided with shield which
;
but the tube, the case Of the instrument
and the wall or post against which it
hangs, are heated by the sun, and some
of their heat is conducted TO the mer-
curry through the glass
In order, then, TO ascertain the tenn
perature of the air (out Of doors), ii is,
first Of all, necessary to place the thcr-
momcter in the shade.
Rt1r 4h;C i NOT enpnOh E-ar, in n
shade the instrument may be sur
rounded by substances warmer than
the air.
In one direction IS a patch OF ground
or wall, on which the sun is shining
brightly, in other directions are the
bodies of men and animals. All these
heated objects radiatc heat TO the ther-
mometer, and raise its temperature
above that of the air.
Even the shaded ground may be hot
ter or colder than the air, and in either
case it will exert disturbing influence.
How can all these causes of error be
avoided, and the true temperature OF
the air obtained?
They can not be entirely avoided, but
it is quite easy to diminish their effects
so greatly that they are of no practical
importance. II will be readily seen,
that if the mir were q very good con
ductor of heat, the effect of the sun OF
other hot body upon the thermometer
would be very slight, because the ain
which surrounds the thermometer
would carry off the heat SO rapidly,
that the temperature of the mercury
could never rise far above that of the
air.
But air is very poor conductor-in
deed it can scarcely be called a con
ductor at all. Air cools hot bodies by
the process called convection. The ali
in contact with the hot body become:
heated, expands, and, being lighter thar
the surrounding air, rises and is re
placed by the latter. This is heated ir
turn, and rises in the same way, and SC
a current of air is formed, which con
tinually carries heat from the hot body
But this process is a very slow one
and the convection current is a very
gentle breeze indeed.
Hence a heated body cools very slow.
ly in still air. If the air is not still
however, but a brisk breeze is blowing
the cooling is much more rapid. as tho
wind changes the air about the hot body
far more rapidly than a mere convec
ton current can do. Hence a thermom
eter exposed to u gale of wind viii soor
attain the temperature of the air very
nearly and viii remain practically n
the air temperature, no matter what no
bodies in the vicinity are striving 4
make il blunder.
The heat radiated by these bodies Z,
the thermometer will be carried off b,
the wind as fast as il arrives. In tn
absence of a natural gale. il is only neces
airy 10 create an artificial one with
bellows or fan.