Barbour County index. [volume] (Medicine Lodge, Kan.) 1880-current, 1891-04-22
USING A THERMOMETER.
Things Not Generally Known of
Temperature-Tes Devices.

Temperature Testing Devices.
There are some peculiarities about
the ordinary thermometer that are not,
j think, as well understood as they
should be. Every one knows, Of course,
that a thermometer is an instrument
designed to measure temperature by
the expansion of n quantity of mercury
contained in a bulb with a fine stem.
So, we hang up our thermometer in
convenient place, look at the fine thread
of mercury when the humoruseizes us,
and say that the temperature at that
place is go degrees, 40 degrees or so de-
Rrees, as the case may be.
The temperature of what?
All that the thermometer really re-
cords is it's own temperature; that is to
say, the temperature of the mercury in
its bulb. Now this mercury is inclosed
in glass, which is very poor conductor
OF heat.
If a cold Thermometer is put in pail
of hot water, although the mercury will
begin to rise quite rapidly, its motion
will gradually become slower until at
last the column seems to be at rest at
(say) 95 degrees.
But the mercury is still rising imper-
ceptibly, and later will be seen to stand
at 95H degrees, or, perhaps SO 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, so that sew
eral minutes may elapse before the
column of mercury becomes quite sta
tionary When it 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
to rest for long time. An ordinary
house thermometer, when taken from
cold to a warm place, or rice UerSd, will
not attain the temperature of the air
surrounding it in its new position for
many minutes.
1t will not do so at 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 will be heated above the tenn
perature of the air, just as stone or a
piece of iron would be. Most thermom-
eters are provided with shield which
shades the bulb from the suns rays;
but the tube, the case of the instrument
md 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 tout of doors), it is,
first of all, necessary to place the Their
mometer in the shade.
But this is not enough. Even in the
shade the instrument may be sur
rounded by substances warmer than
the air.
In one direction Is patch of ground
or a wall, on which the sun isSshining
brightly, in other directions are the
bodies of men and animals All these
heated objects radiate heat to the Their
mometer, and raise it's 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
II is quite easy to diminish their effects
so greatly that they are of no practical
Importance II will be readily seen,
that IL the air were a very good con-
ductor of heat, the effect of the sun or
other hot body upon the thermometer
would be very slight, because the air
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 IL can scarcely be called a. con-
lductor at all. Air cools hot bodies by
the process called convection. The air
in contact with the hot body becomes
heated, expands, and, being lighter than
the surrounding air, rises and is re-
placed by the latter. This is heated in
turn, and rises I'm 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 heated body cools very slow
Ily In still air. II the air is not still,
however, but a brisk breeze Is blowing,
the cooling is much more rapid. as the
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 gale of wind will soon
attain the temperature of the air very
nearly and viii remain practically at
the air temperature, no matter what hot
bodies in the vicinity are striving TO
make il blunder.
The heat radiated by these bodies to
the thermometer will be carried of by
the wind as fast as 1t arrives. In the
absence of natural gale, it is only neces-
airy to create an artificial one with q
bellows or fan.
But Il evidently does not matter
whether the air moves rapidly across
the thermometer or the latter moves
rapidly through the air, and hence it
sufices to attach the instrument to a
short cord and swing it rapidly around
ina circle.