PBINCIPLES OF DRSIGN. 49 



The resistance of a cubic naut of insulating material is easily 

 calculated from measurements taken on sheets or plates of any 

 definite dimensions. In Clark and Sabine's work, referred to 

 above, the resistance at 75°F. between the faces of a sheet of 

 gutta-percha 1 sq. ft. in area and 1 mil in thickness is given as 

 1,066 megohms after one minute's electrification. 



The resistance of a cubic foot is therefore 



1,066 X 1,000 X 12 = 12-8 x 10« megohms, 



and of a cubic naut 



12-8xl0« 

 Q Qoj — = 2,100 megohms. 



(The number 6,087 is the number of feet in one naut. The 

 resistance of a cubic foot is multiplied by this number and 

 divided by its square to obtain the resistance of a cubic naut^ 

 and the process is therefore the same as dividing by the 

 number.) 



By compounding the gutta-percha, that is, mixing together 

 different natural kinds of guttas, the specific resistance may be 

 largely increased (from 80 to 120 per cent.) and the inductive 

 capacity reduced (from 10 to 20 per cent.). Using a quality 

 ordinarily obtained, having a resistance after maturing of 

 3,830 megohms per cubic naut, we have the resistance of any 

 cable of I nauts equal to 



3,830 , D 



"2^ log, -^megohms; 



that is, the insulation resistance of a cable having the diameters 

 in the ratio of 2-718 to 1 isM^ megohms per naut at 75°F. 



Using common logarithms instead of Napierian and apply- 

 ing the correction, we have the insulation equal to 



2-3026 D 

 3,830 X — /?.o log"; megohms per naut ; 



that IS, 3,830 x 0-3661og-7 megohms per naut, 



which is 1,400 log ^ megohms per naut. . (6) 



