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Cage M.E. Dziuba R.F. Van Degrift C.T. Yu D. 《IEEE transactions on instrumentation and measurement》1989,38(2):263-269
The quantum Hall effect is being used to monitor the US legal representation of the ohm, or as-maintained ohm, ΩNBS. 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 ΩNBS 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 ΩNBS is: R H=25812.8[1+(1.842±0.012)×10-6 =(0.0529±0.0040)(t -0.7785)×10-6 /year] ΩNBS, where t is measured in years from January 1, 1987. The value of ΩNBS is, therefore, decreasing at the rate of (0.0529±0.0040) p.p.m./year 相似文献
3.
Shida K. Wada T. Nishinaka H. Segawa K. Igarashi T. 《IEEE transactions on instrumentation and measurement》1989,38(2):252-255
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 /e 2 is estimated to be 25,812.8064 ΩSI 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) 相似文献
4.
Williams E.R. Jones G.R. Jr. Ye S. Liu R. Sasaki H. Olsen P.T. Phillips W.D. Layer H.P. 《IEEE transactions on instrumentation and measurement》1989,38(2):233-237
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 相似文献
5.
《IEEE transactions on instrumentation and measurement》1989,38(1):64-73
A method is described for the accurate measurement of the equivalent parallel capacitance C x and resistance R x of very high loss materials at high frequencies. The important characteristic of the method is its capability of measuring the value of C x extending from 10-2 to 106 pF and R x from 10 to 100×106 Ω. The authors measure the humidity of tea leaves and rockwool by measuring the capacitance C x of specimens irrespective of the value of R x 相似文献
6.
Small G.W. Ricketts B.W. Coogan P.C. 《IEEE transactions on instrumentation and measurement》1989,38(2):245-248
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 相似文献
7.
A measurement of the NBS electrical watt in SI units 总被引:2,自引:0,他引:2
Olsen P.T. Elmquist R.E. Phillips W.D. Williams E.R. Jones G.R. Jr. Bower V.E. 《IEEE transactions on instrumentation and measurement》1989,38(2):238-244
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: ΩNBS/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 E 0=483, 594 GHz/V 相似文献
8.
The effects of partial substitution of light rare earths for Sm in (Sm1-xRx)2 (Co,Fe,Cu,Zr)17 have been investigated where R =Ce, Pr, Nd, Pr0.5Nd 0.5, Ce0.2Pr0.4Nd0.4 and x =0-0.5. Experiments show that both remanence and maximum energy product are improved for x =0.1-0.2, depending upon compositions and heat treatments. For a composition Sm0.8R 0.2(Co0.633Fe0.286 Cu0.061Zr0.020)7.59 with R =Ce0.2Pr0.4Nd0.4, the authors obtained B r=11.57 kG, MH c =15.5 kOe, BH c=10.33 kOe, H k=10.03 kOe, and (BH )max=30 MGOe 相似文献
9.
Edge-elements for scattering problems 总被引:3,自引:0,他引:3
It is shown that edge elements, as vectorial finite-element approximations fields like e and h , can be used to solve scattering problems. This can be done in essentially two ways, according to which of the two fields d or b one most wishes to get in divergence-free form. Apparent drawback of edge elements (due to the increased number of degrees of freedom in comparison with nodal elements) are more than compensated by economies due to greater sparsity. Moreover, they could help get rid of unwanted spurious modes when resonance frequencies of waveguides of complicated shape are computed 相似文献
10.
Amorphous ribbons of composition Fe74.5-xCuxNb3Si13.5B9 (x =0, 1 at.%) have been annealed between about 500°C and 900°C. This produced a series of crystallized samples with grain sizes between about 10 nm and 300 nm and with coercivities H c and initial permeabilities μi varying over several orders of magnitude. The best soft magnetic properties (H c≈0.01 A/cm and μi≈80×103 ) were observed for the smallest grain sized of about 10 nm. With increasing grain size D , coercivity steeply increases following a D 6-power law (up to D≈50 nm). H c then runs through a maximum of H c≈30 A/cm and decreases again for grain sizes above 150 nm according to the well-known 1/D law for polycrystalline magnets. The initial permeability was found to vary in a similar manner, essentially being inversely proportional to coercivity. The variation of the soft magnetic properties with the average grain size is discussed and compared with the predictions of the random anisotropy model and other theories for the magnetization reversal 相似文献
11.
Investigations were conducted on the properties of 24 circuits for which the impedance in the ideal case is equal to s 2 E , where s is the complex frequency and E is a constant. For harmonic excitation each circuit forms a negative resistance-ω2E and is hence called a frequency-dependent negative resistance (FDNR). From these 24 circuits, four that are stable were selected; measurements were made on these, and their equivalent circuit diagrams were determined. A resonant bridge was constructed with one branch containing an FDNR. Using this bridge it is possible to measure inductances with dissipation factors, tan δ, as high as 104 相似文献
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W-type Pb-hexaferrites were prepared by standard ceramic methods. The lattice constants found by refinement were a =0.59140±0.00006 nm and c =3.29209±0.00041 nm. The X-ray density of a typical composition PbZn1.9Fe15.3O25.8 was ρ=5.32 g/cm3 and the Vickers microhardness value h v=6 kN/mm2. A plot of the saturation magnetization versus temperature is given. The extrapolated value of the saturation magnetization (H →∞, T →0) was σs=108 emu×g-1, and the Curie temperature was T c=600±20 K 相似文献
13.
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=Is-b/H 2-c/H3 . . ., or alternatively in terms of the reversible permeability μrev=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/Is[K+1.5(λ100-λ 111)σ]2, where σ is the applied stress and others are magnetic constants. The values of reversible permeability μrev 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 相似文献
14.
Fabbricatore P. Musenich R. Parodi R. Vaccarone R. 《IEEE transactions on magnetics》1990,26(6):3046-3051
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: R αl n 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 相似文献
15.
Gelinas S. Tran V.N. Vaillancourt R. 《IEEE transactions on instrumentation and measurement》1990,39(4):615-620
A global method is presented for solving the permittivity equations for open- and short-circuited coaxial lines of general length for broadband measurements by iterating the recurrence schemes z n+1=c cot z z and z n+1=c tan z n, respectively, and their inverses. The global iteration theory of Fatou and Julia (see J. L. Howland and R. Vaillancourt, Num. Math., vol.46, 323-337, 1985), coupled with linear extrapolation and interpolation and Steffensen's acceleration procedure, supplies starting values and guarantees convergence even near double roots. When R Z⩾0 for open, and R Z⩽0 for short circuit terminations Newton's method, with appropriate starting values, converges to the desired roots. A combination of the three iterative schemes results in an almost global method of solution. Numerical results are quoted 相似文献
16.
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 Mn0.6Zn0.4-xNixFe2 O4 and Mn0.6Zn0.4-yCoyFe2O4 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 相似文献
17.
Sm2Fe17Nx powders were prepared through metallurgical processes such as melting, casting, pulverizing, and gas nitrogenation. Epoxy resin and zinc-bonded magnets were prepared from the powder by a compression molding process. The magnetic properties of magnetically aligned Sm2Fe17Nx powder were (BH )max=170 kJ/m3 (21.4 MG-Oe), Br =1.20 T, and H cJ=547 kA/m (6.87 kOe). Those of an epoxy-bonded magnet were (BH )max =103 kJ/m3 (13.0 MG-Oe) and Br =0.835 T. H cJ values in excess of 2 MA/m (25 kOe) were observed in the case of a zinc-bonded magnet 相似文献
18.
Frizzell LA 《IEEE transactions on ultrasonics, ferroelectrics, and frequency control》1988,35(5):578-581
The threshold dosages for high-intensity focal lesion production were determined at 3 MHz in the cat liver for exposure durations covering the range 0.003-35 s. The liver threshold was found to parallel that for the brain over the range of exposure durations 0.1 to 10 s, but to be more than twice the intensity level, approximately following the relation IT 0.5=460 W-cm2/s2, where I is the peak intensity and T is the exposure duration. At shorter exposure durations the threshold curve deviates from this relation, probably due to a change to a transient cavitation mechanism of damage 相似文献
19.
Hybrid method for computing demagnetizing fields 总被引:1,自引:0,他引:1
A hybrid finite-element-boundary integral method is proposed for computing demagnetizing field. For a three-dimensional (3-D) mesh with N nodes in the magnetic region, the method requires storage O( N 4/3), and no nodes outside the magnetic region need to be considered. The method is suitable for nonlinear calculations, can be adapted to handle curved boundaries, and is especially convenient when the magnetic region consists of several parts whose relative positions are variable 相似文献
20.
High precision phase measurement using adaptive sampling 总被引:4,自引:0,他引:4
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 相似文献