350 THE ELECTRIC TELEGRAPH. 



arid at least an equal length has been lost in the same form, 

 partly by failure in the submersion, principally by the deve- 

 lopment of faults after the lapse of time. 



73. Tests of the Finished Core at the Gutta-percha Works. 

 The insulated conductor, for some days after it has left the 

 last covering machine, has a lower insulation than when the 

 material has become consolidated by age. A peculiarity of 

 gutta-percha, which it is necessary to take into account in 

 measuring the insulation resistance of a cable at different 

 temperatures, is that the conducting power of this material 

 does not immediately alter with the temperature, but follows 

 it leisurely. Mr. Yarley observed this to be the case with 

 the Atlantic cable at Greenwich ; and we have repeatedly 

 observed also that, in changing the temperature of the water 

 in which cable- core is immersed, the altered resistance of the 

 copper conductor will indicate the change to a nicety some 

 time before the gutta-percha is in the least affected by it. 

 For this reason the core which is made at the Wharf Road 

 Gutta-percha Works, is kept in a bath at an uniform tem- 

 perature of 24 C., for a period of twenty- four hours before 

 it is submitted to the electrical tests. Were it allowed to be 

 in at varying temperatures, the numbers obtained for insu- 

 lation resistance at any moment might easily express the 

 resistance due to a temperature which the cable had had hours 

 before, and be quite wrong for its observed temperature. 



The core is submerged either in hanks or coiled upon iron 

 drums, according to its size, and is allowed to remain in the 

 water for the prescribed twenty- four hours, during which 

 time the water is frequently stirred to equalise its tempera 

 ture. The lengths of the hanks or coils tested are between 

 one and two knots, in which lengths the electricians have 

 to measure its conducting power of copper and insulating 

 capacity. 



74. When a cable is submerged in the sea, it is evident 

 that it becomes subject, in every point, to the presence of a 

 superincumbent column of water. In order to produce 

 during the testing, as nearly as possible, the conditions under 

 which a cable is placed when submerged, Mr. Reid conceived 



