Professor W. F. Barrett, in 1879,[42] made some experiments on the buttons of compressed lamp-black used in Edison’s transmitter, and found that when an intimate contact was satisfactorily secured at the beginning, “pressure makes no change in the resistance.”
In the face of all this precise evidence, it is impossible to maintain the theory that the electric resistance of plumbago or of any other such conductor varies under pressure. The only person who has seriously spoken in favour of the theory is Professor T. C. Mendenhall, but in his experiments he took no precautions against variability of contacts, so that his conclusions are invalid.
More recently still, Mr. O. Heaviside and Mr. Shelford Bidwell have experimented on the variations of resistance at points of contact.[43] Mr. Heaviside’s experiments were confined to contacts between pieces of carbon, and though extremely interesting as showing that the resistance of such contacts are not the same, even under constant pressure, when currents of different strength are flowing, do not throw much light on the general question, because they leave out the parallel case of the metals. Mr. Bidwell’s very careful researches were chiefly confined to carbon and bismuth. The choice is unfortunate, because bismuth the most fusible and worst conductor amongst metals (save only quicksilver) is the one metal least suited for use in a telephone transmitter. Mr. Bidwell’s conclusions, so far as they are comparative between carbon and “the metals,” are therefore necessarily incomplete.
Professor D. E. Hughes, whose beautiful invention, the Microphone, attracted so much attention in 1878, has lately thrown the weight of his opinion in favour of the view that with carbon contacts the effect is due chiefly to an electric discharge or arc between the loosely-contiguous parts. But Professor Hughes’s innumerable experiments entirely upset the false doctrine that a “semi-conductor” is necessarily required for the contact-parts. Speaking recently,[44] he has said: “I tried everything, and everything that was a conductor of electricity spoke.” In 1878, in a paper “On the Physical Action of the Microphone,” Professor Hughes stated:[45] “the best results as regards the human voice were obtained from two surfaces of solid gold.” Hughes also found carbon impregnated with quicksilver in its pores to increase its conducting power to work better than non-metallised carbon of inferior conductivity. Quite lately Mr. J. Munro has constructed successful transmitters of metal gauze, having many points of loose-contact between them.
It seems, therefore, much the most probable in the present state of investigations, that the electric resistance of a contact for telephonic purposes is determined solely by the number of molecules in contact at the surface, and by the specific conductivity of those molecules. The element of fusibility comes in to spoil the constancy of the surfaces in action; and hence the inadmissibility of general conclusions with respect to all metals drawn from the behaviour of the most fusible of them. At a mere point in contact physically with another point, there may be hundreds or even millions of molecules in contact with one another, all acting as so many paths for the flow of the electric current. An extremely small motion of approach or recession may suffice to alter very greatly the number of molecules in contact, and the higher the specific conductivity of the substance, and the denser its molecules, the shorter need be the actual range of motion to bring about a given variation in the resistance offered. Just as in a system of electric lamps in parallel arc, the resistance of the system of lamps increases when the number of lamps through which the current is flowing is diminished, and diminishes when the number of lamps connecting the parallel mains is increased; so it is with the molecules at the two surfaces of contact. Diminishing the number of molecules in contact increases the resistance, and vice versâ. Each molecule as it makes contact with a molecule of the opposite surface diminishes, by so much relatively to the number of molecules previously in contact, the resistance between the surfaces. Each molecule as it breaks from contact with its opposite neighbour adds to the resistance between the contact-surfaces. It may therefore be that the variations of resistance which are observed at contacts between all conductors, from the best to the worst, are all made up, though they appear to pass through gradual and continuous changes, of innumerable minute makes-and-breaks of molecular contact. The very minuteness of each molecular make-or-break, and the immense number that actually must occur at every physical “point” of contact, explain why the effect seems to us continuous. We owe, moreover, to Mr. Edison[46] the experimental proof that actual abrupt makes-and-breaks of contact can produce an undulating current when they recur very rapidly. Whether the heating action of the current itself may not also operate in changing the conductivity of the molecules which happen at the moment to be in contact is another matter. It may be so; but if this should hereafter be demonstrated, it will but confirm the contact-theory of these actions as a whole.
Assuming, then, broadly, that the observed resistance at a point of contact is due to the number of molecules in contact and to their individual resistances, it is evident that the property of varying resistance at contact ought to be most evident, ceteris paribus, in those substances which are the best conductors of electricity. Unfortunately, the cetera are not paria, for the question of fusibility comes in to spoil the comparison; and carbon, which has less fusibility than the metals, is commonly credited with giving a better result than any. This common opinion is, however, based on comparisons made without taking into consideration the question of range of motion between the parts in contact, and without taking into consideration the point that whilst some forms of carbon are excellent conductors, others do not conduct at all. In a telephonic transmitter so arranged that the actual range of motion shall be very small, the metals are just as good as carbon—some of them better. I have heard from a transmitter with contacts of pure bright silver better articulation than with any carbon transmitter. And this is exactly what theory would lead one to expect. As to the suggestion that plumbago makes a successful transmitter, because it is a “semi-conductor”—whatever that term may mean[47]—it is one of those suggestions which are peculiarly fitted to catch the unscientific mind as affording an easy explanation for an obscure fact; unfortunately, like a good many other similarly catching suggestions, it is not true. The very best conductor—silver—will serve to transmit articulate speech: and so will the one of the very worst conductors—lamp-black! So much for this fallacious doctrine of semi-conductors!
Reis used for his contact-points substances which, by reason of their non-liability to fuse or oxidize, were customary in electrical apparatus, and chiefly platinum. In his earliest transmitter (model ear), and in his last, platinum was used. In his lever-form of transmitter, so minutely described by von Legat, the material is not specified. The lever-shaped contact-piece was to be a conductor, and as light as possible, and since all metallic parts are particularly described as metallic, whilst this is not so described, the obvious inference is that this was non-metallic. The number of light, non-metallic conductors is so few that the description practically limits choice to some form of hard carbon. No other materials are named by Reis, but Pisko says (p. 103) that brass, steel, or iron might be used for contacts. Any one of these materials is quite competent, when made up into properly-adjusted contact-points, to vary the resistance of a circuit by opening and closing it in proportion to the vibrations imparted to the contact-points. That is what Reis’s transmitter was intended to do, and did. That is what all the modern transmitters—Blake’s, Berliner’s, Crossley’s, Gower-Bell’s, Theiler’s, Johnson’s, Hunning’s do, even including Edison’s now obsolete lamp-black button transmitter. Mr. Shelford Bidwell has very well summarized the action of the current-regulator in the following words: “The varying pressure produces alterations in the resistance at the points of contact in exact correspondence with the phases of the sound-waves, and the strength of a current passing through the system is thus regulated in such a manner as to fit it for reproducing the original sound in a telephone.”
Reis constructed an apparatus consisting of a tympanum in combination with a current-contact-regulator, or “interruptor,” which worked on this principle of variable contact, and he called it “The Telephone” (see pp. 57, 85). The very same apparatus we now-a-days call a “Telephone-transmitter,” or simply a “transmitter.” It is curious to note that Reis seems to have regarded his receiver or “reproducing-apparatus” as no new thing. He says explicitly (p. 56) that his receiver might be replaced by “any apparatus that produces the well-known galvanic tones.” “The Telephone” was with Reis emphatically the transmitter. Bell in 1876 invented an instrument which would act either as transmitter or receiver, and which, though never now used as transmitter, is still called “a Telephone.” Edison’s “sound-telegraph,” or “telegraphic apparatus operated by sound,” was patented in 1877. In his specification he never called his transmitter a “telephone;” that name he reserved exclusively for his receiver. He found it, however, convenient a year later to rechristen his transmitter as the “carbon telephone,” though throughout the whole of his specification neither “carbon” nor “telephone” are mentioned in connection with the transmitter! Within that year Hughes had brought out another instrument—“The Microphone”—which, like Reis’s instrument, embodied the principle of variable contact. Hughes’s instrument, usually constructed with contacts made of loose bits of coke-carbon, was simply a Reis’s Telephone minus the circular tympanum; and the really important new fact it revealed, was that very minute vibrations, such as those produced by the movements of an insect, when transmitted immediately through the wooden supports, sufficed to vary the resistance of a telephonic circuit, though far too slight in themselves to affect it if they had to be first communicated to the air and then collected by a tympanum. Put a specific tympanum to a Hughes’s microphone, and you get a Reis’s telephone. Take away the tympanum from a Reis’s telephone, and you get a Hughes’s microphone. Hughes is not limited to one material, nor is Reis. But the fundamental principle of the electrical part of each is identical. The Blake transmitter ([Fig. 44]), and the Berliner transmitter, and also Lüdtge’s microphone,[48] which was even earlier than that of Hughes, are all embodiments of the same fundamental principle of variable contact which Reis embodied in his “Telephone.”