Copper, Gold, and Silver.
Copper required a much smaller E.M.F. The sensitiveness was only moderate.
Gold was more difficult to adjust, but the action is a little stronger.
Silver.—The receiver was extremely unstable. It exhibited sometimes a diminution and at other times an increase of resistance.
It will be seen from the above that all metals exhibit contact sensitiveness to electric radiation, the general tendency being towards a diminution of resistance.
The most interesting and typically exceptional case, however, is the receiver made with potassium, which not only exhibits an increase of resistance by the action of radiation, but also a remarkable power of self-recovery. In the accidental instances of increase of resistance exhibited by other metals, an increase of pressure or E.M.F. generally brought the coherer to the normal condition, which showed a diminution of contact resistance by the action of electric waves. With potassium I gradually increased the pressure till the receiver grew insensitive. All along it indicated an increase of resistance, even when one piece was partially flattened against the other. I increased the E.M.F. many times the normal value; this increase (till the limit of sensitiveness was reached) rather augmented the sensibility and power of automatic recovery. I allowed the receiver a period of rest, the nature of response remaining the same. As far as I have tried, potassium receivers always gave an increase of resistance, a property which seems to be characteristic of this metal, and to a less extent, of the allied metals.
It will thus be seen that the action of potassium receiver is not, strictly speaking, a cohering one. For it is difficult to see how a cohering action and consequent better contact could produce an increase of resistance. It may be thought that the sudden increase of current may, by something like a Trevelyan rocker action, produce an interruption of contact. But such a supposition does not explain the instantaneous action, and the equally instantaneous recovery.
In arranging the metals according to their property of change of contact resistance, I was struck by the similarity of action of electric radiation on potassium in increasing the contact resistance, and the checking action of visible radiation of the spark discharge. In the latter case too potassium is also photo-electrically the most sensitive. But the action is confined to visible radiation, and is most efficient in the ultra-violet region. I was indeed apprehensive that the action on potassium receiver which I observed might be in some way due to the ultraviolet radiation of the oscillatory spark. But this misgiving was put to rest from the consideration that the receiver was placed in a
