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!