七年级的英语故事,要紧抠教材

However, when actively driving biological materials it is important to understand their
dispersive behavior. A great deal of work has been done in this area by H.P. Schwan who
examined the permittivity and conductivity of tissue and cell suspensions. His work
showed that there are three distinct regions of dispersion which must be considered when
measuring the impedance characteristics of biological materials. At low frequencies (50 to
200 Hz) the counter-ion cloud surrounding the surface of the cellular membrane becomes
polarized, resulting in what is called the a-dispersion. From approximately 60 kHz to 5
MHz the cell membrane becomes electrically charged (through intra- and extra-cellular
pathways) causing what is known as the b-dispersion. At much higher frequencies
(1 GHz), the dispersive behavior of water causes the measured permittivity to change
(g-dispersion) [Schwan, 1988]. In each case, the permittivity of the ma