_____________________________________________________________________ Special Amplifiers
Figure 7-27. Varying Coil Inductance With a Moveable Coil
7-108. The system, shown in Figure 7-27, is not too practical. Even if a motor were used
in place of the hand that is shown, the resulting amplifier would be large, expensive, and
not easily controlled. If the permeability of a core could be changed by electrical means
rather than mechanical, a more practical system would result.
7-109. High permeability depends on there being many molecular magnets (or electron
spin directions) that can be aligned to provide a path for magnetic lines of force. If almost
all of these available paths are already being used, the material is magnetized and there are
no more paths for additional lines of force. The "flux density" (number of lines of force
passing through a given area) is as high as it can be. This means that the permeability of
the material has decreased. When this condition is reached, the core is said to be saturated,
because it is saturated (filled) with all the magnetic lines of force it can pass. At this point,
the core has almost the same value of permeability as air (1) instead of the much higher
value of permeability (5,000) that it had when it was unmagnetized.
7-110. Of course, the permeability does not suddenly change from 5,000 to 1. The
permeability changes as the magnetizing force changes until saturation is reached. At
saturation, permeability remains very low no matter how much the magnetizing force
increases. If you were to draw a graph of the flux density compared to the magnetizing
force, you would have something similar to the graph shown in Figure 7-28. Figure 7-28
also includes a curve representing the value of permeability as the magnetizing force
increases. Point "S" in Figure 7-28 is the point of saturation. The flux density does not
increase above point "S," and the permeability is at a steady, low value.
Figure 7-28. Magnetization and Permeability Curves
23 June 2005
TC 9-62
7-39
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