Evening star. [volume] (Washington, D.C.) 1854-1972, 1910-05-29

WHY is it that when we put our hand on
w piece of steel or marble it feels much
colder than a piece of wood Immediately
next to it? A thermometer will show that
the steel and the wood have the same tenn
perature, and it would seem that they should
feel the same.
The explanation lies in what are called
the relative heat conductivities of the two
substances. Steel is a better conductor of
heat than wood, and will conduct away the
heat from the hand faster than wood. This
can be shown by placing poker of steel and
one of w 4 into fire; the handle of the
steel poker se.s hot more quickly.
The first mentioned phenomercn takes
place only when the temperature of the
object is less than that of the human body.
When the objects are hotter than the body,
just the reverse takes place; in, for instance.
Turkish bath, where the room and its con-
tents are much warmer than the body, the
steel feels very hot and the wood only slightly
warm. The explanation is that the hand
tends to cool them both, that is, bring
them both to its own temperature, and the
steel can and does conduct heat faster from
the heated room than can the wood, and
consequently is not cooled as is the wood.
An interesting experiment may be per-
formed by applying this principle. 1t fol-
lows that good conductor of heat ought
actually to make hot body colder if placed
near or in contact with it, Such is the case.
By placing spiral of copper wire in a candle
fame. the heat will be conducted away so
swiftly that it will put the candle out. the
temperature having been reduced below the
ignition point of the wax.
If piece of wire gauze be placed two
or three inches above a gas jet, the gas
turned on and lighted match applied above
the gauze, we shall have the strange appari-
ton of detached flame; for only the gas
above the gauze will ignite, the gauze con-
Quoting the heat that is on it away so rapidly
that the gas below fails to ignite. Inferno
effects on the stage are sometimes managed
on this plan.
The greatest economic application of this
principle, however, is that of Davy's safety
lamp for miners. 1t consists merely of
flame Gagged in with wire gauze, the gauze
conducting and radiating away the heat of
the internal fame SO fast that it (the gauze)
never attains temperature enough to ignite
firedamp or other gases met with in mines
on the outside of the gauze. The firedamp
that leaks in through the gauze will, how
ever, suffer slight explosion, SO the miner
can tell when he is in an atmosphere of the
stuff.
There are possible many other interesting
experiments on conductivity. The handle of
silver spoon gets hot more quickly than
that of a pewter spoon when thrust into a
cup of hot coffee: in fact, this is an easy and
quick way of detecting imitation silver. Sill
yer itself has been found to be the best con-
ductor of heat known.
If a handkerchief or bit of linen is tightly
and smoothly wrapped round a metal ball, a
candle fame can be held on it for several sec
onds without scorching it. the metal con-
Quoting the heat away fast enough to pre-
vent this. Or if the brightly burning end of
a cigar is pressed against silver quarter or
half dollar similarly incased in bit of mus
lin or linen, the fabric is not even scorched.
If person desires to show up the wrong
impressions of his nerves. the following will
do nicely: Put one hand into pan of hot
water and the other into pan of ice water.
After leaving them in for a few seconds thrust