202 Prof. W. Thomson and Mr. F. Jenkin on the True and 



assertion were, (1) M. Siemens himself assumes in his first paper 

 that the conducting power of his copper (etalon) varies 0*1 per 

 cent, with each degree Centigrade; and (2) M. C. W. Siemens*, 

 in describing his resistance thermometer, assumes also the same; 

 in fact he bases his calculation on Arndtsen's formula without 

 stating the sort of copper he uses. 



That my statement regarding the difference of the coefficients 

 of the increase of resistance for different temperatures of coppers 

 is correct, maybe deduced from the following data: — M. Siemens 

 finds (2nd paper) the resistances of a commercial copper he 

 tested to vary between 0° and 100° C. 32-9 per cent. ; Arndtsen 

 finds copper containing traces of iron to vary 36 per cent.; von 

 Bose and myself have found pure copper to vary 42 per cent.; 

 and lastly, one commercial copper I have tested varies only about 

 8 per cent. 



XXV. On the True and False Discharge of a Coiled Electric 

 ' Cable. By Professor W. Thomson, LL.D., F.R.S., and Mr. 

 Fleeming Jenkin, C.E.f 



IN an article in the last Jvlay Number of this Magazine, " On 

 the Galvanic Polarization of buried Metal Plates," translated 

 from PoggendorfFs Annalen, No. 10, 1860, Dr. Carl describes 

 certain interesting experiments on the electro-polarization pro- 

 duced between two large zinc plates buried in the garden of the 

 Observatory of Munich, by opposing and by augmenting the 

 natural earth-current between them by the application of a single 

 element of DanielPs; and concludes with the following re- 

 mark : — 



" The above experiments disclose nothing at variance with the 

 known laws of galvanism; but it nevertheless appeared to me ad- 

 visable to make them known, as they afford a simple explanation 

 of certain phenomena which Professor Thomson has described 

 (Report of the Twenty-ninth Meeting of the British Association, 

 Aberdeen, 1859, Trans, of Sections, p. 26), and which he seems 

 to attribute to entirely different causes." 



In the report of Prof. Thomson's communication to the Bri- 

 tish Association here referred to, it is stated that (after mention- 

 ing certain experiments by Mr. F. Jenkin on submarine cables 

 coiled in the manufactory of Messrs. Newall and Co., Birkenhead, 

 in which one end of the battery used, and one end of the cable 

 experimented on, in each case was kept in connexion with the earth 

 while the other end of the cable, after having been for a time in 



* Phil. Mag. January 1861. 



t Communicated by the Authors 



