4 SUBMAEINE CABLE LAYING AND REPAIRING. 



in the form of a plate, cable or cube. Thus, taking the product 

 of insulation resistance and capacity per naut, we have 



D 0-1084 

 1,400 log^x ^ = 152. 



This constant is known as the " megohms per microfarad " of 



the particular dielectric. It may be used to determine the 



capacity per naut from the resistance per naut, which in this 



152 

 case is —— =0'426mfd. For pure gutta-percha it is 144*4 at 

 356 



standard conditions, and for compounded gutta-percha it varies 



from about 130 to 260. 



Eef erring to formula (12), it is convenient to express the 



capacity per naut in terms of the ratio of weights of conductor 



and insulator. Substituting the value for ratio of diameters in 



formula (10) we obtain 



Capacity per naut (Jc) = . mfds. . . . (13) 



log^^ + l 

 V n 



Curve C (Fig. 29) is plotted from this formula, and shows the 

 capacity per naut in microfarads for any ratio (n) of weight of 

 copper to gutta-percha between 0-6 and 1*6. The figures in 

 the vertical ordinate read in microfarads by putting a decimal 

 point before them. 



Taking the values of megohms per naut and microfarads per 

 naut from curves B and C respectively for any particular weight 

 ratio, it will be seen that the product of the two values always 

 comes to the same amount — viz., 151'7, or 152 as nearly as can 

 be observed by the curves. This is the "megohms per micro- 

 farad " constant referred to above. 



The capacity per naut of the core may be found from 

 formula (13) or by inspection of the curve C for the weight 

 ratio of l'-35, and is 0-426 mfd. 



The conductor resistance r per naut in terms of the weight 

 w in pounds per naut is 



1,164 , 



w 

 where the constant 1,164 is the resistance in ohms of 1 naut-lb. 



