vibration prevention, do not apply to static field systems, which have no vibrations, with the possible exception of on and off switching. In experiments involving static magnetic fields, the earth's magnetic field needs to be measured and controlled locally. Depending on the hypothesis to be tested, the earth's magnetic field might need to be controlled for sham-exposed cells and for the field-exposed cells. In addition to the exposure specifications listed in Tables 2-10 and 2-12, Table 2-14 lists the specifications needed for in vitro exposures.

INDUCED FIELDS AND CURRENTS

Placement of a biologic system or a cell preparation in an ELF electromagnetic field induces internal electric currents and fields and surface charges at the interfaces of electrically dissimilar media. That behavior is described by Maxwell's equations. In the case of ELF fields, major simplifications to the solution of the equations can be made. The solutions are quasi-static. Because of the size of the objects and the electric properties of biologic tissue, consideration of penetration depth can be neglected. Furthermore, when electric permittivity of tissues is evaluated, it becomes apparent that, for frequencies up to a few kilohertz, the induced conduction current is much greater than the induced displacement current, because σ/we >> 1, where σ is the volume conductivity, ε is the media permittivity, and ω is given by 2π times the frequency of the radiation (consult Foster and Schwan (1986) for the dielectric properties of tissues and cells). Therefore, an ELF electromagnetic field produces currents and electric fields in the exposed biologic system and causes oscillating (at ELF) charges at interfaces (i.e., for the interface between the external biologic body and air and for internal interfaces, such as those between different tissues and the cell and cell medium). The magnitudes and spatial patterns of those currents and fields depend on the type of exposure field, its characteristics (frequency, magnitude,

TABLE 2-14 Typical Specifications for In Vitro Exposures to the Magnetic Field

Critical Specifications

Desirable Specifications

Dimensions of cell culture dishes

Dimensions (height) of the medium in the exposure dish

Orientation of the field with respect to the culture dish

CO2

dc field level and orientation where applicable to the experimental design

Computed or measured currents (electric fields) in the cell preparation

Medium conductivity

Ambient 60-Hz field in all areas occupied by the cell preparation during the experiment



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