Evening star., 1915-09-12
CURIOUS PHASES OF
ELECTRICAL CONDUCTING

MOST substances possess the COM
AVU mon quality or property of being
better conductors of electricity when
cold than when hot. Thus. while cop
per is ordinarily a much better conduc-
tor than iron. copper wire at a bright-
red heat would not be much better IN
this respect than an iron wire at ord'-
nary temperatures. Conversely. iron'
by cooling to liquid-air temperature.
may become almost as efficient a car
rier of current copper under usaai
and If it was feasible TO COO
transmission wires. cables. etc, with
LIQUID air the cheaper metal might even
be used in place of copper
A certain class of substances, now
ever of which glass is an example, are
insulators under ordinary conditions.
but lose this property and become con-
Quoting when heated to redness- in.
Nernst glover is the best example OF
this 1t is little tube or rod com
posed of oxide of certain of the rare
chemical elements such as zirconium'
and, when heated by the electric cur
rent to its normal temperature OF % s0me
1000 F.. or over. gives out the intense
white light that is characteristic OF the
Nernst lamp.
While at this high temperature the
glover is fair conductor OF electric
ity but it has this curious property or
being insulating under ordinary con-
ditions and of requiring heating 70
make it conducting. In the commerciai
type of this lamp the preliminary neat
ing, which may be observed whenever
such a lamp is turned on, is done by
the aid of little special electrical heat
ing devices placed very near the glow
ers, but for many scientihc purposes
Il is convenient to have a single glow
er mounted in as simple form as pos
sible, and in such cases 1t Is easy TO
start it with an ordinary match. This
suffices to bring 1t to a low-red heat.
where it begins to become conducting
and quickly comes up to Its normal
brilliance.
Conversely, since it loses its conduct
ing property on cooling. IL it be vigor-
ously blown at 1T may be cooled to such
an extent as to become practically non
conducting and SO go out. Hence, the
being lighted with a match as well as
being blown out by the breath.
Many other substances also show this
effect of losing their Insulating proper
ties on heating. Even ordinary glass;
as mentioned. acts in this way. and
when handled in the form of small
rod with a wire twisted round each
end may. on heating. be made to- carry
considerable current. substances IKE
glass, however, become soft and plastic
when heated sufficiently to make them
conducting, and so could not be suit
ably used in place Of the glover for
illuminating purposes The latter re-
tains it's shape even under the intense
heat and conveniently serves for the
study of many high temperature effects
on a small scale. Thus. small parti-
cles of gold, palladium and even plati-
mum are easily melted into little glob-
ules on their surface, and by tncreas-
ing the current and hence the tenn
perature sufficiently IT is possible not
only to melt but actually to volatilize
or evaporate such extremely refrac-
tory metals as rhodium and iridium in
this way.
When the vision is aided by a hand
magnifier or other low-power micro
scope. such melted globules on the
Nernst glover show extremely inter-
esting and curious actions. When heat
ed considerably above its melting point
OF 1,948 degrees F. the gold globule.
for instance, shows signs of violent
agitation. Curious whirls develop on
its surface and sometimes the whole
drop seems to be spinning. 1t is inter-
estipg to watch the ever-changing ef-
fects, and no one with imagination can
escape the feeling that here, on a mi
croscopic scale, is a world in its molten
condition, turning on its axis. The very
curious motions that are to be observed
with particles of cobalt (solid rather
than molten) and in some other metals
have interested and fascinated observ-
erS.
Moreover. these molten globules ex-
hibit in a beautiful way what is known
as the phenomenon of ''undercooling"
1t is a matter of common observation
that water in small vessels, when not
disturbed. will still keep fluid at tenn
peratures considerably below the freez-
ing point. If it is stirred or If q
small piece of ice is dropped in. solidifi-
cation starts at once and the whole
rises rapidly to the freezing point. The
amount of undercooling possible varies
greatly with the circumstances, being
limited usually to few degrees; but
in case of these globules it has been
observed that they may remain molten
even if cooled several hundred degrees
below their normal melting points.
When this Is carried too far. however,
solidifcation suddenly occurs, accom-
panied by little flash of light. prob
a bly, due to the sudden rise of tempera-
ture and a roughening or corrugation
of the surface.