The behavior of radiometers at low temperatures, besides being of some interest to the theoretical physicist,(2) is of considerable importance to one desiring to carry radiant energy measurements to an extreme limit. Various workers have already pushed the sensitivity of such instruments as the thermocouple, radio micrometer, and radiometer into the region in which Brownian movement begins to mask observations so that any great gain in the sensitivity of existing instruments must come from some change in technique rather than from mere refinement of our instruments

Smith, Sinclair

English

Caltech Authors - Main

PHYSICS: S. SMITH
not entirely independent of temperature, varies but
tween 0C. and 100'C.
METAL
Aluminium
Bismuth
Cobalt
Copper
Gold
Iron
Magnesium
Molybdenum
Nickel
VALUE OP f
AT 0C. AT 100C.
1.588 1.587
1.359 1.358
1.568 1.568
2.257 2.255
2.322 2.322
2.852 2.853
2.127 2.125
2.450 2.448
1.335 1.334
METAL
Palladium
Platinum
Silver
Thallium
Tin
Tungsten
Zinc
Constantan
Manganin
little, if at all, be-
VALUE OF f
AT 00C. AT 1000C.
2.040 2.039
2.121 2.120
2.203 2.204
2.203 2.204
2.287 22.286
2.841 2.837
2.213 2.211
2.135 2.135
3.136 3.135
Equation
becomes
(1) as amended, in accordance with what has here been shown,
d In KU = f.kT2
dT (7)
1 Theoretische Chemie, 7th ed., p. 675.
2 See Proc. Nat. Acad. Sci., 16 (1930), p. 49, Table 1.
THE EFFECT OF LOW TEMPERATURES ON THE SENSITIVITY
OF RADIOMETERS'
BY SINCLAIR SMITH
CARNUGIS INSTITUTION OF WASHINGTON, MOUNT WILSON OBSURVATORY
Read before the Academy, April 29, 1930
The behavior of radiometers at low temperatures, besides being of some
interest to the theoretical physicist,2 is of considerable importance to one
desiring to carry radiant energy measurements to an extreme limit. Vari-
ous workers have already pushed the sensitivity of such instruments as
the thermocouple, radio micrometer, and radiometer into the region in
which Brownian movement begins to mask observations so that any great
gain in the sensitivity of existing instruments must come from some change
in technique rather than from mere refinement of our instruments. Some
hope lies in reducing the effect of Brownian movement by working at
greatly reduced temperatures, and for this purpose the radiometer has
much in its favor. In the case of the thermocouple, not only must the
couple be at a low temperature, but also the galvanometer, and this offers
numerous technical difficulties. The radiomicrometer is an instrument
very thoroughly insulated against heat, and the temperature is not easily
VOL. 16, 1930 373
PHYSICS: S. SMITH
controlled. The radiometer, however, working as it does at a gas pressure
high enough to have a fair heat conductivity, can be depended upon to
assume the temperature of the walls
of its container.
A chamber suitable for the study
of radiometers at low temperature
was built as shown in figure 1. The
outside case consists of a vacuum-
tight silicon-aluminum casting,3 closed
F77 at the top by a brass plate seated
on a rubber gasket. The inner double-
walled chamber is hung from the
brass cover by the two brass tubes,
>4< z a, a, which serve as supports and
provide a means for introducing the
cooling medium (liquid air in these
a. a--[ &experiments) into the hollow walls
of the inner chamber. These tubes
41|----qiiare made as thin as possible in order
-T--------- to reduce heat conductivity. As the
space between the inner chamber and
the outer case is highly evacuated
during an experiment, the only heat
path from the inner chamber to the
"outside" other than the two support-
ing tubes, is the "sylphon" bellows
which gives access to the radiometer
chamber proper. An extension of
IsIF the radiometer chamber is carried
!IrIIthrough the cover plate, and a section
i1'i|including a glass ground-joint is ce-
I,!ji!1,mented to this extension as shown in
the figure. This enables one to ro-
tate the radiometer without opening
the chamber as the rod which carries
the radiometer ends in a hard rubber
Fo X spline which engages in a slotted
piece of brass attached to the upper
member of the ground joint. The
hard rubber spline also enables one to
remove the radiometer, for the cover
Cross-section of the radiometer (c) of the radiometer chamber proper,
chamber. the rod, and the radiometer itself
374 PROc. N. A. S.
PHYSICS: S. SMITH
form one assembly which can be lifted out when the upper member of
the ground joint is removed.
Windows are double throughout, for the inner windows which are cooled
during the experiments would "frost" if exposed to the atmosphere.
Some difficulty was experienced in finding a method of attaching these
inner windows. Khotinsky wax, sealing wax, and beeswax and resin,
in fact all common waxes, are useless at liquid air temperatures, and while
1X0
140
Io l ,,,II-
20~~~~~~~~~~~~~~~~~
100 . 01 O
60
40
.001 .010 0.100 1.000
PRESSURE IN MM. OF Hg.
FIGURE 2
Dupont cement usually holds at low temperatures, in this apparatus it
generally broke the windows by pulling fragments out of the surface.
The windows were finally held in place by short lengths of rubber tubing,
moistened with a few drops of castor oil. This method worked perfectly,
for the radiometer chamber showed no loss of gas when the pressure in
the region between the cooling chamber and the outer case was kept less
than 10-4 mm. for a period of several hours.
The procedure was as follows. The radiometers to be studied was
VrOL. 16, 1930 375
3'SIty(sIS. S.SMITR A
mounted in the inner- chamber,- and after the chamber had been thoroughly
evacuated, the desired gas was introduced, with precautions as to purity
and dryness. Light from a controlled lamp was then allowed to fall on
one of the radiometer vanes, and deflections were recorded for a series of
different pressures. The cooling chamber was then filled with liquid air
and the series repeated.
The results for a typical radiometer in H2, He, and air are shown in
figure 2. As may be seen from the curves, the maximum sensitivity in
each case increases at low temperatures and shifts toward lower pressures.
As to be expected, the radiometer becomes very sensitive to stray radiation
at low temperatures; but stray radiation can easily be eliminated. In
other respects the behavior is very good. The zero is remarkable steady,
and at a meter scale distance deflections can be duplicated to about 0.1 mm.
1 This work was undertaken as a part of a cooperative study of astrophysical instru-
ments and methods, made in connection with the development of the Astrophysical
Observatory and Laboratory of the California Institute of Technology.
2 For a full discussion of the theory of radiometers see Epstein, Zs. Phys., 54, p.
537,1928.
3Nichols, J. 0. S. A., 19, p. 164.
4 The radiometer used was a sensitive double, vane radiometer built along the lines
described by Tear (Phys. Rev., 23, 641). A full description of the technique of building
sensitive radiometers will appear elsewhere.
37B PRoc. 14. . A.


The effect of low temperatures on the sensitivity of radiometers

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The effect of low temperatures on the sensitivity of radiometers

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