TRANSACTIONS OE THE SECTIONS. 33 



allowed to remain in the gutta percha coverings of a submarine conductor, so long will 

 their insulation fail by slow degrees." 



Great improvements have of late been effected, which may be estimated by the 

 fact that the covering of the Rangoon and Singapore cable, now in process of manu- 

 facture, insulates ten times better if reduced to the same thickness of coating than 

 the covering of the Red Sea and India cable did before it was laid ; and these 

 marked improvements are due to the greater care used by the Gutta Percha Com- 

 pany, assisted by stringent electrical tests which the authors are charged by the 

 British Government to apply. 



The chief characteristic of these tests is, that the conductivity of both the con- 

 ducting wires and the surrounding coating, which is regarded in the light of an in- 

 ferior conductor, is expressed in numerical units, capable of direct comparison. The 

 unit of resistance adopted is that of a column of mercury, I metre in length and of 

 one square millimetre sectional area, taken at the freezing-point of water (as de- 

 scribed by Werner Siemens in Poggendorff's ' Annalen,' vol. ex.). In expressing 

 the degrees of conductivity of both the wire and the insulating medium in definite 

 units of resistance, not only the advantage of a more accurate comparison between 

 the results of different indication is obtained, but subsequently, when the separate 

 coils are united together to a single cable, it affords an admirable means of judging 

 its electrical condition in comparing the total resistances of both the conductor and 

 insulating medium with the sum of the resistance previously obtained in testing each 

 coil separately ; but the principal advantage derived from this system of measuring, 

 consists in the facilities it affords in determining the position of a fault in a cable 

 while it is being laid and after submersion. In carrying this system into practice, 

 MM. Siemens constructed coils of definite resistance variable from 1 to 50,000 units 

 of resistance. 



The cables to be tested are placed for twenty-four hours in water regulated to 

 75° F. ; they are then removed into the testing tank of the same temperature, which 

 is hermetically closed, and hydraulic pressure of at least 600 lbs. per square inch 

 applied, in order to force the water into the cavities or fissures that may present 

 themselves. 



It is a remarkable fact, which is also borne out by observation upon cables in 

 process of submersion, that the application of hydrostatic pressure sensibly decreases 

 the conductivity of gutta percha ; which, however, increases again slightly beyond 

 the former rate when the pressure is relieved. 



For a full description of the methods of testing employed, we must refer our 

 readers to the paper itself. 



The authors give a description of a new instrument by means of which they test 

 the inductive capacity of cables, which has also to be accurately ascertained for the 

 purpose of detecting faults ; and have affixed a Table containing many satisfactory 

 results, and proving the correctness of a formula for calculating the specific induc- 

 tion of cables, which was obtained by Professor Thomson and M. Werner Siemens 

 by different scientific deductions. 



The specific inductive capacity of all gutta percha is shown to be nearly the same, 

 and to be entirely independent of the specific conductivity of the gutta percha ; while 

 India-rubber and Wray's mixture are far inferior in specific inductive capacity, being 

 equal to 07 and0 - 8 respectively, gutta percha being taken = 1. 



In this way the cable is examined repeatedly at the earliest stages of its manu- 

 facture, in lengths of one knot, during the joining and covering of the cable, and 

 finally during the paying out. 



The paper next gives a full description of the electrical tests to be applied during 

 the paying out, and numerous formulae by means of which faults in the cable are 

 ascertained under various circumstances. By these means Messrs. Siemens were 

 enabled to determine with great accuracy faults in the Indian cable, both during the 

 paying out and afterwards, which enabled the contractors, Messrs. Newall and Co., 

 to effect the necessary repairs with a certainty which could not formerly be obtained. 



Respecting the prospects of success of new lines of submarine cables, the paper 

 states that, owing to the great care used, the conductor of the Rangoon and Singa- 

 pore cable is fully ten times more perfectly insulated than the best cable hitherto 

 submerged j and that it mav confidently be expected that the result in practice will 



1860. 3 



