Evening star., 1940-07-04
Of Stors, Men
And Atoms
Notebook Of Science Progress
In Field, Laboratory
And Study
BIZ Thomas r. Henri.

BIZ Thomas r. HENRI.
A thermometer which measures cold
down to $50 degrees centigrade below
zero has just been perfected at the
United States Bureau Of Standards.
1t goes to within four degrees Of the
absolute bottom of cold-the point where
all molecular motion would stop and
which is reached nowhere in the uni
ably vast CULTS of nothingness between
the galaxies.
The thermometer depends on the fact
that the resistance of coil of platinum
wire to the passage OF an electric cur
rent varies with the heat of the wire.
The colder it is the less the resistance
At 259 degrees below zero the resistance
is less than three-thousands of that
at zero. The planet Pluto, where COM
parable temperatures may be reached.
would be an ideal location for electric
power companies if there were anybody
there to buy the current.
The platinum wire thermometer for
some years has measured temperatures
down to 1913 degrees below zero cent?
grade, the point where air turns into a
liquid. This now is accepted in all the
major countries by an international
agreement and it is hoped to have the
newly perfected scale accepted in the
same way as soon as the war is over so
that the scientists of different countries
can get together. 1t now goes down
10 the temperature at which the highly
explosive gas hydrogen turns to ice.
Hitherto, it is explained by DR.. F. G.
BrickWedde, Bureau of Standards physi-
cist, measurements in these depths of cold
have been possible only by cumbersome
method depending on the pressure of
helium gas which varies directly with
the temperature. Compared to the Job
of making almost any kind of heat
thermometer, the celebration of the cold
thermometer" was quite complex. The
former depends on the expansion of some
substance, such as mercury, when it is
heated 1t expands definite amount
for each degree of heat, or fraction of
degree
The platinum wire resistance, how
ever, does not change by regular steps as
the temperature declines. The exact de-
gree of resistance had to be determined
for each fraction of degree change In
temperature. A good deal depends on
the purity of the platinum used.
For the present the little coil Of wire
at the Bureau of Standards is the stand
ard Thermometer with which all others
intended for the same purpose must be
compared BY means of it, DR.. Brick
wedde said, eventually it will be pos
sible to determine exactly the freez-
ing or melting points of various gasses.
When this is known other laboratories
can calibrate their own thermometers.
For the present, DR.. Brickwedde ex-
planned, the cold thermometer" has
limited practical application. Manu-
facture of liquid air is about as far as
industry has gone, down into the depths
of cold. The Bureau of Standards,
however, must be prepared to measure
anything that is measurable. Besides
there may be wide use for the ther-
mometer in scientific laboratories where
numerous researches now are under way
regarding the properties of substances
at extremely low temperatures
The only rival of this thermometer
as a measurer of low temperature, he
said, is the thermo- couple, which is used
to take the temperature of limited areas
on the planets of millions of miles
away. This is based on the variation of
an electric current with the degree Of
heat applied to the junction of two
wires of different materials. It's ad-
vantage lies in the fact that only the
junction, practically a geometric line,
need be subjected to the temperature
which is to be measured. Thus all but
a small fraction of the disk of the
planet Mars can be cut out and the
thermo- couple junction exposed to the
remainder through a telescope. The heat
Of that particular part of the planet can
be determined by the difference be-
tween this junction and one exposed
only to room temperature.
But actually, DR.. Brickwedde said, the
thermo- couple is relatively inaccur-
ate thermometer. The platinum re-
sistance device gives much finer meas
urements of cold. It's only disadvantage
is that the whole wire must be exposed
to the temperature which it is desired to
measure. This would be practically i'm
possible in astronomical observations.
Another disadvantage of the platinum
wire thermometer is that it never will
reach zero, or within few fractions Of
a degree of it. The ideal would be wire
which had no electrical resistance what
soever at the ultima Thule of cold. Only
one substance has yet been found where
this is true. It is combination Of the
rare element columbium and carbon.
Although its resistance varies irregularly
with various degrees of heat 1t would be
possible to calibrate with it thermom-
eter which would take temperatures in
the critical region between no heat at
all and the freezing point of liquid hydro
gen.