Lawrence democrat. (Lawrenceburg, Tenn.) 1884-1925, 1891-03-13
USING A THERMOMETER.
Things Not Generally Known of
Temperature-Testing Devices.

I think, as well understood as they
should be. Every one knows, of course,
that thermometer Is an instrument
designed to measure temperature by
the expansion of quantity of mercury
contained In bulb with a fine stem.
So, we hang up our thermometer in a
convenient place, look at the fine thread
OF mercury when the humoruseizes us,
and say that the temperature at that
place is 90 degrees, 10 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 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 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
ut 95H degrees, or, perhaps 90 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 80, 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 a
cold to warm place, or ctce persa, 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 a 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
and the wall or post against which II
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 TEN.
perature of the air tout of doors), 1t is
first of all, necessary to place the ther
mometer in the shade.
But this is not enough. Even in the
shade the instrument may be sur
rounded by substances warmer thar
the air
In one direction is patch of GROUND
or a 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 the air, and in either
case 1t will exert disturbing influence
How can all these causes of error be
avoided, and tho true temperature o.
the air obtained?
They can not be entirely avoided, but
~ Is quite easy to diminish their effects
; so greatly that they are of no practica
himportance. II will be readily seen
that II the air were q very good con
ductor of heat, the effect of the sun o.
other hot body upon the thermometel
would be very slight, because the ali
which surrounds the thermometel
would carry Off the heat so rapidly
that the temperature of the mercury
could never rise far above that of thu
air.
But air is very poor conductor-in
deed II can scarcely be called con
ductor at all. Air cools hot bodies BY
the process called convection. The al
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 il
turn, and rises in the same way, and S,
q current of air is formed, which con
tinually carries heat from the hot body
But this process is very slow one
and the convection current is a yer,
gentle breeze indeed.
Hence heated body cools very slow
" in still air. If the air is not still
however, but brisk breeze is blowing
the cooling is much more rapid. as tn
wind changes the air about the hot bod.
far moro rapidly than a mere convec
ton current can do. Hence thermom
eter exposed to u gale Of wind viii sool
attain the temperature Of the air yer'