PRINCIPLES OF DESIGN. 47 



The pressure of sea water amounts to about 1 ton per square 

 inch for every 832 fathoms depth, and, therefore, at the depth 

 in this case (1,800 fathoms) the pressure will be 



1,800 ^,«, . , 



-^^ — = 2*1 tons per square inch. 



832 



The increase of resistance over that at atmospheric pressure of 

 pure gutta-percha is about 50 per cent, per ton per square inch 

 pressure. Compounded gutta-percha is subject to less varia- 

 tion in this respect. Employing a quality of compounded 

 gutta-percha having, say, 37 per cent, rise in resistance per 

 ton per square inch pressure, the percentage increase of resis- 

 tance at the above depth over the resistance at atmospheric 

 pressure is 



37x2-16 = 80 percent. 



In other words, the resistance at atmospheric pressure must be 

 multiplied by 1"8 to obtain the resistance at 1,800 fathoms 

 depth. 



The variation in resistance (after one minute's electrifica 

 tion) of gutta-percha with temperature has been determined by 

 Messrs. Bright and Clark, Willoughby Smith, Hockin and others. 

 Messrs. Bright and Clark's experiments were made on the core 

 of the 1863 Persian Gulf cable, and Mr. Hockin's on that of 

 the French Atlantic cable of 1869. Mr. Willoughby Smith's 

 experiments corroborated Messrs. Brigho and Clark's figures in 

 cores with gutta-percha exceeding 0*1 lin. in thickness, but for 

 less thicknesses he found the variation greater [see Clark and 

 Sabine's "Electrical Tables and Formulae," pp. 116 to 120). 

 By Hockin's formula the resistance at 40°F is 11*52 times that 

 at 75°F., by Bright and Clark's tables it is 8-76 times, and by 

 Willoughby Smith's tables for smaller cores it is 13*116 times 

 that at 75°F. Taking the mean of the first two coefficients — 

 viz., 10*14 — as that to be worked to in this case, the resistance 

 of the cable due to the pressure and temperature respectively 

 at this depth will be 1*8 times and 10*1 4 times its ^value at atmo- 

 spheric pressure and 75°F. respectively, or 1*8 x 10*14 = 18*25 

 times its value due to these combined causes. 



The insulation resistance of the laid cable is specified not to 

 be less than 6,500 megohms per nautical mile at the first 



