Oliver Heaviside, who had the advantage of being born later, had a better grasp of electromagnetics than did Faraday or Maxwell, and his view of how a digital signal travels is well worth study.
Whereas the conventional approach to the subject today is to concentrate on the electric current in wires, with some additional consideration of voltages between wires, Heaviside concentrates primarily on what he calls 'energy current', this being the electromagnetic field which travels in the dielectric between the wires. In the quotation below, Heaviside's phrase, "We reverse this;" point to the great watershed in the history of electromagnetic theory - between the 'etherials', who with Heaviside believe that the signal is an 'energy current' which travels in the dielectric between the wires, and the 'practical electricians', who like John T. Sprague believe that the signal is an electric current which travels down copper wires, and that if there is a 'field' in the space between the wires, this is only the result of what is happening in the conductors.
Heaviside wrote (Ref.22);
"Now in Maxwell's theory there is the potential energy of the displacement produced in the dielectric parts by the electric force, and there is the kinetic or magnetic energy of the magnetic induction due to the magnetic force in all parts of the field, including the conducting parts. They are supposed to be set up by the current in the wire. We reverse this; the current in the wire is set up by the energy transmitted through the medium around it...."
The importance of Heaviside's phrase, "We reverse this;" cannot be overstated for digital designers. It points to the watershed between the 'practical electricians', who have held sway for the last half century, promulgating their theory - which we shall call "Theory N", the Normal Theory: that the cause is electric currents in wires and electromagnetic fields are merely an effect - and the 'ethereals', who believe what we shall call "Theory H": that the travelling field is the cause, and electric currents are merely an effect of these fields.
The situation is of course obscured by the many who claim that it is immaterial which causes which. However, experience shows that it is damaging to ignore causality when trying to assemble reliable digital systems.
Before continuing with Theory H, we shall quote one early Theory N man, a 'practical electrician' named John T. Sprague. In his book, ref.23, he ridicules Theory H;
"A new doctrine is becoming fashionable of late years, devised chefly in order to bring the now important phenomena of alternating currents under the mathematical system. It is purely imaginary ... based on Clerk-Maxwell's electromagnetic theory of light, itself correctly described by a favourable reviewer as 'a daring stroke of scientific speculation,' alleged to be proved by the very little understood experiments of Hertz, and supported by a host of assumptions and assertions for which no kind of evidence is offered; but its advocates call it the 'orthodix' theory.
"This theory separates the two factors of electricity ..., and declares that the 'current', the material action, is carried by the 'so-called conductor' (which according to Dr Lodge contains nothing, not even an impulse, and according to Mr. O. Heaviside is to be regarded as an obstructor), but the energy leaves the 'source' (battery or dynamo) 'radiant in exactly the same sense as light is radiant', according to Professor Sylvanus P. Thompson, and is carried in space by the ether: that it 'swirls' round (cause for such swirling no one exaplains) and finds its way to the conductor in which it then produces the current which is apparently merely an agency for clearing the ether of energy which tends to 'choke' it, while the conductor serves no other purpose than that of a 'waste pipe' to get rid of this energy ...
"This much, however, is certain; that if the 'ether' or medium, or di-electrics carry the energy, the practical electrician must not imagine he can get nature to do his work for him; the ether, &c., play no part whatsoever in the calculations he has to make; whether copper wire is a conductor or a waste pipe, that is what he has to provide in quantity and quality to do the work; if gutta percha, &c., really carry the energy, he need not trouble about providing for that purpose; he must see to it that he provides it according to the belief that it prevents loss of current. In other words, let theoretical mathematicians devise what new theories they please, the practical electrician must work upon the old theory that the condctor does his work and the insulation prevents its being wasted. Ohm's law (based on the old theory) is still his safe guide.
"For this reason I would urge all practical electricians, and all students who desire to gain a clear conception of the actual operation of electricity, to dismiss from their minds the new unproved hypotheses about the ether and the abstract theory of conduction, and to completely master the old, the practical, and common sense theory which links matter and energy together..."
Sprague accurately described Theory N, which has been used in practice by virtually every digital designer, with disastrous results. They must now turn to Theory H to get them out of their difficulties.
In his book (ref.24), J. A. Fleming argued for Theory H;
"It is important that the student should bear in mind that, although we are accustomed to speak of the current as flowing in the wire in one direction or the other, this is a mere form of words. What we call the current in the wire is, to a very large extent, a process going on in the space or material outside the wire. Just as we familiarly speak of the sun rising and setting, when the effect is really due to the rotation of the earth, so the ordinary language we use in speaking about electric currents flowing in conductors retains the form impressed upon it by older and erroneous assumptions as to their nature."
The reader will have surmised by now that "energy current", the primary signal which travels down the dielectric from one logic gate to the next, has an amplitude equal to the Poynting vestor, E x H.
We shall end this qualitative discussion with some of the more important quotations from Heaviside, the man who a century ago brilliantly used the concept of energy current to solve telegraph problems which closely parallel present-day problems in high speed digital logic.
Heaviside wrote (ref.22);
"It becomes important to find the paths along which the energy is being transmitted. First define the energy-current at a point to be the amount of energy transferred in unit time across unit area perpendicular to the direction of transmission ... This is true universally, irrespective of the nature of the medium as to conductivity, capacity and permeability ... and is true in transient as well as in steady states. A line of energy-current is perpendicular to the electric and the magnetic force, and is a line of pressure, We here give a few general notions.
"Return to our wire from London to Edinburgh with a steady current from the battery in London The energy is poured out of the battery sideways into the dielectric at a steady rate ... Most of the energy is transmitted parallel to the wire nearly ... But some of the outer tubes go out into space to an immense distance ... If there is an instrument in the circuit at Edinburgh, it is worked by energy that has travelled wholly through the dielectric, then finding its way into the instrument, ... where it enters ... and is there dissipated ...
"In a circular circuit, with the battery at one end of a diameter, its other end is the neutral point; the lines of energy-current are distributed symmetrically with respect to the diameter.
"On closing the battery circuit (i.e. switching the logic output) there is an immediate rush of energy into the dielectric ..."
A number of different dualisms obtain within or in the vicinity of electromagnetic theory as it is developing. the student needs to be warned against thinking that only one dualism is involved, and that he is merely seeing different expressions of the same dualism. The mutually distinct dualisms include:
Theory N - Theory H (ref.18a)
The Rolling Wave - The Heaviside Signal (ref.18b p51)
It will be seen later that one of these is in fact a three-way split between Theory N, Theory H and Theory C.
The transition from classical, wireless-based electromagnetic theory, loosely equivalent to Theory N, to one of the preferred theoretical positions for the digital electronic designer, Theory H or Theory C, is via a complex development shown in Figure 63.
Chapter previously published in Ref.3(a) p65.