IV.
LOW TEMPERATURE PHYSICS
Prof. M. A. Herlin
Prof. S. C. Collins
B. Bernstein
R. P. Cavileer
E. E. Huber, Jr.
L. D. Jennings, Jr.
A.
W. H. Kleiner
H. H. Kolm
I. Manning
R. D. Maurer
V. Mayper, Jr.
D.
H.
A.
E.
S.
J.
H. Rogers
E. Rorschach, Jr.
R. Sears
CG. Sharkoff
Waldron
W. Wright
MAGNETIC DIPOLE INTERACTIONS IN CRYSTALS
The equipment used by M. B. Prince for measuring the curves of magnetization vs.
magnetic field at constant entropy (Quarterly Progress Report, July 15, 1951.) has been
revised for use in an iron core magnet available in the low temperature laboratory.
This
magnet will be used for obtaining curves at higher temperatures than those reached by
Prince.
L. D. Jennings, Jr.
B.
RESISTANCE MINIMUM MEASUREMENTS
The mutual inductance method of measuring resistivity has been refined according
to the proposals in the Quarterly Progress Report, July 15, 1951.
relating the mutual inductance changes to the conductivity,
In the calculation
the next two higher order
terms have been calculated and included in the resistivity determinations.
Another
source of considerable error in the experimental apparatus has also been remedied.
It
was found that most of the measurements yielded a value for the change in the real component of the mutual inductance that was too large.
This was traced to capacitive
coupling between the primary and secondary of the mutual inductance coil used. A
copper shield was inserted between the two windings, and a considerable improvement in the consistency of the measurements resulted.
Resistivity measurements of
various samples at liquid nitrogen temperatures by the determination of the real and
imaginary components of mutual inductance have now been found to agree within 4 percent.
Measurements on superconducting tin have also given close agreement.
(The
calculation on superconductors mentioned in the Quarterly Progress Report, July 15,
1951,
has not been completed.)
Some discrepancies have appeared in a few measure-
ments on magnesium in the liquid helium range, amounting to approximately 10 percent.
It is believed that this is still caused by insufficient shielding in the mutual inductance
coil.
The stability of the mutual inductance bridge has been greatly improved by
I. Manning through impregnating the fixed coils with beeswax.
Mr. Manning has also
undertaken the growth of aluminum crystals in the furnace constructed in this laboratory
and has grown an aluminum sample containing a large single crystal and one small
satellite.
This crystal was grown from stock believed to be very pure.
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Measurements
(IV.
LOW TEMPERATURE PHYSICS)
on the crystal indicate reasonably high purity, as indicated by the sharpness of the
superconducting transition.
Resistivity measurements just above the transition temper-
ature do not show a minimum; however, measurements at lower temperatures through
destruction of the superconducting state by a magnetic field would be desirable.
The previously investigated samples of magnesium are being measured with the
corrected apparatus.
Some measurements in liquid helium above the lambda point in-
dicate the presence of a thermal gradient in the helium column, rendering the temperature determination uncertain.
It is hoped that this can be remedied by the construction
of an agitator for the helium bath.
Preparatory to measurements above the liquid helium range, the properties of a
constantan resistance thermometer have been investigated.
It was found that it exhibited
a rise in resistance as the temperature was lowered from 4°K to 1 °K.
reproducible discontinuities in the resistance were also present.
Some disturbing
The temperature coef-
ficient of resistivity was found to be too small to serve as a resistance thermometer.
As
an alternative, a new helium gas thermometer is under construction.
H. E. Rorschach, Jr.
C.
AUTOMATIC CONTROL OF LIQUID HELIUM BATH TEMPERATURE
The temperatures of liquid helium baths have been controlled in this laboratory by
two means:
(1) an electronic, pressure-operated mechanism to regulate the pumping
rate, for use at temperatures above 1. 7 "K; and (2) an electronic device for use at lower
temperatures, utilizing a resistance thermometer and a heating coil in the bath itself.
After developing this second device to a certain efficiency, it was decided that work
should be done on a galvanometer-type control system.
The new system has been built
and consists of a carbon resistance thermometer in a Wheatstone bridge. The light from
a galvanometer turns on a heater in the bath by means of a phototube circuit. Preliminary tests have been carried out, and comparison runs at helium temperature between
the two control systems are about to be made.
D. H. Rogers
D.
TEMPERATURE AND PRESSURE DEPENDENCE OF THE VISCOSITY OF LIQUID
HELIUM
It was found that the improved electronic equipment described in the Quarterly
Progress Report, July 15, 1951, is capable of measuring amplitudes of oscillation of the
disc which are smaller by an order of magnitude.
and stems from lateral vibrations of the disc.
The limiting noise is now mechanical
These were found to increase consider-
ably when the equipment was cooled to liquid helium temperatures due to the fact that the
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(IV.
LOW TEMPERATURE PHYSICS)
sliding upper head (see Fig. IV-1 in the Quarterly Progress Report, July 15, 1951)
failed to maintain tension of the suspension wire, probably due to frozen impurities on
the sliding parts.
A new suspension frame was constructed,
incorporating these improvements:
(1)
Tension is applied to the suspension wire directly by a leaf spring; this arrangement
avoids sliding parts but decreases the maximum permissible slack due to differences
in contraction which the spring will take up. (2) The plane of the square detecting coils
has been rotated by 90* with respect to the frame, and the mounting capillaries to which
the suspension wire is soldered pass through open slots rather than holes; this arrangement makes it possible to preassemble the disc, suspension wire and mounting capillaries outside the frame so as to avoid damage to the coil insulation by the heat of
soldering.
(It will be recalled that abnormal damping observed previously was caused
by a single short-circuited turn of the detecting coils. ) (3)
The frame is constructed
entirely of plexiglass so as to decrease electromagnetic damping due to the proximity of
brass parts.
One series of measurements has been made with the new suspension, but it was
found that the take-up stroke of the leaf spring (one-eighth inch) is inadequate to compensate for the very considerable thermal contraction of plexiglass, and the suspension
was again loosened.
Measurements consequently showed the usual excessive scattering
due to mechanical noise.
The situation is being remedied by changing the design of the
leaf spring so as to double its stroke.
H. H. Kolm
E.
MEASUREMENT OF SECOND SOUND PULSE HEIGHTS IN LIQUID HELIUM II
Some changes have been made in the design of the keyed amplifier in order to make
its operation linear in the desired range.
The keyed amplifier scans the base and top
of the pulse, thus giving a measure of its amplitude, and averages to zero any random
voltage fluctuations and any periodic voltage with repetition frequency differing from
that of the pulse.
B. Bernstein
F.
THE SPECIFIC HEAT OF MAGNESIUM
In order to measure the specific heat of magnesium at low temperatures, a calori-
meter containing a sample of magnesium has been constructed.
An a-c bridge circuit
is now being developed for the purpose of measuring a carbon resistance thermometer
to five-place accuracy with 0. 1 volts across the bridge.
By this method it is hoped to
measure temperature differences to within 0. 001 * when a known amount of heat is
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(IV.
LOW TEMPERATURE PHYSICS)
applied to the sample.
E. E. Huber, Jr.
G.
MEASUREMENT OF THERMAL CONDUCTIVITY OF MAGNESIUM AT LIQUID
HELIUM TEMPERATURES
The magnesium sample has been constructed and all necessary resistance thermo-
meters placed on it.
A vacuum system has been constructed to eliminate errors due to
gas conduction from the sample.
A-c bridges and allied electronic measuring equipment have been assembled to measure the thermal conductivity of the sample.
As soon as better accuracy can be obtained in the measuring circuits a qualitative
measurement can be made, and a quantitative measurement of the thermal conductivity
can be made with a slightly additional amount of equipment.
Developmental work is being performed on a pressure-regulating device to eliminate
some random effects in the experiment.
E. G. Sharkoff
H.
THERMOELECTRIC FORCES
Relatively crude measurements of the output of thermocouples of Mg-Ag, Mg-Au,
and Ag-Au have been made as a pilot experiment. Only small variations (on the order
of 10 lvolts) were encountered when the low temperature junction was varied between
1.5 "K and 4. 2"K.
While the voltages observed varied from run to run, the thermoelec-
tric power seemed to remain unchanged and the values of this variable between the three
couples were qualitatively consistent.
It is planned to extend the present measurements
to the temperature region of the minimum in resistance of magnesium before refining
the technique.
R. D. Maurer
I.
STUDY OF THERMAL PROPERTIES OF SOLIDS BY A PULSE TECHNIQUE
In this study pulses of heat are applied to the end of a rod-shaped sample.
The heat
from each pulse diffuses along the rod and causes, at a distance x, a rise in temperature
followed by a decay.
The shape of the temperature vs. time curve is a function of x and
of both the specific heat and thermal conductivity of the sample.
In measurements taken in May the sample temperature was returned to the reference level between pulses by means of a transfer gas.
From the decreasing portion of
the temperature vs. time curve the rate of heat transfer by gas could be calculated.
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It
(IV.
LOW TEMPERATURE PHYSICS)
was assumed that this transfer rate acted during the period of temperature rise at x.
This assumption, though inapplicable, was used to obtain the temperature-rise vs. time
curve compared with theory.
to follow a T
3
In this manner the specific heat of magnesium was found
law between 1. 5"K and 4. 20 K.
In the present design temperature is returned to the reference level between pulses,
not by gas conduction but by having heat flow to the end of the rod which contacts the
bath, essentially at infinity. To insure that heat flows only along the rod, the boundaries
must be adiabatic.
The requisite vacuum system is now being tested.
Heat input to the sample is from a carbon sheet resistor which unfortunately has to
be supported.
The heat capacity of the support is a source of error.
have been designed and tested.
Several supports
Further knowledge of their behavior in use will be ob-
tained from projected electronic apparatus.
Temperature variation is measured by another carbon sheet.
Electronic equipment
to measure the resistance very accurately and to obviate the effect of random fluctuations has been constructed and tested.
As soon as the high vacuum system is in proper operation, measurements can be
made to determine the characteristics of the heater and the thermal properties of the
sample.
S. Waldron
J.
MAGNETIC DETERMINATION OF THE THERMODYNAMIC TEMPERATURE SCALE
Work has been started on the determination of the thermodynamic temperature scale
at very low temperatures by a magnetic method.
The paramagnetic salt to be used,
initially at least, will be potassium chrome alum; some progress has been made toward
obtaining large single crystals of the salt.
The necessary apparatus has not yet been
fully constructed.
J. W. Wright
K.
PRESSURE DEPENDENCE OF SECOND SOUND VELOCITY IN THE ADIABATIC
DEMAGNETIZATION REGION
Construction of apparatus for the magnetic cooling of a sample of helium under a
known pressure is nearing completion.
When it has been completed and is working with
reasonable reliability further apparatus for the measurement of the velocity of second
sound in the cooled helium will be added.
The demagnetization will be carried out in
the usual manner, using ferric ammonium alum as the refrigerant and condensing in the
helium under pressure through a stainless steel capillary into the salt chamber.
Making
and breaking of heat contact to the bath will be by the standard method of filling with
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(IV.
LOW TEMPERATURE PHYSICS)
helium gas and then evacuating the space around the salt chamber.
Tentative calcula-
tions indicate that if there are no unexpected sources of heat leak, warm-up rates of
0. 1* to 1* per minute are to be expected in the region 0.1 * - 0. 7", with the lower rates
occurring at the lower temperatures.
This indicates that there is some possibility that
the experiment will work.
V. Mayper, Jr.
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