402 ELEMENTARY LESSONS ON [CHAP. xn. 



bottom of the sea, which carry conducting wires care- 

 fully protected by an outer sheath of insulating and 

 protecting materials. The conductor is usually of purest 

 copper wire, weighing from 70 to 400 Ibs. per nauti- 

 cal mile, made in a sevenfold strand to lessen risk 

 of breaking. Fig. 164 shows, in their natural size, 

 portions of the Atlantic cables laid in 1857 and- 1866 

 respectively. In the latter cable, which is of the usual 

 type of cable for long lines, the core is protected first by 

 a stout layer of guttapercha, then by a woven coating of 

 jute, and outside all an external sheath made of ten iron 

 wires, each covered with hemp. The shore ends are even 

 more strongly protected by external wires. 



43O. Speed of Signalling through Cables. 

 Signals transmitted through long cables are retarded, the 

 retardation being due to two causes. 



Firstly p , The self-induction of the circuit may prevent 

 the current from rising at once to its height, the retarda- 

 tion being expressed by Helmholtz's equations, given in 

 Art. 405. 



Secondly, The cable in its insulating sheath, when 

 immersed in water, acts like a Leyden jar of enormous 

 capacity (as explained in Art. 274), and the first portions 

 of the current, instead of flowing through, remain in the 

 cable as an electrostatic charge. For every separate 

 signal the cable must be at least partially charged and 

 then discharged. Culley states that when a current is 

 sent through an Atlantic cable from Ireland to New- 

 foundland no effect is produced on the most delicate 

 instrument at the receiving end for two-tenths of a 

 second, and that it requires three seconds for the current 

 to gain its full strength, rising in an electric wave which 

 travels forward through the cable. The strength of the 

 current falls gradually also when the circuit is broken. 

 The greater part of this retardation is due to electrostatic 

 charge, not to electromagnetic self-induction ; the re- 

 tardation being proportional to the square of the length 



