SUBMARINE TELEGRAPHY 205 



In 1857, in a lecture delivered to the British Association at 

 Dublin, he pointed out that a so-called insulator was really a 

 conductor of enormous resistance ; that this resistance, though 

 large, was measurable in terms of the same units as measured 

 the resistance of conductors, and he then gave an estimate that 

 the gutta-percha of the first Atlantic Cable had a specific resist- 

 ance twenty million million million times greater than that of 

 copper at about 24 C. At his suggestion Mr. Fleeming Jenkin 

 made systematic measurements of the resistance of the insulating 

 sheath of the Red Sea Cable ; and, independently, Dr. Siemens 

 of Berlin had made similar arrangements for those measurements 

 during the submersion of the cable. Unfortunately this cable 

 was not tested under water, and these tests were therefore of 

 little use, except to determine the properties of gutta-percha. 

 Since 1859, every important cable has been tested on a similar 

 system. The methods used have varied, but they have always 

 resulted in determining the resistance per knot of the insulator. 

 Attention has been paid to the temperature, any rise in which 

 rapidly diminishes the resistance of gutta-percha. The necessary 

 allowance for the different dimensions of various cables has also 

 been made, and no test is now counted of any value unless made 

 under water. The result is that definite numerical results are 

 obtained, comparable one with another, whatever be the dimen- 

 sions, length, or temperature of the cable, and whatever be the 

 variations in the batteries or galvanometers employed. The 

 work of one day is comparable with that of another ; the re- 

 sults obtained in various factories, and by various engineers, are 

 all comparable, and no considerable variation in the resistance 

 of the insulator, such as would be caused even by a small fault, 

 can possibly escape detection. The improvements in the tests 

 have here also been followed by a great improvement in the 

 quality of the materials, as well as by increased security against 

 faults. The specific resistance of the gutta-percha of last Atlantic 

 Cable is twelve-fold that of the Eed Sea gutta-percha ; and at 

 24 C. may be roughly said to be 200,000,000,000,000,000,000 

 times that of copper (referred to equal dimensions). 



It is difficult to find any comparison which will give a 

 tolerably clear idea of the extraordinary difference between the 

 electrical resistance of these" two materials: it is about as great 



