1909] on Modern Submarine Telegraphy. 525 



the minimum K.R. This critical limit, or the point when the size of 

 the copper is reached to give the lowest K.K., is when the diameter 

 of the copper is to the diameter of the core as 1 : 1 ' 65. 



There is another advantage in keeping the resistance low for any 

 K.R : the time constant only determines the time when the current 

 at the far end reaches a certain percentage of the possible maximum 

 after the application of the voltage at the sending end. Of course, 

 the quantity of current after any given time is determined again by 

 the voltage of the sending battery and is inversely as the resistance 

 of the cable. 



For instance, if two cables were constructed of equal K.R. but 

 one had a larger copper of half the resistance of the other, with equal 

 sending batteries the one with the lower resistance would deliver 

 twice the current at the receiving end, at the ends of equal times, 

 and could therefore be made to work at a faster rate. It should also 

 be a cheaper cable, because copper is less expensive than gutta-percha. 



Against these electrical advantages should be placed several 

 mechanical disadvantages : the reduction of the thickness of the 

 insulation might result in a greater liability to faults developing 

 after the cable was laid. 



With such a heavy wire, which would naturally have to be well 

 stranded, to reduce the stiffness, the liabihty of the decentralisation 

 during manufacture would be greater than with existing cores. 



These mechanical difficulties could, I feel sure, be overcome, say 

 by greater care being taken in the manufacture, or by substitution 

 for the present yielding gutta-percha of dry cotton or similar material 

 well impregnated with gutta-percha compound. 



Fig. 1. — Atlantic 1894 Cable. 



I take an Atlantic cable laid in 1894 (Fig. 1) as having the 

 greatest size of copper for size of core ; I take this core to illustrate 

 the improvement that might result by increasing the copper up to 

 the largest size electrically permissible : — 



1894 Gable. 



Diameter of core . . . . . • 466 inch 



„ of copper ..... 0-202 inch 



Kesistance per nautical mile . . . 1 • 684 ohm 



Capacity „ „ . . . • 420 microfarad 



The cable is 1852 nautical miles long, and its K.R. is 2*41, and 

 its speed of working under the capacity block system of duplex 

 about 205 letters per minute. 



