\//i' \VlI.l.lA.\f SIEMENS, F.R.S. 67 



degree as india-rubber. The accompanying table shows the re- 

 spective non-conducting or insulating power of gutta-percha, 

 india-rubber, and Wray's mixture, which last is a compound of 

 india-rubber with shellac and pounded flint ; and of the two 

 latter substances combined : 



SPECIFIC NON-CONDUCTING AND INDUCTIVE POWER OF GUTTA- 

 PERCHA, INDIA-RUBBER, &c. 



The great superiority of india-rubber and its compounds over 

 gutta-percha in insulating power is at once apparent, india-rubber 

 itself being 16 times better than gutta-percha as a non-conductor 

 at a temperature of 52, and 70 times better at 92 ; and the 

 combination of india-rubber and Wray's mixture is on the average 

 as good a non-conductor as india-rubber, while its inductive 

 power, which causes retardation of the electric current in its 

 passage along the wire, is only three quarters that of gutta-percha. 

 To these advantages the greater tenacity of india-rubber and its 

 greater power to resist heat have to be added. 



India-rubber was tried for the purpose of insulating tele- 

 graph conductors more than twenty years ago, when it was 

 employed by Jacobi of St. Petersburg for underground telegraphic 

 lines. In 1846 Dr. Werner Siemens employed it for the same 

 purpose, previous to his application of gutta-percha. About the 

 same time india-rubber was put to the same use in this country, 

 and it is said remains still in good condition in Portsmouth 

 harbour. There is nothing new therefore in substituting india- 

 rubber and its compounds for gutta-percha in insulating sub- 

 marine or other telegraph conductors : the present paper has 

 special reference to a new method of effecting the covering. The 



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