CURRENT ATTENUATION 113 



is "00918 microfarad, the capacity of a wire to earth 

 '0153 per mile. 



If we compare a mile of the submarine cable with 

 a mile of the overhead as above, the capacities are 

 roughly as 0*24 to '0245 ; and there being twenty- 

 three miles of cable in the Paris -London line, it 

 will be seen that the difficulties of working over 

 it are considerably greater than those experienced 

 in our own country. 



In the case of an alternating current passing 

 through the lines, there is a definite attenuation 

 factor according to Pupin = e~P m , where e is the 

 base of Naperian logarithms, /3 the attenuation 

 constant, and m the mileage. /3 is the fraction of 

 the current at any moment lost in the passage of that 

 current through a mile of line. The current em- 

 ployed in the telectrograph is, as already ex- 

 plained, of a character somewhat comparable with 

 an alternating current, but with a continuous cur- 

 rent impressed on it. How far these figures are 

 applicable to the photo-telegraphic work remains 

 to be found ; the contrary current used in the telec- 

 trograph is especially a " wipe-out" current, and 

 prevents the latter part of each cable discharge. 



The practical effect of the line capacity is to 

 lengthen the time during which a discharge takes 

 place in the receiving apparatus. Thus if one very 

 brief current be sent through a high -capacity line 

 to the receiver an ideal oscillograph absolutely 



P.T. I 



