230 ELEMENTARY LESSONS ON [CHAP. iv. 



on, though diminishing, until the cable is fully charged. 

 Now remove the battery, and instead join up a and b by 

 a wire ; the charge in the cable will rush out through the 



Fig. 105. 



galvanometer, which will show an opposite deflection, and 

 the residual charge will continue " soaking out " for a 

 long time. 



Since the speed of signalling, and therefore the 

 economical working through a cable, depends upon its 

 u capacity " as a condenser, 1 and since its capacity 

 depends upon the specific inductive power of the in- 

 sulating substance used, it would be well if some other 

 suitable substance could be found instead of the gutta- 

 percha now used, which is expensive, and has a specific 

 inductive capacity as high as 2-46. 



275. Use of Condensers. To avoid this retarda. 

 tion and increase the speed of signalling in cables several 

 devices are adopted. Very delicate receiving instruments 

 are used, requiring only a feeble current ; for with the 

 feebler batteries the actual charge given to the cable is 

 less. In some cases a key is employed which, after 

 every signal, immediately sends into the cable a charge 

 of opposite sign, to sweep out, as it were, the charge left 

 behind. In duplex signalling (Lesson XXXIX.) the 



1 The capacity of the " Direct" Atlantic cable from Ballinskelligs (Ireland) 

 to Nova Scotia is 992 microfarads. 



