870 Dr. W. H. Eccles on Coherers. 



familiar ten years ago, where the alteration of resistance 

 is permanent, that explanation seemed plausible. 



Within the past six years the problem of the coherer has 

 often been attacked, and each observer has in turn found all 

 the theories of the action of coherers insufficient to explain 

 his experiments, and has added some suggestion of his 

 own. For example, A. Blanc * in 1904 concluded that 

 neither oxide nor condensed gases played a part in certain 

 coherers, and believed that particles of the two surfaces 

 diffuse at a rate depending on the current, temperature, and 

 pressure. On the other nancl, Robinson f decided that in 

 single point coherers the resistance is that of an elastic film 

 of oxide, and that permanent coherence implied injury or 

 thickening of the oxide ; while Huth J attributes coherence 

 to the ionization of the film rather than to the thickening of 

 the oxide. From the point of view of the results of some 

 of the experiments to be described in this paper, the obser- 

 vations of A. H. Taylor § are of special interest. He found 

 that the current through a single point steel coherer shows 

 three stages as the gap is diminished. In the first stage 

 ihere is a feeble leakage current clue, he thinks, to vapour 

 conductivity ; in the second, conduction due to metallic 

 ions (in this stage a small rise of E.M.F. produces a great 

 increase of current) j in the third stage ordinary conduction 

 occurs. In a paper entitled " Recherches sur les Contacts 

 Impnrfaits" (Journ. de Physique, ser. 4, iii. 1904, p. 350), 

 A. Fisch describes his experiments on the voltage-current 

 curves for a coherer consisting* of two spherical surfaces 

 of steel whose separation was capable of fine adjustment. 

 He found, from a study of numerous curves, that the average 

 curve exhibits three stages : — (1) As the current is raised 

 the potential difference across the contact increases more 

 and more slowly to a critical value constituting stage (2), 

 where, though the current increases, the potential difference 

 remains constant. Then (3) increasing current is accompanied 

 by continually rising potential difference. This curve of 

 Fisch's is precisely the curve II. of fig. 8, below. Besides 

 these investigators there are others who have advocated that 

 the electrostatic attraction between the conducting surfaces 

 may account for the phenomena. 



Very few, if any, experiments with electrical oscilla- 

 tions of very minute measured voltages have so far been 



* Science Abstracts, vol. viii. 1923, 2248. 



t Science Abs. vol. vii. No. 1135; Ann. d. Phys. xi. 4, p. 754 (1903). 

 | Science Abs. vol. vii. No. 1136 ; Phys. Zelt. iv. p. 594 (1903). 

 § Science Abs. vol. vii. No. 157 ; Phvs. Review, xvi. p. 199 (1903). 



