(_b_) To a change in the resistance of the substance of the conductor itself. (_c_) To the formation of a minute voltaic “arc,” or electric discharge. (_d_) To the change in the thickness of the intervening film of air. (_e_) To the change in resistance of the parts in contact consequent on the evolution of heat by the current. It is admitted that this last suggestion, though it might account for a difference between different substances, in so far as they differ from one another in the effect of heat upon their specific resistance, implies as a preliminary fact that the amount of surface in contact shall be varied by the pressure. No convincing proof has yet been given that the alleged layer of air or other gases has any real part to play in the phenomena under discussion. Nor can the hypothesis, that minute voltaic arcs are formed at the contact be regarded as either proven or probable. The only two theories that have really been investigated are (_a_) and (_b_) of the above series. Of these two (_b_) is certainly false, and (_a_) is probably, at least to a very large extent, true. It is often said by persons imperfectly acquainted with the scientific facts of the case, that carbon is used in telephone-transmitters, because the resistance of that substance varies with the pressure brought to bear upon it, whilst with metals no such effect is observed. This statement, taken broadly, is simply false. Mr. Edison has, indeed, laid claim to the “discovery” (_vide_ Prescott’s ‘Speaking Telephone,’ p. 223), that “semi-conductors,” including powdered carbon and plumbago, vary their resistance with pressure. All that Mr. Edison did discover was that certain substances, whose properties of being conductors of electricity had been known for years, conducted better when the contact between them was screwed up tightly than when loose. The experiments made to test this alleged “property” of carbon are absolutely conclusive. The author of this book has shown[39] that when a rod of dense artificial coke-carbon, such as is used in many forms of telephone transmitters, such as Crossley’s for example, is subjected to pressure varying from less than one dyne per square centimetre up to twenty-three million times that amount, the resistance of the rod did not decrease by so much as one per cent. of the whole. In this case any doubt that might have been introduced by variable contact was eliminated at the outset by taking the precaution of electro-plating the contacts. In 1879, Professors Naccari and Pagliani, of the University of Turin, published an elaborate series of researches[40] on the conductivity of graphite and of several varieties of coke-carbon, and found, even with great changes of pressure, that the changes of electric resistance were practically too small to be capable of being measured, and that the only changes in resistance appreciable were due to changes of contact.