Low frequency fields

The connection between the molar polarization Pm and the molar refraction Rm is through Maxwell s theory of electromagnetism, according to which e = (at low-frequency fields). This is the basis for considering the molar refraction a measure of polarizability. 

Finally, the intrinsic features of the ultrasonic field, frequency and power, should also be taken into account in the design of the experimental arrangement. It is obvious that the mechanical and chemical effects derived from a low frequency field are quite different than those provided by high frequency fields, and these features should match with features of the electrode materials such dimension, structure and physical and chemical properties [30]. 

The LMMP-GPI represents a tissue that allows studying enteric nerve - smooth muscle interactions. Low-frequency field stimulation of the LMMP-GPI results in... 

For instance, for a nematic polymer with positive anisotropy of dielectric constant (Ae > 0) orientation of mesogenic groups along the applied field takes place (homeo-tropic orientation). The fact of orientation is illustrated in Fig. 25, which shows that under crossed polarizers the optical transmittance I of a film of nematic polymer with optically anisotropic texture (taken for 100%) falls practically to zero when a low-frequency field is switched on. 

Later studies involving low-frequency exposure have not clearly demonstrated a correlation between low-level fields and effects on human health. One study observed a slight increase in nervous system tumors for people living within 500 m (=1600 ft) of overhead power lines, while most recent studies in this field have not found any clear evidence to relate exposure to low-frequency fields with childhood leukemia. 

Table 7.3 shows the EMFs produced by some common household electrical appliances. While the EMF levels can be considered high, the exposure duration is low in most cases. It is important to realize that the effects of exposure to low-frequency fields are not clearly known, thus it is prudent to exercise caution and avoid prolonged exposure to electrical and magnetic fields. One way to minimize exposure is to maintain sufficient distance between the EMF source and people in the environment. As we saw earlier, electrical and magnetic fields diminish as the square of the distance from the source. For example, instead of sitting 1 ft away from a table lamp, one can move 2 ft away and reduce EMF exposure to approximately one fourth the level found at 1 ft. It is expected that studies conducted in the future will reveal more about the effects, if any, of low-frequency electromagnetic fields. 

McLeod, B. R., Liboff, A. R., and Smith, S. D. (1992) Biological Systems in Transition Sensitivity to Extremely Low Frequency Field, Electro Magnetbiology, 11, 29-42. 

In addition to these results, we have observed also another interesting phenomenon, i.e., resonance of polymer impedance under high intensity but low frequency fields (1 KHz to 10 KHz). At the present time, we are checking the reproducibility of this phenomenon. We hope to publish this in a subsequent article. 

Wever, R. Effects of lov-level, low-frequency fields on human circadian rhythms. Neurosci. Res. Program Bull. 1977, 15, 39-45. 

Sheppard, A. R. Adey, W. R. The role of cell surface polarization in biological effects of extremely low frequency fields. U. S. Department of Energy Symposium Series 50, Biological Effects of Extremely Low Frequency Electromagnetic Fields" DOE Washington, D. C., 1979 pp. 147-158. 

Dumont and coworkers [136, 155, 156] have observed that shining doped (or functionalized) polymer thin hlms with noncentrosymmetric dipolar chromo-phores, induces a significant increase of electro-optic coefficient in the chromo-phore absorption band, corresponding to a better, polar orientation of chromo-phores. The measurements have been done by using the attenuated total reflection technique, and the optical field polarization was perpendicular to the applied low-frequency external electric field to the thin film (Fig. 33). A better stability of induced orientation was observed in the case of functionalized polymers than in guest-host system, as is usually the case with the static field poled polymers. The chromophores orient with dipolar moments perpendicular to the optical field (and parallel to the applied static (or low frequency) field. As will be discussed later, the chromophore orientation undergoes a trans-cis isomerization process (Fig. 34). 

Direct detection, which is useful when the repetition rate of the transient signal is greater than about 10 per second. A superimposed very low frequency field modulation sometimes yields improved baseline stability. 

Space charge polarization occurs when the material contains free electrons whose displacement is restricted by grain boundaries. Hence, entire macroscopic regions of the material become either positive or negative. This mechanism is often called the Maxwell-Wagner effect and it takes place in low frequency fields. 

T. A. Buchhold and P. J. Molenda, Measurement of the Electric Surface Losses of Superconductors in Low Frequency Field, Prepublication Kept., General Electric (March 30,1962). 

R. A. Luben, C. D. Cain, M. C.-Y. Chen, D. M. Rosen, and W. R. Adey, Effects of Electromagnetic Stimuli on Bone and Bone Cells in Vitro Inhibition of Responses to Parathyroid Hormone by Low-Energy Low-Frequency Fields, Proc. Natl. Acad. Sci. USA 79, 4180 184 (1982). 

These effects are classified as either electrophoresis (EP) or dielectrophoresis (DEP). Whereas EP arises from the interaction of a fixed charge and an electric field, DEP results from the interaction of an induced charge and the spatial or temporal gradient of the electric field. In EP, direct current (DC) or low-frequency fields, usually homogeneous, are applied. On the other hand, in DEP, alternating current (AC) fields over a wide range of frequencies are used. 

World Health Organization. Extremely Low Frequency Fields Environmental Health Criteria, No. 238, WHO 2007 available at http // www.whojnt/peh-emf/publications/elf ehc/en/index.html (accessed January 27, 2009). 

The optical reorientation processes discussed up to now were qualitatively similar to the corresponding low-frequency field effects. As mentioned earlier this is not always the case. A breakdown of the analogy with static fields was first reported by Zolotko et al. who observed in a homeotropic layer a drastic increase of the Freedericksz threshold power for an o-ray as the angle of incidence was increased. Durbin et al. mentioned that in a planar cell Freedericksz transition cannot be induced by a light beam polarized perpendicularly to the director. From a simple analogy one would expect for these cases a threshold not deviating significantly... 

If the cell walls are treated to produce a homeotropic orientation in a cholesteric liquid crystal with As < 0, for a particular relation between the thickness and the helical pitch (1 < Po/d < 2) a new texture appears under the influence of the electric fleld. These are the bubble domains [77] mentioned in Section 6.1.1. When there is no field present the liquid crystal has a homeotropic nematic structure, since the helix is untwisted by the walls. The application of a low-frequency field induces an electrohydrodynamic instability. After the removal of the field a new stable texture appears in the form of cholesteric bubbles dispersed through the homeotropic nematic phase. Thus a memory is created, while erasure of the information can be performed either by a high-frequency field or by the mechanical... 

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