SUBMARINE TELEGRAPHY 207 



large tanks ; the resistance measurements are repeated daily, 

 and the results compared with those calculated from the length 

 and temperature of the cables. The effects of an increase of 

 temperature in diminishing the resistance of gutta-percha have 

 been separately examined by Messrs. Siemens, Mr. F. Jenkin, 

 and Messrs. Bright and Clark. The results of the various ex- 

 periments agree very closely. One curious phenomenon de- 

 serves mention : the apparent resistance of insulators increases 

 materially while the battery is applied to them, and it is there- 

 fore necessary to note the time at which the observation is 

 taken. In the earlier cables even this fact escaped notice. This 

 extra resistance is said to be due to electrification ; it ceases 

 gradually after the copper conductor has been discharged by 

 being maintained in electrical connection with the earth, or 

 with the opposite pole of the battery, but in the latter case it 

 reappears as before, increasing as the application of the battery 

 is prolonged. Its cause is not understood. It seems to be a 

 kind of electrical absorption, and is first mentioned by Faraday 

 in experiments on induction. 



Enough has been said to explain the care and accuracy with 

 which the insulation of a cable is now measured. The results 

 obtained may be understood from the following facts. Not one- 

 third per cent, of a current entering either the 1865 or 1866 

 Atlantic Cables is lost by defective insulation before reaching 

 Newfoundland. Such loss as does occur indicates no fault, but 

 is simply due to the uniform but very minute conducting power 

 of the gutta-percha. 



Again, if one of the cables be charged with electricity, and 

 its two ends insulated, at the end of an hour more than half 

 the charge will still be found in the cable. The conducting 

 power of the two thousand miles of gutta-percha has been in- 

 sufficient in one hour to convey half the charge from the copper 

 to the water outside. Those who have tried to insulate the 

 conductor of a common electrical machine well enough to 

 retain a charge for a few minutes will appreciate the degree of 

 insulation implied by the above statement. Contrast these 

 facts with the following extract from the lecture delivered before 

 the British Association by Sir W. Thomson in 1857, at Dublin, 

 and good reason will be seen for believing that the rapid 



