A study of the effects of vibration on the performance of standard cells showed that an a-c voltage was produced during vibration, but that no permanent damage resulted (page 36).

instrument includes two equal thermoelements, one supplied by an unknown. voltage and the other supplied by a highly stable reference voltage. The thermoelement supplied by the unknown voltage is in series with a decade resistor. The dials of this resistor are read directly in volts and so can be set to indicate the nominal value of the unknown voltage. The percentage difference between the unknown voltage and its nominal value as determined from comparison with the reference voltage is then read directly on a galvanometer.

Magnetism. The nuclear magnetic resonance of Ni was observed in 99.97 percent pure nickel metal. It was found that the resonance occurs

at a frequency of 26.1 Mc/s at room temperature with a line width of about 50 kc/s. The temperature dependence of the frequency was measured over a temperature range from 77 to 536 °K, and the effect of an externally applied field on the intensity of resonance was observed. The Ni1 resonance was also observed in 99.7 percent pure nickel with substantially the same results as with the purer sample. Studies are in progress on the resonance in nickel-rich alloys.

A special susceptibility apparatus was constructed for measuring the absolute susceptibility of small samples (less than 0.5 gram) and single crystals by means of a quartz beam balance. This apparatus is designed to measure the primary static susceptibilities of paramagnetic substances while a given microwave magnetic field is being applied. The apparatus also provides for relative measurement of greater sensitivity of susceptibilities by use of a vibrating magnetometer method. Preliminary measurements are now being made with the apparatus of the primary susceptibility of single crystals grown in this laboratory.

Analysis of the Melting Point of Polychlorotrifluoroethylene. The observed melting point of the linear polymer polychlorotrifluoroethylene was experimentally determined to depend strongly on its original crystallization temperature. The observed melting point rises with crystallization temperature. (Results of this type have been known in other polymers for many years.) This interesting phenomenon was explained in terms of the behavior of chain-folded crystals, and the experimental data were used to estimate the equilibrium melting temperature of the polymer.

Analysis of the Dielectric Properties of Polychlorotrifluoroethylene. Data obtained from a previous detailed experimental study on polychlorotrifluoroethylene were analyzed to reveal the contributions of the crystalline and amorphous regions of this semicrystalline polymer. One of the loss processes in the polymer is associated with the freezing out of longrange molecular motions that are associated with the onset of the glass transition in the amorphous component at 52 °C. At a given frequency, say 1 c/s, the dielectric loss peak associated with this mechanism corresponds closely with a mechanical loss peak observed by others. The activation energy exhibited by this process is large and strongly dependent on temperature. At low temperatures, far below the glass transition, the amorphous component exhibits an additional and very prominent dielectric loss mechanism, which is a result of short-range motions in the glassy state of the polymer. The activation energy of this process is small (about 60 kj/mole) and independent of temperature. This process also possesses a mechanical analog-the crystals in the polymer exhibit a marked dielectric polarization which is associated with a very rapid dipolar re-orientation process, probably a twisting mode. Evidence exists that in highly crystalline specimens the amorphous material still remaining has somewhat abnormal properties. Foremost among these is an elevated glass transition temperature. The properties of the liquid polymer above the melting point, which is 221 °C, exhibit the customary negative temperature coefficient.

Finding the frequency response of a differential thermocouple voltmeter. This instrument is a recent contribution to the solution of the problem of transfer from d-c and a-c measurements at power and higher frequencies (page 38).

Dielectric Properties of Polyparachlorostyrene and Polymetachlorostyrene. Polyparachlorostyrene is a molecule which, in effect, has a large dipole moment that is attached at a right angle to the main polymer chain backbone. Thus, a motion of the main chain is required to permit reorientation of the dipole moment and dielectric loss. Dielectric measurements on this noncrystalline polymer in the glassy state reveal no evidence of molecular motion, even at low frequencies.

The situation is radically different in the case of polymetachlorostyrene. In this material, dipolar orientation can be effected by allowing the benzene ring side group to turn about the bond connecting it to the polymer chain backbone. Dielectric measurements have confirmed that such a motion exists even well below the glass transition temperature, since a large dielectric loss is observed. This work discloses an excellent example of dielectric loss due to hindered internal rotation in a polar side group substituent on a polymer chain.

2.1.4. RADIO STANDARDS

The Bureau program in radio standards, centered at the Boulder Laboratories, consists of basic research and development of national standards of fundamental electromagnetic quantities, measurement techniques, and properties of materials. A large calibration service is provided from direct

current through microwave frequencies, and radio broadcasts are made of the national primary standards of frequency and time intervals.

Probably the most striking developments during the past year, scientifically and internationally, were in the area of frequency standards. International comparisions of the atomic beam frequency standards of the United States, England, and Switzerland indicate continued agreement to within 1 or 2 parts in 1010. The United States standard has also been compared daily with commercial standards and has operated on a routine basis throughout the year. All indications are that the United States Frequency Standard is performing at the estimated level of stability. This performance, and the performance of frequency standards in other parts of the world, has led to active consideration on the international level of a redefinition of the second in terms of an atomic transition.

During the year several conferences designed to define and resolve major problems in electronic measurement were held. A series of measurement research conferences between industry and NBS, initiated by the Aerospace Industries Association, is continuing to probe each field of measurement in depth. In the area of electronic measurement the first such conference was held during May 1960, and considered measurement and calibration. problems in the fields of microwave power and attenuation. Additional conferences in this area were held during January and June 1961, and considered the same problems in the fields of pulsed signals, sinewave rf signals, and rf and microwave noise and impedance. These conferences help define what ranges and accuracies are most needed in these fields, why they are needed, and what immediate and long-range action might best be taken by the Bureau, industry, the military, and scientific organizations.

At the suggestion of several defense agencies, the Electronic Calibration Center provided a 5-day workshop covering microwave frequencies-for about 40 technical supervisors from a large number of standards laboratories in the Department of Defense. Basic theory of the precision measurement of power, impedance, frequency, attenuation, and noise was covered in halfday sessions for each quantity. Equal time was spent in the laboratory to provide the opporunity of viewing measurement equipment in actual use. This kind of effort, by improving measurement competence in laboratories of industry and defense, multiplies the effectiveness of NBS.

The Bureau also served as host for a meeting on high precision connectors. It is expected that the ideas expressed at this meeting-by dif ferent manufacturers and NBS-will lead to much better measurement equipment in this field and to a standardization of the instruments and techniques involved.

Theoretical Physics. Studies in theoretical physics and applied mathematics (including numerical analysis) contribute to the more basic work of the entire radio standards program. These studies are an end product in themselves and, on occasion, provide key theoretical developments upon which further work in other projects may be based.

Perturbation formulas, based on a "compensation theorem" stated for waveguide junctions, were used to obtain approximate results in a variety of waveguide problems. One such problem, for example, was that of reflection at the junction of a perfectly rectangular waveguide with a filleted rectangular waveguide of the same main dimensions. The theoretical results of this problem are being compared with NBS experimental results. The perturbation formulas were also applied to theoretical results which are already available and it was found that the common expedient of using an unperturbed field, as an approximation to an unknown field, does not always lead to a correct lowest-order approximation.

Radio Plasmas. Radio waves and plasmas are intimately connected in that each can be used as a tool to study th other. For example, radio waves are used to create plasmas and to determine their properties; plasmas are used in such microwave devices as TR switches, noise sources, modulators, couplers, and harmonic generators.

A current objective in this area is to understand all modes for the propagation of energy through plasmas, including both electromagnetic and plasma waves. Theoretical and experimental progress was made in understanding the many possible electromagnetic modes in a cylindrical plasma, and especially the effect of placing bounding surfaces on a medium supporting the "whistler" mode.

A Fabry-Perot Interferometer, designed to operate at millimeter wavelengths, is being used as the cavity resonator for a hydrogen cyanide gaseous maser. The device shows promise for frequency standard applications, and in microwave spectroscopy. Bi-conical, spherical cavity (inset) illustrates the design flexibility of the perforated cavities (page 44).