.S7A' WILLIAM SIEMENS, F.R.S. 49 



It was necessary for the purpose to determine in the first place 

 the specific conductivity of the material which experience has 

 proved to be sufficiently uniform at constant temperatures. 



The effect of temperature upon the conductivity of gutta-percha 

 and other insulators has lately been fully investigated by the 

 Scientific Telegraph Committee of the British Government, whose 

 report is however not yet published. 



It suffices for our present purpose to state that between the 

 limits of 41 and 80 Fahrenheit we found the conductivity of the 

 insulating covering of the Rangoon and Singapore Cable to 

 increase nearly in the ratio of 1 to 7. The ratio of this enormous 

 increase is, however, by no means constant, and in the absence of 

 very elaborate and reliable experimental results we thought it 

 advisable to test at a uniform temperature of 75 Fahrenheit 

 (20 Cent.) This comparatively high degree of temperature has 

 the advantage that it is seldom exceeded naturally, and that the 

 conductivity being seven times greater at that temperature than 

 at the winter temperature of 41, the effect of minute faults 

 upon the measuring instrument will also be proportionately 

 exaggerated. 



In order to insure uniformity of temperature the coils to be 

 tested are placed for twenty-four hours in tanks containing water 

 regulated to 75 ; they are then removed into the testing tank of 

 the same temperature, which is hermetically closed, and hydraulic 

 pressure of at least GOO Ib. 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 borne out by 

 observation upon cables in process of submersion, that the applica- 

 tion of hydrostatic pressure sensibly decreases the conductivity of 

 gutta-percha, which however increases again slightly above the 

 former ratio when the pressure is relieved. 



In slightly defective coils the increase of external pressure 

 produces, on the contrary, no increase, or even a decrease of 

 insulating property, and a clue is thus obtained to ascertain other- 

 wise inappreciable defects. The methods usually employed of 

 measuring the conductivity and insulation of conductors in degrees, 

 by simple galvanometer tests, would be insufficient for the purposes 

 here intended. 



It was necessary to express the conductivity of both the con- 



VOL. II. E 



