Another recent achievement was the successful development of a method for the complete combustion in a bomb calorimeter of a metal in fluorine when the product is relatively non-volatile.
This work gave a heat of formation of aluminum fluoride which closely substantiates a value which had been determined by a less direct method, and raises this property to 15 percent above that accepted a few years ago.
Similar measurements are being initiated to resolve a large discrepancy in the heat of formation of another important combustion product, beryllium fluoride.

The development and testing of new apparatus to measure other properties is nearing completion.
In one of these, an exploding-wire device to study systems thermodynamically up to 6,000 Af and 100 atmospheres pressure, a major goal was achieved.
The accuracy of measuring the total electrical energy entering an exploding wire during a few microseconds was verified when two independent types of comparison with the heat energy produced had an uncertainty of less than 2 percent.
This agreement is considered very good for such short time intervals.
The method of calibration employs a fixed resistance element as a calorimeter.
The element is inserted in the discharge circuit in place of the exploding wire, and the calorimetric heating of the element is measured with high accuracy.
This is used as a reference for comparing the ohmic heating and the electrical energy obtained from the measured current through the element and the measured voltage across the element.

A high-speed shutter has been developed in order to permit photographic observation of any portion of the electrical wire explosion.
The shutter consists of two parts: a fast-opening part and a fast-closing part.
Using Edgerton's method, the fast-closing action is obtained from the blackening of a window by exploding a series of parallel lead wires.
The fast-opening of the shutter consists of a piece of aluminum foil (approximately Af) placed directly in front of the camera lens so that no light may pass into the camera.
The opening action is obtained when a capacitor, charged to high voltage, is suddenly discharged through the foil.
During the discharge the magnetic forces set up by the passage of current cause the edges of the foil to roll inward toward its center line, thus allowing light to pass into the camera.
Experiments have shown that the shutter is 75 percent open in about 60-80 microseconds.
The shutter aperture may be made larger or smaller by changing the foil area and adjusting the electrical energy input to the foil.
Laboratory measurements of interstellar radio spectra.

Besides the well-known hydrogen line at 21 cm wavelength, the spectra of extraterrestrial radio sources may contain sharp lines characteristic of other atoms, ions, and small molecules.
The detection and study of such line spectra would add considerably to present information on interstellar gas clouds and, perhaps, planetary atmospheres.
Among the most likely producers of detectable radio line spectra are the light diatomic hydrides OH and CH;
somewhat less likely sources are the heavier hydrides SH, SiH, and Aj.
Very small concentrations of these hydrides should be detectable;
in interstellar gas, concentrations as low as Af molecules per Af may be sufficient, as compared to the Af hydrogen atom's Af required for detection of the 21-cm line.

High sensitivity in radio telescopes is achieved by reducing the bandwidth of the receiver;
therefore, only with precise foreknowledge of the line frequencies is an astronomical search for the radio spectra of these molecules feasible.
To secure precise measurements of these frequencies, a research program in free radical microwave spectroscopy has been started.
Since conventional methods are insensitive at the low frequencies of these molecular transitions, the paramagnetic resonance method is being used instead.
This involves the application of a strong magnetic field to the radical vapor, which shifts the low-frequency spectra to a conveniently high microwave range, where they may be measured with optimum sensitivity.

The first diatomic hydride investigated by the paramagnetic resonance method was the OH radical.
Results of this experiment include the frequencies of the two strong spectral lines by which OH may be identified in interstellar gas;
the frequencies are 1665.32 and 1667.36 Af, with an uncertainty of 0.10 Af.
Success in observing these spectral lines has so far, apparently, been confined to the laboratory;
extraterrestrial observations have yet to be reported.
Preparations are being made for similar experiments on CH and SH radicals.
Low temperature thermometry.

The Bureau is pursuing an active program to provide a temperature scale and thermometer calibration services in the range 1.5 to 20 Af.
The efforts and accomplishments fall into three main categories: absolute thermometry based upon the velocity of sound in helium gas, secondary thermometry involving principally studies of the behavior of germanium resistors, and helium-4 vapor-pressure measurements (see p. 144).
Acoustical interferometer.

An acoustical interferometer has been constructed and used, with helium gas as the thermometric fluid, to measure temperatures near 4.2 and 2.1 Af.
Such an interferometer provides a means of absolute temperature measurement, and may be used as an alternative to the gas thermometer.
When values of temperature derived with this instrument were compared with the accepted values associated with liquid helium-4 vapor pressures, differences of about 10 and 7 millidegrees respectively were found.
This result is preliminary, and work is continuing.
Resistance thermometers.

Carbon resistors and impurity-doped germanium resistors have been investigated for use as precision secondary thermometers in the liquid helium temperature region.
Several germanium resistors have been thermally cycled from 300 to 4.2 Af and their resistances have been found to be reproducible within 1/3 millidegree when temperatures were derived from a vapor pressure thermometer whose tubing is jacketed through most of the liquid helium.
Preliminary calibrations of the resistors have been made from 4.21 to 2.16 Af at every 0.1 Af.
The estimated standard deviations of the data for two of the resistors were 1 millidegree;
and for the third resistor, 3.3 millidegrees.
Vapor pressure method.

The reproducibilities of helium vapor-pressure thermometers have been investigated in conjunction with a "constant temperature" liquid helium bath from 4.2 to 1.8 Af.
Surface temperature gradients have been found to exist in liquid helium baths contained in 15- and 25-liter metallic storage dewars.
The gradient was about one half of a millidegree at 4.2 Af but increased to several millidegrees for bath temperatures slightly greater than the **yl point.
A hydrostatic head correction has been neither necessary nor applicable in the determination of vapor pressures or temperatures for the bulk liquid helium.
However, the surface temperature gradient can produce erroneous vapor-pressure measurements for the bulk liquid helium unless precautions are taken to isolate the tube (which passes through the surface to the vapor pressure bulb) from the liquid helium surface.
It has also been observed, in helium 2, that large discrepancies can exist between surface vapor pressures and those pressures measured by a vapor pressure thermometer.
This has been attributed to helium film flow in the vapor pressure thermometer.
In this case also the design of the thermometer can be modified to reduce the helium film flow.
Pressure transducer for pvt measurements.

Precise pressure-volume-temperature measurements on corrosive gases are dependent on a sensitive yet rugged pressure transducer.
A prototype which fulfills the requirements was developed and thoroughly tested.
The transducer is a null-type instrument and employs a stretched diaphragm, 0.001 in. thick and 1 in. in diameter.
A small pressure unbalance displaces the diaphragm and changes the capacitance between the diaphragm and an electrically insulated plate spaced 0.001 in. apart (for Af).
Spherical concave backing surfaces support the diaphragm when excessive pressures are applied and prevent the stresses within the diaphragm from exceeding the elastic limit.
Over a temperature range from 25 to 200 Af and at pressures up to 250 atm, an overload of 300 psi, applied for a period of one day, results in an uncertainty in the pressure of, at most, one millimeter of mercury.
Transport properties of air.

A 6-year study of the transport properties of air at elevated temperatures has been completed.
This project was carried out under sponsorship of the Ballistic Missile Division of the Air Research and Development Command, U.S. Air Force, and had as its goal the investigation of the transport by diffusion of the heat energy of chemical binding.
A significant effect discovered during the study is the existence of Prandtl numbers reaching values of more than unity in the nitrogen dissociation region.
Another effect discovered is the large coefficient of thermal diffusion tending to separate nitrogen from the oxygen when temperature differences straddling the nitrogen dissociation region are present.
The results of the study, based on collision integrals computed from the latest critically evaluated data on intermolecular forces in air, will be reported in the form of a table of viscosity, thermal conductivity, thermal diffusion, and diffusion coefficients at temperatures of 1,000 to 10,000 Af and of logarithm of pressure in atmospheres from Af to Af times normal density.
International cooperative activities.

In March, 1961, representatives of the national laboratories of Australia, Canada, The Netherlands, United Kingdom, U.S.S.R., United States, and West Germany, met at the NBS to devise means for reaching international agreement on a temperature scale between 10 and 90 Af.
As a first step toward this goal, arrangements were worked out for comparing the scales now in use through circulation of a group of standard platinum resistance thermometers for calibration by each national laboratory.
Such a group of thermometers was obtained and calibrated at the Aj.
These thermometers have now been sent to the United Kingdom for calibration at the National Physical Laboratory.
Temperature symposium.

During the last week of march 1961, Columbus, Ohio was the site of the Fourth Symposium on Temperature, Its Measurement And Control In Science And Industry.
The Symposium, which was jointly sponsored by the American Institute of Physics, the Instrument Society of America, and the National Bureau of Standards, attracted nearly one thousand registrants, including many from abroad.
The Bureau contributed to the planning and success of the Symposium through the efforts of Mr. W. A. Wildhack, General Chairman, and Dr. C. M. Herzfeld, Program Chairman.
Dr. A. V. Astin, NBS Director, opened the 5-day session with introductory remarks, following which a total of twenty-six papers were given throughout the week by NBS scientists, from both the Washington and Boulder Laboratories.

2.1.6.

Atomic physics

In addition to the basic programs in wavelength standards, spectroscopy, solid state physics, interactions of the free electron and atomic constants which are necessary to provide the foundation for technological progress, the Bureau has strengthened its activities in laboratory astrophysics.
The programs in infrared spectroscopy are undergoing reorientation toward wavelength standards in the far infrared, the application of infrared techniques to solid state studies, and increased emphasis on high resolution instrumentation.
Two data centers have been established for the collection, indexing, critical evaluation, and dissemination of bibliographies and critical values in the fields of transition probabilities and collision cross sections.
Laboratory astrophysics.

Transition probabilities.

Under the sponsorship of the Office of Naval Research and the Advanced Research Projects Agency, a data center was established to gather and index all published information on atomic transition probabilities.
An exhaustive survey was made of the literature, and a primary reference file of approximately 600 references was catalogued.
Selected bibliographies and tables of available data are now in preparation.

A wall-stabilized high-current arc source was constructed and used to study transition probabilities of atomic hydrogen and oxygen.
This apparatus will also be used to measure transition probabilities of a large number of other elements.
A study of the hydrogen line profiles indicates that a measurement of these profiles can be used to calculate a temperature for the arc plasma that is reliable to about Af percent.

A set of tables containing spectral intensities for 39,000 lines of 70 elements, as observed in a copper matrix in a d-c arc, was completed and published.
Studies of the intensity data indicate that they may be converted to approximate transition probabilities.
These data are not of the precision obtainable by the methods previously mentioned, but the vast number of approximate values available will be useful in many areas.
Atomic energy levels.

Research continues on the very complex spectra of the rare earth elements.
New computer and automation techniques were applied to these spectra with considerable success.