Magneto-inductive element

A novel sensitive magnetic head using a magnetoinductive (MI) effect in zero- or negative-magnetostrictive amorphous wires is described. The MI effect refers to the change of an inductance L for an external magnetic field in a ferromagnetic wire element magnetized with a wire AC current I_AC. A 5-mm-length MI element using a folded almost zero-magnetostrictive amorphous wire having a 120-μm diameter showed a sensitive rate of change of its inductance of more than 50% for an external low frequency field of about 10 Oe (800 A/m) applied in parallel with the element. The a-wire MI element works for a wire current having frequencies up to 1 MHz     

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     

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     

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     

High precision phase measurement using reduced sine and cosinetables

A high-precision phase measurement technique, which can accurately determine the phase even when the signals are distorted by harmonics, was presented previously by the author (ibid., vol.38, p.954-60, Oct. 1989). He proposes a microprocessor-based hardware implementation of the technique. The hardware implementation requires two reduced tables: a sine table and a cosine table. If each table has i(i+5)/2 entries, then the phase can be accurately determined even if the signals are distorted by any number of harmonics in the range of second through ith harmonics     

High precision phase measurement using adaptive sampling

The conventional phase measurement techniques introduce error in the phase when the input signals are distorted by harmonics. A novel technique, known as adaptive sampling, for high-precision phase measurement is introduced. A digital signal-processing approach is used in this technique. The maximum sampling rate required for this technique is h+2 samples/cycle of the input signals, i.e. (h+2) f sampless, where h, is the highest harmonic present in the signals and f is the fundamental frequency of the signals. This sampling rate is way below the Nyquist sampling rate (more than 2hf samples/s) when h is a large number. In the adaptive sampling technique the sampling rate is started from three samples/cycle and then is gradually increased until the phase is correctly measured. This phase measurement technique has been verified using synthesized signals     

Analysis of the noise sources in an optically pumped cesium beamresonator

The main noise sources that affect the clock signal detection in an optically pumped cesium beam resonator are determined and their influence on the clock S/N ratio is evaluated. The following noise sources are taken into account: atomic shot noise, fluorescence photon noise, laser frequency noise, and clock signal detection noise. A theoretical model that predicts the variations of the S/N ratio as a function of different parameters characterizing the atom-laser interaction is developed. The main conclusion concerns the saturation of S/N value for high atomic flux due to laser frequency fluctuations. All the theoretical predictions were experimentally verified     

Automatic technique for measuring the Polder resonance in ferritespheres

An automatic technique for measuring the parameters of Polder resonance in polycrystalline ferrite spheres is proposed. The diagonal elements of the external susceptibility tensor versus DC magnetic field are calculated from the changes in resonance frequency and unloaded Q factor of a microwave cavity by perturbation theory. From these diagonal elements, all the elements of the intrinsic permeability tensor can be obtained. By fitting the theoretical curves to experimental data, the parameters of Polder resonance, ΔH M_s, and the g factor, are simultaneously calculated. The whole measurement procedure is controlled by a desktop computer. An accuracy of 5% is obtained in ΔH and M _s, and of 0.1% in the g factor     

A low field determination of the proton gyromagnetic ratio in water

The proton gyromagnetic ratio in H₂O is measured by the low-field method. γ'_p(low)=2.67513376×10⁸ s^-1 T^-1 (0.11 p.p.m.), leads to a value of the fine structure constant of α^-1=137.0359840 (0.037 ppm) and a value for the quantized Hall resistance in SI units of R_H=25812.80460 Ω (0.037 p.p.m.). To achieve this result, the dimensions of a 2.1-m solenoid were measured with an accuracy of 0.04 μm, and the NMR (nuclear magnetic resonance) frequency of a water sample was measured in the field of the solenoid     

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     

Hot-pressed Mn-Zn-Ni and Mn-Zn-Co ferrites for magnetic recordingheads

Ni²⁺ and Co²⁺ substituted Mn-Zn ferrites have been developed as high-density magnetic recording materials by hot pressing using optimum sintering parameters. Ferrite series of the composition Mn_0.6Zn_0.4-xNi_xFe₂ O₄ and Mn_0.6Zn_0.4-yCo_yFe₂O₄ were prepared by the hot-pressing technique wherein x and y varied from 0.0 to 0.4 in steps of 0.05. It yielded ferrites with improved magnetic properties having higher hardness (⩾650 Vickers units), low porosity (<0.1%), and small grain size (≈10 μm). The initial permeability increased, whereas the coercive field decreased for an Ni²⁺ concentration x or a Co ²⁺ concentration y equal to 0.05. Maximum values of saturation magnetization equal to 4850 and 5250 G were obtained for x and y equal to 0.225 and 0.275, respectively. The Curie temperature increased appreciably, whereas the DC resistivity decreased for a larger substitution of Ni²⁺ and Co²⁺ ions     

A transformer-ratio bridge for the measurement of small loss angles

A development in the architecture of transformer-ratio bridges for the measurement of the loss angles of low-pass dielectrics is described. This has led to a sensitive and accurate bridge capable of measuring loss angles with a detection limit of 1×10^-8 rad (S /N=1 at a bandwidth of 1 Hz of the detector) and an accuracy of a few percent in the 10^-6 and higher ranges in the frequency range from 200 Hz to 200 kHz. The readout of the bridge is computer controlled and has an automatic capacitive balance. The most important characteristics of the bridge and their influence on the sensitivity and accuracy are discussed. Measurements as a function of frequency are presented as an example. The bridge can be used in numerous applications concerning the characterization of dielectric materials     

High coercivity in melt-spun SmFe10(Ti,M)2 ribbons (M=V/Cr/Mn/Mo)

Magnetic properties of SmFe₁₀(Ti,M)₂ melt-spun ribbons were studied, where M is V, Cr, Mn, and Mo. The ribbons (M=V/Cr/Mo) quenched at 20 m/s exhibit the high coercivities of 4.2-5.5 kOe. Annealing the ribbons quenched at 40 m/s enhances their coercivities in the range of 5.9-10.0 kOe. In particular, SmFe₁₀ (TiV) and SmFe₁₀(TiCr) ribbons yield coercivities of 10.0 kOe and 7.9 kOe, respectively. This is the highest value among the reported melt-spun ThMn₁₂-type structure ribbons. The importance of Sm atmosphere during annealing in minimizing the Sm evaporation from ribbons is also demonstrated     

Towards a frequency measurement of the Rydberg constant using the 2S-8S and 2S-8D transitions ofhydrogen

A very precise measurement of the Rydberg constant is performed using a direct-frequency comparison of the 2S-8S and 2 S-8D two-photon transitions in atomic hydrogen with the difference of two optical standards connected to a frequency chain     

Magnetic properties of GdBa2Cu3O7crystals in high magnetic field

The magnetic properties of single crystals of GdBa₂Cu ₃O₇ were investigated in a magnetic field of up to 20 T applied parallel to the c axis in the 1.7-300 K temperature range. In the superconducting state, the field and temperature dependences of the critical current densities were deduced from the hysteresis of the half-cycle using Bean's critical state model. The Gd³⁺ paramagnetic moment was then studied. Above about 20 K, the M(H) isotherms were found to be given, at different temperatures, by the Brillouin function of the free Gd³⁺ ion. Below 20 K, the average magnetization does not obey the Brillouin law. The normal-state susceptibility was described by the free-ion Curie-Weiss law     

Digital signal processing techniques for high accuracy ultrasonicrange measurements

Several digital signal processing (DSP) methods are analyzed and compared with respect to the expected errors for an ultrasonic range measurement arrangement. These include L1, L2 norms and correlation with different approaches for envelope extraction. The influence of different factors such as signal-to-noise ratio (SNR), sampling frequency, and digitizing resolution on measurement errors is analyzed using a synthetic approach through nearly 40000 simulations. Results show different performance levels involving accuracy, computing time, and cost for the studied methods, although all of them allow reduction of errors by several orders of magnitude     

Effect of the n value and the field inhomogeneity on thequench current of superconducting cables [magnet winding]

A model developed to obtain a relation connecting the critical current and the quench current of superconducting (S/C) cables is described. The model is based on the hypothesis that the heat produced inside the cable is only due to the ohmic dissipation, and it is only removed by thermal exchange with the liquid helium bath. The ohmic dissipation is calculated by supposing that the electrical resistance of the S/C cable at the transition to normal state is: Rαlⁿ where l is the current and n (n value) is an integer number. To calculate the function R(I), the field inhomogeneity at the conductor due to the self-field is taken into account, introducing the effective critical field     

Measurement of frequency response functions in noisy environments

The geometric mean is proposed as an alternative averaging technique for frequency response function (FRF) measurements of a linear system. It is shown that it produces almost unbiased measurements even if the input and output measurements are both disturbed with (normal distributed) noise. The properties of the arithmetic and geometric mean of expression are compared to those of the H₁ and H₂ methods. It is shown that if the S/N ratio on the input and the output measurements is higher than 3 dB, then it is possible to generate measurements without systematic errors     

Extracting μr and εr of solids fromone-port phasor network analyzer measurements

A method for simultaneously and rapidly extracting permittivity, ε, and permeability, μ, of nonmetallic solids over a wide range of frequencies, based on one-port reflection measurements made by an automatic network analyzer (ANA), is described. Reflection measurements are taken of a short, an offset short-circuited air line of length l , and two short-circuited samples of lengths land nl , where n is any integer >1. The information contained in these four measurements is sufficient to extract ε and μ without recourse to any a priori knowledge of the waveguide's cross section, dispersion, or loss. This method is particularly convenient for measuring ε and μ at temperatures significantly different from ambient and in waveguide cross sections whose dispersion and loss are not well known. The presence of higher order modes and of sample inhomogeneities can be detected from an additional measurement of an offset short-circuited air line of length nl. Measurements at Ka-band on polystyrene agree with those of other investigators. A number of practical considerations for successfully implementing the method discussed above, relating to sample length, time-domain gating, mode suppression, and sample fabrication, are discussed