Study of the Cohering Action of different Metals. 171 



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 some- 

 times a diminution and at other times an increase of resistance. 



It will be seen from the above that all metals exhibit contact sensi- 

 tiveness 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 diminu- 

 tion of contact resistance by the action of electric waves. With potas- 

 sium I gradually increased the pressure till the receiver grew insensi- 

 tive. 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 sensitive- 

 ness was reached) rather augmented the sensibility and power of auto- 

 matic 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 in- 

 crease 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 on the spark discharge. In the 

 latter case too potasium. 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 

 ultra-violet radiation of the oscillatory spark. But this misgiving was 

 put to rest from the consideration that the receiver was placed in a 



