282 THE ELECTRIC TELEGRAPH. 



Or, if the two cables have the same length, these conducting 

 powers will be as 



E E' 



tan -7 l S- e TT 



-ir- -IT- 



E E' 



And if, further, the relation of their diameters be - = 



c : c = W : W, 



inversely, therefore, as the resistances. 



With the aid of (I., it is easy to calculate the insulation 

 resistance of a wire covered with any insulating material 

 whose conducting power is known in comparison with that 

 of some other material ; or, when the material is the same, 

 as, for instance, when gutta-percha is used to insulate both 

 cables, for any unit of length (a knot, for example), the 

 resistances will be as 



-W : W = log. nat. -?- : log. nat.-^- 



T> 



log. nat. 

 W=W LT . : . . . (III. 



log. nat. 



39. Variation of the Conducting Power of Gutta-Percha with 

 Temperature. The per-centage variations of the conducting 

 powers of insulating materials are considerably more than 

 those of the metals. The conducting power of gutta-percha, 

 for example, at a temperature of 20 C., is about twelve 

 times as great as at the freezing point of water. 



The conditions under which the cores of submarine cables 

 had been tested were too uncertain, until the date of the 

 Persian Gulf cable, to justify any dependence upon the 

 quantitative results obtained in measuring under various 

 temperatures. A tolerable idea of the curve which the con- 

 ducting power of gutta-percha made when, in a graphic 

 representation of the measurements, the temperatures were 

 taken as abscissae and the conducting powers as ordinates, 



