872 



Dr. W. H. Eccles on Coherers. 



for many years — and chosen for these experiments after 

 careful comparisons with a number of similar coherers of 

 more modern vogue. As is well known, almost any con- 

 ductor that can be coated with a closely adherent, tenacious, 

 and very thin film of solid or liquid of not too high re- 

 sistivity makes a good coherer : for example, oxidized or 

 snlphided copper against any solid conductor ; oxidized iron, 

 steel, nickel, tantalum, or any similar metal, against mercury 

 or other conductor are all good *. I have tried also iron- 

 mercury coherers made by the action of organic acids and 

 other reagents on iron. For example, even the graphite (?) 

 films left on a steel knife that has been unwisely used as a 

 fruit-knife make coherers. Amongst all these and others 

 I have found that the best for my present purposes is the 

 coherer made by dipping the end of a carefully oxidized fine 

 iron wire or steel needle into a pool of clean mercury. It is 

 important that the layer of oxide on the wire should be of 

 uniform very small thickness. Seven or eight years ago 

 I found that this can be accomplished very well by heating 

 the wire in a glass tube carrying steam from a boiling flask. 

 The wire should be watched till the film of oxide yields one 

 of those interference colours familiar in the tempering of 

 steel. For instance, a blue film makes a splendid coherer, 

 which works well for a month or two in the open air. Most 

 of the experiments discussed below were carried out on 

 -coherers made in this way. 



Details of the Apparatus. 

 Fig. .1 is a diagrammatic plan of the apparatus. The 

 primary circuit consists of the coil L/ and condenser C ; the 



Fig. 1. 



4r Pi 



zn. 



J" 



y 



M 



^>T 



secondary of L and C The detector D and condenser K are 



arranged shuntwise to the condenser C. C and C are variable 



* See Branly, Comptes Rendus, cxxxiv. pp. 347, 1197 (1902). 



