Telegram-herald., 1891-02-13
USING A THERMOMETER.
Things Not Generally Known OF
Temperature Testing Devices.

There are some peculiarities about
the ordinary thermometer that are not.
/ think. as well understood as they
should be. Every one knows. OF course,
that Thermometer is an instrument
designed TO measure temperature 10y
the expansion of quantity OF mercury
contained in bnlb with fine stem
So, we hang up our thermometer in a
convenient place, look at the fine thread
of mercury when the humor rcscizes us,
and say that the temperature ut that
! place is 90 degrees, 20 degrees or so de-
| grees, as the case may bc.
The temperature of what?
All that tho thermometer really re-
cords is its own temperature; that is to
say. the temperature of the mercury in
| its bulb. Now this mercury is incloscd
in glass, which is very poor conductor
| of heat.
! II 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 viii be seen to stand
at 951I degrees, or, perhaps 96 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.
/ - 1., n1, ia
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
mir, 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 o
cold to warm place, or rice rerso, will
not attain the temperature of the air
surrounding it in - 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
strunent will be heated above the tenn
perature of the air, just as stone or q
piece of iron would be. Most thermom-
eters are provided with n shield which
shades the bulb from the suns rays;
but the tube, the case of the instrument
and the wall or post against which il
hangs, are heated by the sun, and some
of their heat is conducted to the mer-
curry through the glass.
In order, then, 70 ascertain the tenn
perature of the air tout of doors). it is,
first of all, necessary to place the ther-
uometer 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 n wall, on which the sun is shining
brightly, in other directions are the
bodies Of men and animals. All these
heated objects radiate 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 tho air, and in either
case it win exert disturbing influence
How can all these causes of error be
avoided, and the true temperature oi
the air obtaincd2
They can not be entirely avoided, bu,
Il is quite easy to diminish their effects
so greatly that they are of no practical
4 10
that IL the DIR were very good con
ductor Of heat, the effect of the sun o,
other hot body upon the thermometer
would be very slight, because the all
which surrounds the thermometel
would carry of the heat so rapidly
that the temperature of the mercury
could never rise far above that of the
air.
But air Is q very poor conductor-in
deed IL can scarcely be called a con
iuctor at all Air cools hot bodies b,
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 II
turn. and rises ia the same way. and s
q 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 q verJ
gentle breeze indeed
Hence heated body cools very slow
| ly in still afr. II the cir is not still
however. bat brisk breeze is blowing
The cooling is much more rapid. a. The
wind changes The air about the hot bodJ
far more rapidly thaa q mere convec
ten current can do Hence thermo-a
eter exposed 20 gale Of wind vii soos
attain the sern nperature Of The air yer,
nearly and will remain practically Al
the air temperature, 40 matter what ho>
bodies IN the vicinity are striving "a
make il bluuder-
The heat radisted by these bodies ~
TIE Thermometer viii be carried % ~
the wind as fast as IL arrives In ire
absence Of a natural gale. Il is only necee
sr7 20 create an artificial orc with 9
bellows OF fan
But Il eviSeatly does not mates
whether the air moves rsrsdly s<ros=
The Thermometer or the later moves