Investigation of the Simple Coherer. 287 



After reading the above paper, I looked for reversal, and 

 frequently found rapid changes in resistance of the contact ; 

 thus on one occasion, on sending successive sparks, the 

 following voltmeter readings were recorded : — 



1'42 



0*5 



1-42 



1-42 



0-26 



0-3 



0-28 



0-20 



0*32 



1-42 



0-5 



0-24 



Further sparks produced no change, tbe voltmeter remaining 

 at 0*24. 



T. Mizimo* has recently made a careful study of the 

 change in resistance of coherers made of filings (in one case 

 he used two lead balls touching) as successive sparks were 

 received. This individual-spark method, which I also used, 

 is the obvious way to measure off radiation and to watch its 

 quantitative effect. A set of curves are given showing the 

 relation of number of sparks to resistance. The general 

 tendency after the first spark is for the resistance to fall to a 

 fairly constant value ; but in bismuth (specially), zinc, iron, 

 and antimony the resistance-curve rises distinctly. 



Bismuth gives peculiarly high resistances, confirming my 

 own results ^see later). 



The inference to be drawn from the whole paper seems to 

 be that the bridge formed by the first spark is as a rule 

 strengthened (by acquiring new particles or by molecular 

 change ?) by succeeding sparks, but that for a few metals the 

 bridge is weakened by some contrary action. 



It will be seen in the above summary (1) that most writers 

 agree that a protecting film of some sort exists at the surface ; 

 but whereas they all seem to consider an oxide actually 

 formed, it appears to me that a layer of condensed air might 

 answer all the requirements in some, at any rate, of my own 

 experiments : (2) that many writers uphold, in some cases on 

 direct experimental evidence, that a bridge of particles is laid 

 between the metal surfaces : (3) that a strictly molecular 

 change occurs in the metal is upheld, with ample evidence^ 

 by Bose and by him alone ; but he seems j to have paid little 

 attention to points (1) and (2). 



My own views are expressed at the end of this paper. 



* Phil. Mag. Nov. 1900. 



t Principal Lodge writing, partly in reference to the work of Trof . 

 Bose, in a letter to the ' Electrician,' Oct. 12, 1900, says :— " There 

 seems a tendency to regard the metallic coherence as a theory, whereas 

 it is a fact." He goes on to say that the coherence can be Jelt-, it can 

 also be seen when particles are made to hold together in a string. 



