PBINCIPLES OF DESIGN. 53 



reciprocal of that derived above in tlie case of resistance 

 (formula 5), and is therefore 



which Is proportional to the capacity. The capacity of a cubic 

 naut of gutta-percha is given by Clark and Sabine as 0*0687 

 microfarad at 75°F., but, as mentioned above, compound gutta- 

 percha is made now of less specific capacity. Taking 0"0397 

 as the capacity per cubic naut, we have the capacity of any 

 cable equal to 



0-0397 X 27rZ X r; microfarads, 



that is, the capacity of a cable having the diameters in the ratio 

 of 2-718 to 1 is 0-0397 x27r microfarads per naut at 75°F. 



Putting common logarithms instead of Napierian, we have 

 the capacity equal to 



00397 X o.QQog ^ — D naicrofarads per naut, 



0-0397 1 . , , 

 or o-^(\a ^ — iT microfarads per naut, 



which ig rr- microfarads per naut. ,-,iyx 



That is, the capacity of a dielectric of gutta-percha in which 

 the outer and inner diameters are as 10 ;1 is equal to 0*1084 

 microfarads per naut. 



It is sometimes necessary to calculate the capacity per naut- 

 cube from that per naut (k) as measured on the cable. From 

 the above it is seen that 



Capacity per naut-cube = 0*336 h log-r. 



It is of interest to remark that since insulation resistance and 

 capacity are inversely proportional, their product is a constant 

 for any given material, irrespective of whether the insulator is 



