128 Dr. W. H. Eccles on an Oscillation Detector 



was found to contain 5*05 x 10" 5 gram thorium. As the 

 entire 50 grams should contain 16*3 x 10~ 5 gram thorium, 

 and the soluble part was found to be just 71 per cent, of the 

 rock, there is here a considerable concentration in the 

 insoluble residue. 



It may be remarked as regards the geological significance 

 of: these results, that the calcareous rocks have been estimated 

 as constituting about 5 per cent, of the bulk of the total 

 sedimentaries upon the land*, and although somewhat higher 

 estimates have been made, they certainly constitute a small 

 fraction of the sedimentaries. 



Soils derived from such rocks consist largely of the in- 

 soluble residues, and hence the influence of the surface 

 materials in ionizing the atmosphere over calcareous districts 

 cannot be directly inferred from such results as the fore- 

 going. 



Geological Laboratory, Trinity College, Dublin, 

 May 13, 1910. 



XI. On an Oscillation Detector actuated solely by Resistance- 

 temperature Variations. By W. H. Eccles, D.Sc.f 



IN a recent communication to the Physical Society J the 

 properties of a type of iron-oxide coherer were discussed. 

 The paper described experiments on coherers made by dipping 

 a slightly oxidised iron wire into clean mercury, or by 

 pressing a fine iron wire against a thinly oxidized iron plate, 

 and the results were discussed mathematically. It was shown 

 that in the case of the iron point and oxidized plate the 

 whole of the experiments could be explained qualitatively on 

 the assumption that the only electrical phenomena at play 

 were the Joule effect and the resistance-temperature changes 

 in the small mass of oxide between the metal electrodes. 

 The hypothesis that was put forward must be summarized 

 here. Let p be the resistance of that part of the detector 

 where the current flow is so constricted that the Joule effect 

 produces rise of temperature, and r the resistance of the 

 remainder of the circuit. The resistance p is usually localized 

 at the contact of the conductors that form the detector, and 

 it varies with the temperature of the minute mass of matter 

 at the contact ; let a be the coefficient of decrease of resistance 

 with rise of temperature. The resistance r includes that of 

 the bulk of the substances forming the detector, the leads 



* Van Hise, ' Treatise on Metarnorphism,' p. 940. 

 f Communicated bv the Physical Society : read May 27, 1910. 

 \ W. H. Eccles, "On Coherers," Phil. Mag. June 1910; Proc. Phys. 

 Soc. vol. xxii. 1910. 



