"Electromagnetic theory is a peculiar subject. The peculiarity resides not so much in the stratification - superposed layers of electrostatics. magnetostatics. steady currents and time-varying fields - as in the failure that has attended all attempts to weld these layers into a logical whole. The lowest layer. electrostatics. defines certain concepts. such as E. D, ~, in a way that is generally satisfactory only for the static case. Yet the attempt is made to force these specialised definitions into the higher strata, with ad hoc modifications when necessary. The student, in looking through his text books on electromagnetics, can find general definitions only with difficulty. if at all; and even the most advanced treatises fail to present a rigorously logical development of the subject". 1 So wrote Moon and Spencer some 30 years ago; and their criticism continues to be pertinent today. 2 More recently. a senior physicist of the National Bureau of Standards has expressed his concern in similar terms: "A logically consistent set of definitions of the electromagnetic field quantities is extremely difficult to find in the literature. Most text books either evade the problem or present definitions that are applicable only to special cases".