According to Edison (British Patent, No. 792, 1882), compressed graphite is a substance of great conductivity. According to Faraday (‘Exp. Res.’ vol. i. p. 24), retort-carbon is an excellent conductor. Both graphite and retort-carbon agree with the metals in the property that the electric resistance offered at a point of contact between them varies when the pressure at the contact is varied. It is indeed remarkable through what wide ranges of resistance the contact between two good conductors may vary. The resistance of contact between two pieces of copper may be made to vary in a perfectly continuous manner by changes of pressure through a range, according to Sir W. Thomson, from a small fraction of one ohm, up to a resistance of many thousand ohms. The same is true of silver, brass, and many other good conductors, including graphite and retort-coke, though with the latter materials the range of resistances is not so great. With partial conductors, such as oxide of manganese, sulphide of copper, sulphide of molybdenum, &c., and with bad conductors, such as lamp-black and selenium, whose conductivity is millions of times less than that of graphite, copper, and other good conductors, it is impossible to get equally wide variations of resistance, as the amount of pressure at a point which will bring the bad conductors into intimacy of contact, will not turn them into good conductors. Platinum being in the category of good conductors, is amongst those substances which yield a very wide range of electrical resistances at the contact-points which are submitted to varying pressures.
With the very highest conductors, such as silver and copper, the electrical range of contact-resistance is higher than with those of lesser conductivity, such as lead, platinum, graphite, and retort-coke.
But though the range of variation in electrical resistance at contacts is highest for the best conductors, there comes in another element, namely, the range of distance through which the contact-pieces, or either of them, must be moved in order to pass through the range of variations of resistance. This is quite a different matter, for here the best conductors have the smallest range, and some that are not so good a greater range. In any case the available range of motion is very small—to be measured in minute fractions,—millionth-parts, perhaps,—of an inch. So far as experiments go, however, silver has the smallest range of all, then gold, then copper. Platinum and nickel have a considerably wider range, plumbago and retort-coke a still wider one.
It is an extremely difficult matter to decide what is the precise nature of that which goes on at a point of contact between two conductors when the pressure at the point is altered. The principal suggestions hitherto advanced have been that the change of resistance observed is due:—
- (a) To the mere changes in the amount of surface in contact.
- (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.
In January 1882, Mr. Herbert Tomlinson communicated to the Royal Society[41] the results of experiments on a number of electric conductors. The change of conductivity by the application of stress was found to be excessively small. For carbon it was less than one-thousandth part of one per cent. for an increase of fifteen lbs. on the square inch in the pressure. For iron it was slightly greater, and for lead nearly twice as great, but with all other metals less. If this alleged property were the one on which the action of telephone transmitters depended, then lead ought to be twice as good a substance as graphite; whereas it is not nearly so good.