A waveguide cavity, containing a crystal specimen, is placed between the poles of a magnet in studies of magnetic resonance. The objective is the establishment of standards and measurement techniques, based on a better understanding of the interaction of electromagnetic waves with matter (page 48).

In the measurement of magnetization of materials a significant contribution was the development of an absolute technique for calibrating vibrating sample magnetometers. The new technique greatly improves the accuracy of determining spontaneous magnetization, a significant parameter in fundamental magnetic investigations, as well as a figure of merit in many microwave material engineering problems.

An improved Maxwell bridge was completed to measure resistance as low as 10 ohms, and inductance as low as 10-11 henrys, at frequencies from 1 to 100 kc/s. It is unique in that the sample of material can be inserted without opening the unknown arm of the bridge. Contact resistance is thus eliminated and low permeability measurements can be made with greater accuracy than previously possible.

A study of the dynamic magnetoelastic properties of several ferrites resulted in a new technique for analyzing the mechanisms responsible for magnetization in a material. This method is based on the fact that domain-rotation and domain-wall phenomena are apparently separated when magnetostrictive

measurements are made on selected ferrites subjected to mechanical shock. The dependence of Young's modulus on a static magnetic field was reported for the first time.

The accuracy of the rf permeameter was increased through the development of exact working equations. This has improved measurements on extremely low loss materials by at least an order of magnitude.

Magnetic resonance studies were initiated to determine the magnetic energy levels, relaxation times, and transition probabilities of paramagnetic and antiferromagnetic crystals. This will provide information on internal crystalline fields and exchange interaction. An investigation on the effect of impurities on the spontaneous magnetization of nickel was also instituted. Electronic Calibration Center. The Electronic Calibration Center provides an extensive calibration service for various agencies in the Department of Defense as well as for scores of industrial laboratories. There is a continued effort to improve the instrumentation and thus increase the efficiency, accuracy, and scope of the Center's calibration services.

The method developed for the accurate calibration of inductive voltage dividers using a transformer capacitance bridge has surpassed all expectations of accuracy. By a conservative estimate, this method of measurement is accurate to within 0.2 parts per million of input. Calibration services for inductive voltage dividers were established for several values of ratio at an input voltage of 100 volts and a frequency of 100 c/s.

This year the Center had the opportunity to observe a group of saturated standard cells soon after the group had been measured by the NBS laboratories in Washington. Results indicate that agreement between the two laboratories is within 0.6 millionth of a volt, or well within the estimated limits of accuracy.

A modification of the rf voltmeter calibration consoles, which will improve their accuracy by a factor of 10 for frequencies up to 100 Mc/s, was nearly completed. These consoles cover the range of 30 kc/s to 100 Mc/s from 0.2 to 500 v, and the frequencies of 300 and 400 Mc/s from 0.2 to 100 v.

An attenuation calibration console which will permit very accurate measurement at 1, 10, 30, 60, 100, and 300 Mc/s was essentially completed. Its total dynamic range is 140 db, and it is estimated to be accurate within. 0.07 db at the upper limit. Some of the more precise laboratory standards may be calibrated over a range of 0 to 60 db within an accuracy of + (0.002 db +0.01 percent of the total attenuation in db).

A technique, accurate to within one percent, was developed and services provided for the calibration of dry calorimeters for measurement of microwave power over a range of 10 to 100 milliwatts and a frequency range of 8.2 to 12.4 kilomegacycles.

A new high-temperature oven was designed and constructed, for the microwave noise measurement system, with a control circuit that maintains the temperature of the oven at a given point to 1 degree at approximately 1,000 °C. The hot-body noise standard was redesignated so that its operation is more reliable and its structure is easier to analyze.

2.1.5. HEAT

Heat measurements, standards, and related research play a most important role in modern science and technology. The Bureau discharges important responsibilities in these areas through the maintenance of the National standards for heat measurements. Internationally agreed upon temperature standards are maintained to assure a common scale upon which all temperature measurements are based. A strong research program aims to keep these standards adequate for the expanding National needs. In addition, supporting research on the physical properties of solids and gases at both low and high temperatures includes studies in low temperature physics, in statistical thermodynamics, in high-temperature processes, in high-pressure thermodynamics, and in various aspects of plasma physics.

During the year significant progress was made in the generation and accurate measurement of high temperatures and pressures. An acoustical interferometric method was used successfully to measure very low absolute temperatures in the liquid helium range. The investigation of the thermodynamic properties of light-element substances important in rocket propulsion was continued. In addition, advances were made in long-range experimental and theoretical programs devoted to characterizing and predicting the properties of hot gases and highly ionized gases (plasmas).

High-Temperature Thermocouple Furnace. A tantalum-tube furnace has been designed and constructed to study the high-temperature properties of refractory metal and rare metal thermocouple materials. The furnace has been operated at temperatures up to 2,000 °C and, with minor modifications, temperatures up to 2,200 °C are anticipated. The heating element in the furnace is a tantalum tube heated through its own resistance. Thermocouples to be calibrated in the furnace are placed inside of the tantalum tube and are free from insulating and protection tubes. Blackbody temperatures at the measuring junctions of the thermocouples are determined by a calibrated commercial optical pyrometer with a modified optical system. Thermocouples can be calibrated in a high-purity helium atmosphere or a moderately high vacuum.

Electrical power to the furnace is regulated by a saturable core reactor. Stable furnace temperatures are maintained through the use of an automatic controlling unit which receives a feed-back voltage from the furnace transformer winding. At 1,500 °C the maximum temperature fluctuations indicated by a thermocouple over a 10-min period were less than 1 deg C. A limited amount of data have been obtained on tungsten-rhenium and iridium-iridium-rhodium type thermocouples. Other thermocouple combinations to be investigated include tungsten-tungsten 26 percent rhenium, tungsten-iridium, and tantalum-tungsten 26 percent rhenium (see p. 33).

Photoelectric Pyrometer. Above the freezing point of gold, 1,063 °C, the disappearing filament optical pyrometer is used for the realization of the International Practical Temperature Scale (IPTS). The precision of brightness temperature determinations with this instrument is limited by the

contrast sensitivity of the human eye. This limitation, however, can be reduced significantly by using a physical detector rather than the eye to make brightness matches.

During the past few years NBS has been developing a photoelectric optical pyrometer which uses a photomultiplier tube rather than the eye as a detector. This instrument, now completed, has a precision at 1,063 °C of 0.02 deg C when a time constant of 1 2/3 seconds and a target size of 0.2 mm by 0.6 mm are used. In comparison, the precision of the NBS visual optical pyrometer at 1,063 °C is about 0.3 deg C. Moreover, the higher precision of the photoelectric pyrometer has been achieved with a spectral passband of only 100 A, or about 1/4 that of the visual pyrometer. This is important because the mean effective wavelengths of the pyrometer can

As part of an effort to extend the range and accuracy of temperature measurements, the thermoelectric properties of high-temperature thermocouples such as tungsten-rhenium and tungsten-iridium are studied in this experimental tantalum tube furnace. Operating temperatures of 2,000 °C and higher are obtained (page 51).

be determined more accurately. The increased precision and the more accurate mean effective wavelengths are expected to improve the accuracy with which the IPTS can be realized.

The long-term stability of the photoelectric pyrometer is now being investigated in order to determine how often the instrument will have to be calibrated. The heart of an optical pyrometer is the pyrometer lamp. This lamp serves as a reference standard for the pyrometer much as an electrical standard cell does for a potentiometer. Therefore, the stabilities of various types of pyrometer lamps are being determined. Preliminary results show that some lamps, previously considered excellent, change by an amount equivalent to 0.5 deg C in 150 hours of use at the gold point. These investigations are expected to result in recommended procedures for the optimum design, aging and use of pyrometer lamps.

Specific Heat of Diamond at High Temperatures. Accurate measurements of the specific heat of gem diamonds between 273 and 1,100 °K have recently been completed. These measurements will be used for comparison with values calculated theoretically from lattice dynamics over a wide temperature range. Such investigation should lead to a better understanding of the covalent bonds important to chemistry. It will also be possible to evaluate the energy contribution from nonharmonic vibrations in the diamond crystal. These assume greater importance with increasing temperature.

The high accuracy of this research will permit extrapolation of the measured specific-heat values to higher temperatures with less uncertainty than has been possible in the past. Other thermodynamic properties of diamond derived from this work permit examination of the temperature and pressure relationships which exist when diamond is formed from graphite.

Thermodynamic Properties of Light-Element Compounds. Under the sponsorship of the Department of Defense, the Bureau is continuing its comprehensive interdisciplinary program of thermodynamic research on simple light-element substances which are important in rocket propulsion. The compounds being specially investigated are those of lithium, beryllium, aluminum, and zirconium with hydrogen, oxygen, fluorine, and chlorine, as these compounds are potential fuel components, fuel oxidizers, and combustion products. During the past year the program extended its emphasis to include compounds of "mixed" type (such as intermetallic compounds, double fluorides of two metals, and oxyfluorides) whose use may lead to substantial gains in propulsion efficiency.

Though propulsion efficiency depends on the simultaneous operation of all the thermodynamic properties being separately investigated in the program, the most critical property is the heat of formation. The Bureau has recently contributed reliable values for this property for several important substances. A series of measurements established accurately the heats of formation of three alkali-metal perchlorates and ammonium perchlorate, the last substance in particular being a widely used fuel oxidizer. Nitronium perchlorate may have similar application, and measurements on it are under