brations of elastic force measuring devices, closing the ends of pressure vessels and supporting optical flats during use or measurement.

Nonlinear Constitutive Equations.-New progress has been made in the development of a rheological constitutive equation to describe the mechanical properties of materials which can be classed as elastic fluids (rubber, polymer solutions, etc.) when subjected to large deformations and/or flow velocities. It has proved possible to relate viscosity and normal stress as a function of rate of shear to dynamic viscosity, measured using small sinusoidal deformations as a function of frequency. This formulation predicts that the dynamic viscosity as a function of frequency and the apparent viscosity as a function of rate of shear should coincide over only a limited range, and gives a more complicated relationship between these two functions which holds over a much wider range. It is now clear why previous, empirical attempts to relate these two functions have been successful over only a limited range.

ELECTRICAL QUANTITIES-DC AND LOW FREQUENCY

Thermal Voltage Converters.-A new NBS set of thermal voltage converters, and an emf comparator for evaluating and using them, has greatly simplified high-accuracy a-c voltage measurements and ac-dc difference tests at audio frequencies. The converters consist of two thermoelements (2.5 and 5 mA) and five plug-in resistors, to provide voltage ranges from 1 to 600 V with ac-dc differences less than 10 ppm. With the emf comparator, which has a readout directly in ppm, the differences between adjacent voltage ranges may be intercompared to a few ppm. Six such intercomparisons (less than half those required with the earlier NBS standards) verify the continued stability of the entire set of voltage converters. The set may then be used with the comparator to make ac-dc tests of other transfer standards, and to make voltage measurements by reference to suitable d-c standards.

Capacitance Standards.-The bank of 10-pF fused silica capacitors, constructed in 1964, has now been under observation for more than two years. Most of them appear to be stable to better than a part in 107 over this time interval. Units from the group are being circulated to other national laboratories for international comparisons, and a special bridge was constructed for the International Bureau of Weights and Measures (BIPM) so that they might also compare low-value standards. This bridge has been checked out and installed at BIPM by a member of the NBS staff.

Voltage-Ratio Measurements.-A "boot-strapping" technique was developed to evaluate inductive voltage dividers at 10 kHz with an

accuracy estimated to be 1 part in 106. This represents an improvement in accuracy at this frequency by a factor of about 3 and has the additional advantage of being an "absolute" method in that it does not rely on the calibration of any other ratio devices. It is expected that further refinements will soon yield an accuracy approaching 1 in 107.

ELECTRICAL QUANTITIES-RADIO

Radio standards-from 30 kHz up to and including microwave, millimeter wave, and laser frequencies are provided by the NBS Radio Standards Laboratory in Boulder, Colo. The laboratory conducts basic and applied research leading to standards for all important radio quantities (such as power, pulse quantities, attenuation, and noise), standards of frequency and time, and calibration and broadcast services which transmit these standards to industrial and Government laboratories. The Laboratory also uses its unique competence to measure selected atomic and bulk properties of matter and to improve the determination of fundamental physical constants.

High-Frequency Electrical Standards

International Comparison of Radio-Frequency Standards.-NBS is the supervisory laboratory for an international intercomparison of power standards at 3 GHz. Transfer standards have been furnished by Canada, USSR, Japan, and the United States. The transfer standards will be compared with the primary standards in each participating country, in turn. Measurements on the standards were completed at NBS and shipped to the next country, Canada. Results obtained in each laboratory will be reported directly to the International Bureau of Weights and Measures (BIPM) in Sèvres, France. The BIPM will issue a final report when all results have been received.

Radio-Frequency Voltmeter/Comparator.-A new, wide-band rf voltmeter/comparator has been developed for the frequency range d-c to 1 GHz. The device employs matched hot-carrier diodes in a dualchannel 50-ohm coaxial configuration. Power extraction from the signals being measured is negligible, and the coaxial line sections are impedance compensated to maintain a voltage standing wave ratio of 1.02 or less. The uncertainty in the voltage measurement is 3 percent or less from 0.5 to 15 V peak. Other applications include (a) measurement of differential rf voltage, (b) frequency-independent detector to 1 GHz, and (c) rf voltage comparator monitor of very high resolution.

Improvement in Coaxial Attentuation Measurement.-An automatic hydraulic servo system has been developed to control the displacement of the piston in a waveguide-below-cutoff attenuator. The

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system enables the piston to be positioned with an uncertainty no greater than 10-6 mm. This precision is almost two orders of magnitude better than previous methods and results in a similar improvement in the measurement of attenuation.

High-Frequency Impedance Standards

RF Admittance Measurements.-An informal comparison of admittance standards has been completed by the Radio Standards Laboratory and the Ministery of Aviation in Great Britain. Three capacitors and four conductances were measured at a frequency of 5 MHz. The conductance values obtained agreed to within 0.25 percent, and the capacitance values to within 0.03 percent.

Radio Standards Comparators.-Under contract with the U.S. Air Force, the Radio Standards Laboratory is constructing a comparator for monitoring the calibration of standards of radio-frequency current, voltage, power, and impedance. Intended for use at the Base Laboratory level, the comparator will enable the user to detect, with a high degree of certainty, a standard which has drifted beyond acceptable tolerance limits. The purpose of the comparator is to increase both the accuracy of calibration at the base level and the efficiency of the overall calibration program.

RF-DC Transfer Standards.—A first model of a newly designed rfde transfer standard for rf current measurements has been constructed and is being evaluated. It employs an ellipsoidal reflecting surface to focus infrared energy from a resistance heater to a receptor. The heater may be energized by rf or direct current. The receptor is a thermopile with a direct current output.

Inductive Voltage Dividers.-An experimental binary type rf voltage divider attenuator having 42 dB in seven 6-dB steps was compared with the NBS precision piston attenuator at 1 MHz. The divider and attenuator agreed within 0.003 dB per 6-dB step, which is less than the estimated uncertainty in the piston attenuator at 1 MHz.

High-Frequency Calibration Services

Special Measurement Services.-There has been an increased demand for special measurement services as contrasted to calibrations of interlaboratory standards for basic quantities. In one major effort the U.S. Air Force was assisted in measurements for a large procurement of bolometer-coupler units. Also, a number of nonstandard calibrations, such as time delay calibrations for airline altimeters, were made.

UNIVERSITY

MICHIGAN LIBRARIES

Pulsed Quantities.-Additional pulsed power services were offered from 300 to 500 MHz. Pulse voltage, or base-band pulses, are now calibrated for nanosecond rise times, from 5 to 1000 volts.

Increased Immittance Services.-The large increases in accuracy announced last year have been extended to new frequencies, and to new values of capacitance, inductance, and resistance. NBS capabilities are now being better used in that new standards, which use the new precision connectors, are now being received for calibration. Uncertainties can be reduced up to 100 times on such standards.

Heat Flow.-A theoretical study was made of the dc-rf substitution error of a Wollaston wire bolometer power meter. The study resulted in the solution of the nonlinear differential equation which describes the heat flow by all significant transport mechanisms. Application of this theoretical study to millimeter wave equipment is expected to lead to the use of millimeter wave barretters as power standards.

To improve measurements of power at millimeter wave frequencies, this largescale model of a Wallaston wire bolometer and related components was made approximately 100 times normal size. (In cross section the model is 16" by 8".)

Low Attenuation Measurements.-In the process of selecting lengths of waveguide of uniform specific resistance, microwave low-attenuation measurement capability has been extended to microbel precision. The stability necessary for such measurements was achieved through refinement of existing technology and inclusion of some innovations such as regulation of power output through servo-control of the klystron beam voltage. For eventual use in the system, and other systems, a rotary vane attenuator with microbel sensitivity and repeatability has been developed. An optical readout and improved rf shielding are features of the attenuator.

Reflection Coefficient Studies.-A computer program has been written to enable preparation of tables and graphs to facilitate calculation of reflection coefficients of quarter wavelength, short-circuited waveguide impedence standards. The tables will cover 38 standard sizes of waveguide and three sizes of 50-ohm coaxial line. This will enable anyone to construct his own standards of reflection coefficient for frequencies up through 325 GHz.

Small Attenuations of Waveguide Joints.-Data have been obtained on the small attenuations of waveguide joints versus frequency. Little or no data have previously been available on the frequency dependence of these losses, and these data should reveal more about their nature. The series and shunt losses of waveguide windows have been measured over the frequency range of the waveguide. The method which has been developed is especially convenient and accurate, but not quite as sensitive as cavity reasonance techniques.

Microwave Calibrations

Power Calibration Systems.-A new calibration service was established to fill a strongly needed measurement gap in the area of microwave power measurements for coaxial devices. The service provides continuous frequency coverage from 4 to 10 GHz for low-value bolometric power measurements on interlaboratory standards terminated with Type N coaxial connectors and from 4 to 8.5 GHz for the newer 14-mm precision coaxial connectors. The service is based on existing, recently developed microwave power calibration systems in rectangular waveguide and upon a waveguide-to-coax "adaptor" technique. Microwave power calibration systems were also developed for the measurement of low-value CW power in WR284 waveguide (2.63.95 GHz) utilizing the impedance variation method of power measurement and in WR42 waveguide (18.0-26.5 GHz) using the reflectometer method.

Calibration Accuracy of Bolometer Units.-A significant improvement in the calibration accuracy applied to bolometer units was

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