Determination of the time-dependence of ΩNBS usingthe quantized Hall resistance

The quantum Hall effect is being used to monitor the US legal representation of the ohm, or as-maintained ohm, Ω_NBS. Measurements have been made on a regular basis since August 1983. Individual transfers between the quantized Hall resistance R _H and the five 1-Ω resistors which comprise Ω_NBS can be made with a total of one standard deviation (1σ) uncertainty of ±0.014 p.p.m. This uncertainty is the root-sum-square of 32 individual components. The time-dependent expression for R_H in terms of Ω_NBS is: R_H=25812.8[1+(1.842±0.012)×10^-6 =(0.0529±0.0040)(t-0.7785)×10^-6 /year] Ω_NBS, where t is measured in years from January 1, 1987. The value of Ω_NBS is, therefore, decreasing at the rate of (0.0529±0.0040) p.p.m./year     

An improved straddling method with triaxial guards for thecalibration of inductive voltage dividers at 1592 Hz

Calibrated inductive voltage dividers (IVDs) with the smallest possible uncertainties are required for the determination, maintenance, and dissemination of the ohm and the farad. An improved straddling method is described which uses triaxial guards to decrease systematic errors due to screen currents. The relative uncertainty with which IVD voltage ratios can be determined is calculated to be 1×10^-9 (1 σ) at a frequency of 1592 Hz     

A measurement of the NBS electrical watt in SI units

The National Bureau of Standards (NBS) electric watt in SI units to be: W_NBS/W=K_W =1-(16.69±1.33) p.p.m. The uncertainty of 1.33 p.p.m. has the significance of a standard deviation and includes the best estimate of random and known or suspected systematic uncertainties. The mean time of the measurement is May 15, 1988. Combined with the measurement of the NBS ohm in SI units: Ω_NBS/Ω=K_Ω =1-(1.593±0.022) p.p.m., this leads to a Josephson frequency/voltage quotient of E_J=E₀[1+(7.94±0.67) p.p.m.] where E₀=483, 594 GHz/V     

SI value of quantized Hall resistance based on ETL's calculablecapacitor

The SI value of the quantized Hall resistance based on Electrotechnical Laboratory's (ETL) calculable capacitor is presented. Some improvements for previous measurement systems were made and some of the measurement techniques were changed. Based on measurements of ETL, the value of h/e² is estimated to be 25,812.8064 Ω_SI with a systematic uncertainty of 0.24-p.p.m. root-sum-square (r.s.s.) and a random error of 0.11-p.p.m. at one standard deviation (1σ)     

Quantized Hall resistance measurements

The quantized Hall resistances, R_H(4), of Si MOSFETs were measured at ≈0.5 K in a magnetic field of 15 T. The value of R_H(4) was determined in terms of the Commonwealth Scientific and Industrial Research Organization (CSIRO) realization of the SI ohm. A weighted mean of three determinations gave a value for the quantity R_H(4) of (6453.203,36(52)) Ω_SI-NML which can also be expressed as 6453.2(1.000,000,52(8)) Ω_SI-NML. This R_H (4) value gives a value for h/e² which is about 0.3 p.p.m. larger than the value for h/e² derived from the anomalous moment of the electron, using the quantum electrodynamics (QED) theory     

New international representations of the volt and ohm effectiveJanuary 1, 1990

Starting January 1, 1990, new representations of the volt and ohm based on the Josephson and quantum Hall effects, respectively, are to come into effect worldwide. Their implementation in the US will result in increases in the values of the national representation of the volt and ohm maintained at the National Institute of Standards and Technology (NIST) of 9.264 parts per million (p.p.m.) and 1.69 p.p.m., respectively. The resulting increases in the values of the US representation of the ampere and US electrical representation of the watt will be 7.57 p.p.m. and 16.84 p.p.m., respectively. The reporting of calibration results in terms of the new representations is discussed     

A reevaluation of the NML absolute ohm and quantized Hallresistance determinations

Several electrical and geometrical aspects of the National Measurement Laboratory (NML) calculable capacitor are reexamined for effects of wear and other possible causes of error. A new set of quantized Hall resistance measurements is made and related to the NML realization of the SI ohm based on the calculable capacitor. The results of these measurements may be expressed as R_H=25812.8 (1.000,000,363 (66)) Ω     

Impact of new clock technologies on the stability and accuracy ofthe International Atomic Time TAI

The BIPM Time Section is in charge of the generation of the reference time scales TAI (International Atomic Time) and UTC (Coordinated Universal Time). Both time scales are obtained in deferred-time by combining the data front a number of atomic clocks spread worldwide. Since the end of 1992, the quality of the timing data received at the BIPM has evolved rapidly thanks to the extensive replacement of older designs of commercial cesium clocks by the new HP 5071A units and to the use of active auto-tuned hydrogen masers. Consequently, the stability and the predictability of the reference time scales has improved significantly: these are characterized by an Allan standard deviation σ_y(τ) of 2.6×10^-15 for averaging times τ=40 d. The accuracy of TAI is estimated by the departure between the duration of the TAI scale interval and the SI second as produced on the rotating geoid by primary frequency standards. It is now possible to estimate TAI accuracy through the combination of results obtained from six different primary standards: LPTF-FO1, PTB CS1, PTB CS2, PTB CS3, NIST-7, and SU MCsR 102, all corrected for the black-body radiation shift. This led to a mean departure of the TAI scale interval of +2.0×10^-14 s over 1995, known with a relative uncertainty of 0.5×10^-14 (1 σ)     

A wide-band active current transformer and shunt

An active current transformer and shunt with high accuracy and wide frequency range is described. The transformer, which uses two three-stage core transformers with two two-stage feedback amplifiers, can be used in the audio frequency band. It has a wide range of current ratio (0.01-1) and can dissipate 10 W in the external burden. If the external burden of the transformer is an AC standard resistor, it becomes an AC current to voltage transducer that can be used in many electrical measurements. The relative uncertainty (3σ) in the current ratio of the new transformer is between (1+j1) p.p.m. to (8+j8) p.p.m. over the frequency range from 40 Hz to 10 kHz     

Reducing local oscillator phase noise limitations on the frequencystability of passive frequency standards: tests of a new concept

We report on the experimental test of a new concept for reducing the limitation on short-term frequency stability of passive frequency standards due to local oscillator phase noise. This concept is general and can be applied to many passive frequency standards. Systems that use sinewave modulation to interrogate a stable resonance are limited in short-term frequency stability by phase noise at the second harmonic of the modulation, f_m. This effect limits the fractional frequency stability to approximately σ_v(τ7)=0.9(f _m/ν₀) (S_φ(2f_m))^1/2τ^-1/2, where ν₀ is the carrier frequency and S_φ(2f _m) is the phase noise at twice the modulation frequency. (Contributions from higher even harmonics of the modulation generally can be neglected). This new concept uses notch filters at ±2f_m from the carrier to reduce this effect. Tests on a modified passive rubidium standard demonstrate an improvement of approximately 18 in σ_y(τ). The dual notch filters proved to be feasible and were obtained commercially. Measurements suggest that ultimate performances of less than 2×10^-14τ^-1/2 are possible if the atomic resonance has sufficient quality     

Accuracy of measurements of the quantized Hall resistivity by adirect current comparator type potentiometer: calibration using aJosephson potentiometer

The accuracy of measurements of the quantized Hall resistivity (QHR) by a direct-current-comparator (DCC)-type potentiometer is estimated based on a linearity calibration of the potentiometer using a Josephson potentiometer. The power coefficient contribution to the nonlinearity of the DCC potentiometer was found to be 0.15±0.02 p.p.m./(100 mV)² at 18.5±0.5°C in 1985 and 0.21±0.03 p.p.m./(100 mV)² at 20.5±0.5°C in 1988. The possibility of accurate measurements of the ratio QHR/R _STD with uncertainties less than 0.05 p.p.m. by the DCC potentiometer is discussed     

High spectral purity CO2 laser stabilized using amolecular frequency reference

A CO₂ laser has been frequency stabilized to a molecular absorption line with a bandwidth of 10 kHz, yielding a laser noise spectral density reduction below 1 Hz/√ Hz and a relative accuracy of 10^-10. In the time domain, this corresponds to an Allan standard deviation of σ(2, τ)=3·10^-14 τ^-1/2, for τ>100 μs. This source allows experiments of very high resolution spectroscopy as well as the realization of accurate frequency standards in the 10-μm region     

Hexagonal ferrite particles for perpendicular recording prepared byion exchange

Particles of nonstoichiometric M-type barium hexaferrite with the chemical formula BaFe_10.5M_0.25O_17.05 (M=Mg, Cd, Co, Ni, Nd Zn) have been prepared by ion exchange in Ba containing molten salts from precursor ferrites with the β- or β"-alumina structure. The particles possess a plateletlike shape with diameters as low as 0.2 μm and as diameter-to-thickness ratio between 5 and 10. The Curie temperature is close to 470°C. Saturation magnetization values σ_s up to 64 emu g and coercive fields jH_c ranging from 0.5 to 1.5 kOe were found     

CO2/OsO4 lasers as frequency standards in the29 THz range

The CO₂ lasers, frequency stabilized on OsO₄ transitions around 29 THz, are successfully used as secondary references for visible frequency synthesis and also for high-accuracy spectroscopy in the infrared domain. The BNM-LPTF CO₂/OsO₄ frequency standards exhibit high metrological performances: a long term reproducibility of ±10 Hz (1σ=2×10^-13) and a stability σ_y=6.6×10^-14/√τ with a Flicker plateau of 4×10^-15 (Δν/ν=0.1) for 300 s to 10³ s averaging times. These results and a preliminary study of the main effects which can induce frequency shifts are reported     

Magnetic particles derived from iron nitride

Due to its high saturation magnetization (σ_s(0K)≈208 emu/g) and to the ease of obtaining it as a particulate material, Fe₄N could be a suitable magnetic material for replacing highly reactive metallic iron pigments (σ _s(0K)≈218 emu/g) in high-density recording. Throughout a nitriding process of properly selected precursors Fe₄N can be obtained in either of two morphologies: acicular or isotropic. Further protection of the magnetic particles can be ensured through coating of the precursor particles prior to thermal decomposition in NH₃/H₂ atmosphere or through substitutions with selected elements. The obtained passivated nitrides are characterized by high magnetic properties (H_c≈600 Oe; σ_s≈150 emu/g) and chemical stability well adapted for use as magnetic pigments for high-density recording     

Recent determinations of R/sub H/ in terms of Omega /sub 69-BI/

A system based on a cryogenic current comparator is used to measure the quantized Hall resistance R/sub H/ in terms of Omega /sub 69-BI/, the Bureau International des Poids et Mesures (BIPM) representation of the ohm, with an uncertainty of 1.5*10/sup -8/. This measurement system and the former potentiometric one are in good agreement within their combined uncertainties (5.1*10/sup -8/).<>     

Effect of stress on the law of approach to saturation magnetizationin carbon steels

In high fields where the magnetization is approaching saturation, the resolved magnetic polarization I can be expressed by a power series of the inverse of external field H as: I=I_s-b/H ²-c/H³ . . ., or alternatively in terms of the reversible permeability μ_rev=1+(2b/H³+3c/H⁴+ . . .)/μ₀ where I_s is the spontaneous magnetic polarization and b, c, are constants. These equations express the law of approach to saturation magnetization. The coefficient b for a cubic crystal has been deduced as b=0.0762/I_s[K+1.5(λ₁₀₀-λ ₁₁₁)σ]², where σ is the applied stress and others are magnetic constants. The values of reversible permeability μ_rev under biasing field H were measured for carbon steels with applied stresses. The results showed that the square root of 2b changed linearly with the applied stress, The values of magnetic constant K andλ₁₀₀ -λ₁₁₁ were calculated, They agreed with the standard values, but were affected by chemical composition and heat treatment of materials, especially in λ₁₀₀-λ₁₁₁