Dc Field Strength Research Articles

A scheme for incorporating DC magnetic fields into epidemiological studies of EMF exposure.

Experimental data on calcium-ion release in chicken brain tissue suggest that biological effects of electric and magnetic fields (EMFs) are concentrated near certain "active combinations" of DC magnetic field strength and "effective" AC magnetic field frequencies. We hypothesize that active AC/DC combinations may exist and suggest that epidemiologic data, coupled with DC magnetic field measurement, may be used to identify critical exposure conditions. An empirical model is used to calculate these multiple active combinations at any given DC magnetic field strength and to define a rating system that incorporates the proximity of AC magnetic field frequencies generated by electric power lines to the new, computed effective frequencies. Such an exposure score may be useful in investigating correlations of EMF exposure with disease incidence. For 60 Hz and 50 Hz, the highest EMF exposure scores occurred at DC field strengths of 506 mG and 422 mG, respectively. The exposure score contains a factor which may be adjusted to reflect the importance of harmonics of the AC magnetic field as well as of the fundamental frequency. Using this factor, we consider two important special cases consistent with chick brain data: 1) we consider active pairs associated with all detectable harmonics (up to 660 Hz) without regard to relative intensity of the harmonics, and 2) we use the relative intensities of the AC field frequencies to adjust their contribution to the exposure score.

Investigation of the frequency dependence of Barkhausen emissions for measuring the depth dependence of magnetic properties

Discontinuous displacement of Bloch walls and discontinuous domain rotation are two of the mechanisms which contribute to Barkhausen emissions. Measurements of the pulse height spectra and root mean square ac amplitude of the emissions have been used to investigate the depth dependence of magnetic properties. An ac exciting field was superimposed on a constant dc field and the Barkhausen emissions were analyzed at different dc field strengths. The depth of penetration of the ac field, and hence the depth of the Barkhausen emissions, is controlled by the skin depth.

Preisach diagrams and anhysteresis: do they measure interactions?

Traditionally, it is assumed that the off-diagonal distribution on the Preisach diagram and the inverse anhysteretic susceptibility as a function of DC field strength measure the spectrum of magnetostatic particle interactions in single-domain (SD) samples. To test this assumption, we have measured Preisach diagrams and anhysteretic remanence acquisition curves at a series of temperatures up to the Curie point for samples containing SD and small pseudo-single-domain (PSD) magnetites and maghemites. Preisach diagrams were also determined at room temperature only for a suite of PSD to multi-domain (MD) magnetites with mean sizes ranging from 2 to 100 μm. In the latter Preisach diagrams, the diagonal distributions, which are thought to represent microcoercivity spectra, changed from peaked, SD-like distributions for the smallest grains to typical quasi-exponential MD distributions for the largest grains. The transverse or off-diagonal distributions broadened enormously over the same grain-size range, probably indicating that self-demagnetizing fields become more influential in the larger grains. In the SD and small PSD samples, the diagonal and transverse Preisach distributions contracted about equal amounts at each high-temperature step. Both distributions changed approximately in proportion to bulk coercive force, HC(T), and not in proportion to saturation magnetization, Ms(T). Such a temperature dependence was expected for the diagonal distribution, which depends on crystalline and shape anisotropies. It was not anticipated for the transverse distribution, as magnetostatic interactions should vary with temperature as MS(T). Inverse anhysteretic susceptibilities at first changed like MS(T), but at the higher temperatures they also decreased in proportion to HC(T). Therefore the off-diagonal Preisach distribution seems to be affected more by microcoercivity than by magnetostatic phenomena such as particle interaction or self-demagnetization, at least in this suite of samples, and inverse anhysteretic susceptibility is also a dubious measure of interaction strength. A preliminary theoretical model is presented in which an interaction field acting perpendicular to the easy axis of anisotropy considerably reduces the particle coercivity. Thus the diagonal Preisach distribution is not independent of interactions. However, it remains unclear why the off-diagonal distribution is not independent of coercivity.

Factors controlling the electrofusion of murine embryonic cells

A wide variety of physical and biological factors are involved in determining the success of electrofusion procedures. The optimal conditions for the fusion and survival of mouse two-cell embryos have been determined by manipulating the electric field parameters, medium composition, degree of cell—cell contact and the relationship between current flow and membrane orientation. The experiments demonstrate that the events which initiate embryonic cell fusion are dependent upon a closely defined electric field strength and associated pulse duration. We show further that high cell fusion rates are the product of an inverse relationship between dc field strength and pulse duration and the initiation of pore formation by electric field application is insufficient to induce successful fusion unless accompanied by appropriate post-pulse medium and adequate membrane contact. Manipulation of the direction of current flow, membrane orientation and degree of cell—cell contact have shown that the initiation of pore formation occurs across the entire surface of the cell membrane.

Factors controlling the electrofusion of murine embryonic cells

A wide variety of physical and biological factors are involved in determining the success of electrofusion procedures. The optimal conditions for the fusion and survival of mouse two-cell embryos have been determined by manipulating the electric field parameters, medium composition, degree of cell-cell contact and the relationship between current flow and membrane orientation. The experiments demonstrate that the events which initiate embryonic cell fusion are dependent upon a closely defined electric field strength and associated pulse duration. We show further that high cell fusion rates are the product of an inverse relationship between dc field strength and pulse duration and the initiation of pore formation by electric field application is insufficient to induce successful fusion unless accompanied by appropriate post-pulse medium and adequate membrane contact. Manipulation of the direction of current flow, membrane orientation and degree of cell-cell contact have shown that the initiation of pore formation occurs across the entire surface of the cell membrane.

Influence of AC and DC fields on streaming electrification of transformer oil

A study conducted by flowing transformer oil through a coaxial cylinder system is discussed. In the case of AC voltage, the streaming current increases with the field strength and with temperature. The increase of the streaming current at low AC field strength is attributed to vibration of positive ions, diffused into the oil at the interface with the electrode. In the case of DC voltage application, the polarity of the streaming current varies from slightly positive at low DC field strength to negative at high field strength. The streaming current at low fields may be caused by the difference between positive and negative ion mobilities. >

Dielectric studies of the smectic C*-smectic a transition of a ferroelectric liquid crystal with high spontaneous polarization

The static dielectric constant ∊⊥ in the smectic A and the smectic C* phase of the ferroelectric liquid crystal 4-(3-methyl-2-chlorobutanoyloxy)-4'-heptyloxybiphenyl was determined. Strong pretransitional effects were observed in the smectic A phase. When the Goldstone mode contribution in the smectic C* phase is quenched by the application of a dc electric field, a cusp like behaviour of ∊⊥ at the smectic C*-smectic A transition can be observed which is similar to solid ferroelectrics. The maximum in ∊⊥ was found to shift to higher temperatures with increasing dc field strength in agreement with the electric field dependence of the transition temperatures reported previously.

Measurement of piezoelectric parameters versus bias field strength in polyvinylidene fluoride (PVF2)

Piezoelectric polymers, such as PVF2, being light and flexible, are attractive new materials for transducers. A new all-electrical meaurement method for piezoelectric parameters of PVF2 has been developed. Moreover, the method permits application of a variable bias field going to high values in addition to the small alternating field used for measurement. By using this technique, it has been found that linear performance extends to a very high electric field (30 V/μm) without degradation. This technique is also used to study the behavior of d31 under a range of dc field strengths. Finally, the method is used to investigated various procedures for poling PVF2.

Domain wall dynamics and small-signal permittivity of TGS

Measurements are reported on the small signal complex dielectric constants ε' and ε'' (or ε' and tan δ) for thin samples of single crystal TGS, as functions of frequency, degree of poling, dc and ac field strengths and temperatures. The data strongly suggest that most of the effects are caused by dynamic domain wall effects. At low ac fields there is an enormous dip in tan 6 between 10-20 kHz that indicates an almost complete absence of any loss mechanism at that frequency.

Dependence of the Phonon-Instability Threshold for Parallel Pumping on Crystal Orientation and Magnetic Fieldstrength

The theory of phonon instability under parallel pumping is extended by taking account of magnetocrystalline and magnetoelastic anisotropy. Previous results apply to real materials only when the magnetic field is along a [100] direction. Results are obtained for the instability threshold when the magnetic fields are along a [110] or a [111] direction. When magnetic loss is neglected the calculated threshold (minimum with respect to all directions of propagation) for all three principal crystal directions varies with dc field-strength in such a way that hcrit1/2 is approximately a linear function of the dc field, with the extrapolated intercept (zero threshold) at internal dc fields of −⅔Ha for [111], ½Ha for [110], and Ha for [100]. Here Ha=−2K1/M0 is the anisotropy field and it has been assumed that K1<0. For the [110] case the threshold is approximately hcrit1/2=(2γ/σQe)1/2(H−12Ha),where σ/γ=b22/C44M0 and Qe is the phonon Q. When magnetic loss is taken into account, hcrit1/2 still remains an approximately linear function of dc field, but the extrapolated intercept is moved to lower dc fieldstrengths.

Subthreshold Steady-State Absorption under Parallel Pumping

If the rf and dc magnetic fields are parallel, the absorption coefficient μ″ at low power levels is theoretically expected (for YIG at room temperature, a frequency of 10 Gc/sec, and a dc field of 1 kOe) to be of the order of 2×10−4. We have calculated how the steady-state absorption varies with dc and rf field strength below the instability threshold. Experiments on a high-purity YIG single crystal are in reasonably good agreement with the theoretical expectations.

Experiments on the Carnauba Wax Electret

The strength of the applied electric field, the thickness of the slab being electrified, the time of electrification and the low temperature holding time before removing the field were varied for carnauba wax electrets. After approximately one hour the discharge current varied inversely as a power of the time since turning off the forming field, the power being close to one. The magnitude of this current depended on both the forming field strength and on the thickness of the electret. A very large discharge was obtained when the wax was electrified and discharged while not completely solid. Dielectric constant measurements were made at temperatures between 27°C and 86°C, frequencies between 50 cps and 13 kc, and with superposed dc field strengths in the wax of zero, 3700 and 8100 volts per cm. The application of dc field was found to raise the dielectric constant at the higher temperatures. The case for permanent dipole orientation and that for ionic space charges as the cause of the stored charge of the electret is examined, and it is shown that there are objections to each mechanism. An alternate explanation is suggested.

Radio-Frequency System. Part V—Properties of Ferromagnetic Ferrites

Ferromagnetic ferrites are used in numerous applications in the Cosmotron. For example, in the accelerating unit over a ton of ferrite is included. During the design of the Cosmotron, much information was required about the various electrical and magnetic properties of ferrites. Since this information was not available, a detailed program of investigation of ferrite properties has been carried out. Results are presented of dielectric properties and of magnetic properties as functions of frequency, dc and ac field strengths, and temperature. A general discussion is included of the behavior of electromagnetic waves in ferrite structures.