the mechanical force between current-carrying conductors) was found to be ANBS/A=1.000010±0.000007. The stated uncertainty represents the estimated probable error (50% confidence interval) resulting from all known sources, the estimate being based on measurement repeatability. This result, together with other recent absolute determinations of the electrical units will be considered by an International Committee later this year in reassigning the units of emf which are maintained by the various national laboratories.

International Comparison of Capacitance Standards.-Three stable 10 pF fused-silica capacitors constructed in 1964 continue to be circulated to various national laboratories for comparisons of the unit of capacitance. The most recently completed circuit was NBS, NSL (Australia), ETL (Japan), and NBS. Values in the NBS laboratory at the beginning and end of this circuit agreed within a part in 107, indicating that the construction is adequate for international comparisons. The group of standards of this type held at NBS have now been under observation for more than three years and appear to be stable within this same limit.

Interlaboratory Agreement of Electrical Measurements.-Statistical experiment designs and methods for data analysis were provided as part of the Bureau's collaboration with the Air Force on precision electrical measurements. The designs are used in carrying on an interlaboratory comparison of calibrations and for methods of transporting voltage standards.

Interlaboratory Transport of the Unit of Voltage. The National Bureau of Standards and the U.S. Air Force undertook a joint study of the interlaboratory transfer of the unit of voltage. The two major objectives of the program, both of which were achieved, were to reduce the uncertainty of the assignment of the local unit and to reduce the time required to make the assignment. Using the present method, six to eight weeks are generally required to assign the unit with a stated uncertainty of 1 ppm. The techniques developed in this program reduce the time required to about two to four weeks with an overall uncertainty of approximately 0.5 ppm.

Fundamental Physical Constants

The Faraday.-An analysis was made of differing values of the Faraday-most of which have been determined at NBS. Values obtained by silver deposition, iodide oxidation, oxalate oxidation, the omegatron, and silver dissolution were reviewed. All values were converted to the unified 12C international scale of atomic weights using the international atomic weights of 1967. The value determined

by dissolution of silver in aqueous solutions was shown to be least subject to uncertainties. A value of 96,487.0±1.6 coulombs per gramequivalent was recommended.

Laser Velocity of Light Measurement.-Important progress has been made in the refinement of laser stabilization and control as applied to the measurement of the velocity of light. The method involves measuring the difference in wavelength and frequency of two laser lines very accurately and using these measurements to calculate the velocity of light. Three important steps have been taken: (1) the 51 GHz photobeat between a pair of laser lines at 1.15 um has been measured, (2) the stability of the 30 m interferometer has been studied using a laser which is locked to a stable 30 cm reference cavity, and (3) apparatus has been developed which allows locking an auxiliary laser at a known variable frequency offset from a reference laser.

Mechanical Quantities

Adiabatic Saturation Psychrometer.-NBS has developed an adiabatic saturation psychrometer for measuring the humidity of gases. This instrument can also be used to determine the vapor content of a vapor-gas mixture. The psychrometer behaves in accordance with predictions deduced from thermodynamic considerations, and has considerable potential as a standard for meteorological use. Since it yields a direct measurement of "thermodynamic wet-bulb temperature," it should have application to measurement problems in engineering fields, particularly in air conditioning. In the chemical engineering field, especially in several of the process industries, the instrument can be used to measure the vapor content of various vapor-gas mixtures.

Wide Range Vibration Generator.-Piezoelectric vibration generators (shakers) have been developed which produce motion suitable for the calibration of vibration pickups over a frequency range of 1 to 60 kHz. These shakers incorporate highly elastic, highly damped coupled resonators with overlapping resonant frequencies, and they produce relatively good motion over a wide frequency range. Indications are that these techniques can be extended to frequencies of at least 100 kHz. The measurement of small amplitude, high frequency mechanical vibrations is especially important in the scale model testing of large complex structures such as space vehicles.

Hydrostatic High-Pressure System.-A cylindrical high-pressure liquid container which uses a polyethylene liner to seal the highpressure liquid has been developed. The container can be pressurized in almost any conventional piston and die press and used to perform PVT and ultrasonic measurements on liquids and solids immersed in

liquids at pressures as high as 45 kbar. Electrical leads have been brought into the high-pressure container in order to determine electrical properties of materials as a function of pressure. Ultrasonic measurements with water pressurized in the liquid container were used to compute the density of water at 22 °C to 12.6 kbar. The estimated uncertainty of these measurements is 0.004 kbar.

Microphone Test Calibrations.-A new method for performing test calibrations on standard condenser microphones has been developed. The method makes use of the reciprocal properties of NBS reference standard microphones. Instead of making a direct comparison of the test microphone against the reference standard using a third microphone as a sound source, the standard is used as a source and the test microphone as a receiver. Only one reading is necessary to measure the response of the test microphone at each frequency rather than two as previously required. Test calibration time is reduced by up to one-half.

Fatigue Crack Detector.-Fatigue tests of high strength aeronautical fasteners loaded in double shear require detection of the fatigue crack in the fastener before catastrophic failure of the fastener occurs. A detector with associated circuits was developed which senses the change in stiffness of the fastener while it is subjected to cyclic loads. The heart of the device is a linear-variable-differential transformer whose output is recorded and which automatically causes termination of the test a few hundred cycles after the crack initiates. In most cases, the resulting crack is too small to be visually detected but its presence is verified using a dye penetrant.

Volume Changes Accompanying the Extension of Rubber.-As part of the effort to determine how to include the effect of compressibility in a general, nonlinear, constitutive equation, the volume changes accompanying extension of peroxide vulcanizates of natural gum rubber have been measured with a dilatometer. These volume changes, of the order of 10-4 cm3/g for 100 percent extension, were measured with a precision of 2 percent. Measurements of forceextension behavior and compressibilities were made on the same samples. Though a constant compressibility described the results obtained using isotropic pressure, the volume changes accompanying extension were not proportional to the isotropic part of the stress. Thus, the strain energy in extension cannot be separated into a sum of two parts, one due to the shear and one to the dilatation.

Determination of Structural Damping.-Structures such as turbine blades, air frames, and rocket housings require not only optimum strength but also proper damping characteristics. A method to deter

mine the damping coefficient of structural metals has been developed. Vibration tests of mass-loaded, cantilever-beam specimens were made in air and in vacuum. The method utilized a knowledge of the effects of mass-loading to normalize the experimental data to fit a nondimensional curve of transmissability-the ratio of free-end to fixed-end beam displacements.

Cryogenic Flow Research.-A program is being developed for an analytical and experimental verification of the performance of existing flowmeters under well defined reproducible conditions. Work is also underway to develop recommended measurement practices for use of these meters in cryogenic systems, and to investigate new commercial metering devices. Implementation of these objectives will provide Government and industry with a centralized research facility for the study of cryogenic flowmetering problems. The program was established under sponsorship of NBS and the Compressed Gas Association. The joint sponsorship by CGA provides international support for the program as this trade organization represents six of the largest cryogenic producers in the United States as well as cryogenic producers in England, France, and Germany.

Swirling Flow Study. The presence of swirling flow can at times have significant influence upon the accuracy of flowmeters, including turbine and differential-pressure types. However, the behavior of swirling flow fields is not well known. Cooperative studies with the University of Maryland Department of Mechanical Engineering on turbulent, incompressible swirling fields in long, cylindrical, unobstructed pipes show that the angular momentum flux decays exponentially along the pipe at a rate of about 3 percent per pipe diameter. Also, the rate of decay increases slightly as the Reynolds number decreases in the range from 200,000 to 12,500. Information from these studies will aid in an improved description of swirling fields in which the performance of flowmeters can be evaluated.

Electrical Quantities-DC and Low Frequency Improved 10,000- Resistance Calibrations.-For half a century standard resistors of the "NBS Type" have been widely used in electrical standardizing laboratories in a range of nominal values extending from 10 to 1 Mo. Demands for better standards at kilohm resistance levels having stability comparable to the 1-2 level are being met with improved 10,000- standards recently developed by several manufacturers. NBS has developed methods and constructed apparatus for calibrating these improved standards with a total measurement uncertainty of 1 ppm. This uncertainty applies to the calibrated value (given to the nearest 0.001 ) in terms of the legal unit of resistance maintained with 1-0 standards. To obtain this increased accuracy,

better methods for scaling resistance values have been developed utilizing so-called "Hamon Devices" for multiplying resistance values 100-fold from 1 to 100 to 10,000 . In practice, the accuracy of these devices approaches the theoretical limit (ratio uncertainty a few parts in 10o).

Cryogenic Parametric Amplifier-Detector.-A null detector for audio-frequency capacitance bridges has been developed. The device, which utilizes solid-state varactor diodes, permits an increase in measurement resolution up to a factor of five over conventional units. Operated at room temperature using a signal frequency of 101 rad/s, a minimum noise figure of 0.01 dB was obtained; at liquid nitrogen temperatures, noise figures below 0.001 dB (referred to a room temperature source) were measured.

Standard Cells.-Seventeen Vosburgh-type, saturated Weston standard cells were constructed and their emf's measured from 5 to 40 °C. This type of cell (Bi added to Cd-Hg anode and double salt CdSO.. Na2SO, 2H2O added at both electrodes) has an emf (1.018690 V) at 25 °C which is almost the same as the conventional, saturated Weston cell (1.018393 V), but it has a lower emf-temperature coefficient: +13.5 μV/°C compared with -49 μV/°C for the conventional type. This cell is expected to have a long-range emf stability.

Voltage-Ratio Measurements.-The "boot strapping" technique for evaluation of inductive voltage dividers has been further improved in accuracy and expanded in frequency coverage. It is now estimated that an accuracy of 2 parts in 10' is obtainable for 5 and 10 kHz calibrations, and a few parts in 108 for 400 and 1000 Hz calibrations. Since it is a self-calibration method, the technique does not rely on any national standards, so it may be used by any laboratory with equal validity.

High-Voltage Pulse Measurements.—A Kerr electro-optical system for measurement of high-voltage pulses has been developed and calibrated. This system permits time-resolved measurements of pulses with peak amplitudes as high as 100kV. Simultaneous pulse divider and Kerr system measurements of pulses as high as 100 kV have indicated that the Kerr system is accurate at least to the same degree as the pulse divider (to within 1 percent). With further refinements, it is anticipated that the system will be useful as a standard for calibration of pulse dividers and that it will permit measurment of transient voltages considerably in excess of 100 kV.

Stable 10 kV Power Supply.—A stabilized adjustable high-voltage supply with a direct current capacity of 10 mA has been built for use