[ SM ] 



XCI. On Coherers. By W. H. Eccles, D.Sc, A.R.C.S* 



T is well known that an imperfect contact between almost 



i 



any pair of! good electrical conductors suffers a change 

 of resistance when electrical oscillations of sufficient intensity 

 occur in the circuit containing the contact. Prof. Branly, in 

 1896, concluded after much investigation that the effects 

 observed were due to a modification of the electrical con- 

 ductivity of the film of dielectric that happened to separate 

 the conductors. For example, a mass of metal filings melted 

 down in resin showed appreciable alteration of resistance when 

 a spark discharge occurred near it ; and a pair of pieces of 

 metal in very light contact, yet separated by their surface 

 films of condensed gas, showed the phenomenon excellently. 

 Sir Oliver Lodge at that date held the view that the alteration 

 of resistance, usually a diminution, was due to the piercing of 

 the dielectric and a consequent cohesion of the metals. He 

 gave the name " coherer " to instruments exhibiting or 

 utilising the phenomenon, and this name has been uni- 

 versally adopted and the suggested explanation widely 

 accepted. It has often been pointed out that Lodge's 

 explanation is inapplicable to known cases where the resist- 

 ance of the coherer is increased by the passage of electrical 

 oscillations. In the present paper it will be shown that 

 there is no need to adopt the " cohesion " hypothesis, even 

 in the case of typical coherers where the resistance is lowered 

 by the action of electrical oscillations. 



Ten years ago the known ways of investigating a coherer 

 required the device to submit itself to powerful electric 

 discharges. We find investigators of filings coherers watching 

 the sparks between the filings and boasting of fusing the 

 filings into chains of quite considerable tenacity. But the 

 engineers who were at that date nursing the infant wireless 

 telegraphy were, on the whole, more gentle with the coherer 

 than were the men of science. At most, the engineers 

 required that the current through the coherer when it 

 "cohered" should work a delicate relay. A few years 

 later they merely asked that the coherence current should 

 move the diaphragm of a telephone receiver. The oscillatory 

 voltages that need be applied to a coherer in order to produce 

 a resistance change perceptible by an ear at the telephone 

 are very small indeed ; and in these cases a good coherer 

 spontaneously and very perfectly recovers its original re- 

 sistance when the oscillations cease. The " cohesion " 

 explanation appears improbable here, though in the cases 



* Communicated by the Physical Society: read March 11, 1910. 



