Lumped Electromechanical Elements terminal pair is excited by the current source i and that the terminal pair is in a region of space in which the time rate of change of magnetic flux density is negligible. This restriction is necessary if we are to be able to describe a terminal voltage unambiguously. The perfect conductor that connects the two terminals is often wound into a coil and the coil may encircle an iron core. The drawing in Fig. 2.1.1 is simplified to illustrate the principles involved. We must include the possibility that the perfect conductor in Fig. 2.1.1 is moving. We define a contour C that passes through and is fixed to the perfect conductor. That portion of the contour which goes from b to a outside the perfect conductor is fixed and in a region of negligible magnetic flux density. The terminal voltage v (see Fig. 2.1.1) is defined in the usual way* as v = -E dl; (2.1.5) it is understood that this line integral is evaluated along the path from b to a that is external to the perfect conductor. We now consider the line integral
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