•336 Mr, J. N. Hearder on a New Form of Telegraph Cable 



length from a conductor, or by discharges from a unit-jar, be 

 assumed to be ten measures. Take now a jar or plate exposing 

 precisely the same sui-face but whose glass is only half as thick, 

 and it will be found that twenty measiu'es will now be required 

 to produce a discharge of the same length, or in other words, to 

 produce the same amount of tension. Now the tension is the 

 same in both cases, but the quantities are as 2 to 1. 



Again, unite these two plates or jars so as to form a battery 

 of double surface. Thirty measures will now be required to 

 produce the same tension or discharge, but the relative quanti- 

 ties on the two plates respectively will still be as 2 to 1. We 

 thus have an analogue of one-half of the cable ; the thick glass 

 representing the outer half, and the thin glass corresponding to 

 the portion of dielectric between the two conductors. 



It will thus be seen that one common conductor in connexion 

 with two surfaces of dielectrics of diflPerent charging capacities 

 will exhibit one uniform degree of tension, whilst the surfaces 

 with which it is in contact may be taking up widely different 

 quantities of electricity. In a double-wire cable, then, the ne- 

 cessary diminution of thickness between the two conductors will 

 give rise to a correspondingly increased charging capacity of one- 

 half of the entire charging surface. 



But the mischief does not end here. It is a well-established 

 law in electricity, and one which I have had abundant opportu- 

 nities of verifying, that the effects of Leyden discharges are as 

 the squares of the accumulated quantities irrespective of ten- 

 sions ; therefore whatever tends to increase the charging capacity 

 of the surface of the dielectric, augments the mischievous effects 

 in the ratio of the square of that accumulation : hence I cannot 

 imagine that placing two wires in the same sheath can be any- 

 thing but injurious, since one-half at least of the internal charg- 

 ing surface of the dielectric has its capacity to take up charge 

 very considerably increased, by the greater facility afforded for 

 inductive action by the diminished thickness of the intervening 

 dielectric. 



In my last communication I alluded to the advantage which 

 might be derived from the employment of a return wire, espe- 

 cially of large dimensions, for the current instead of working to 

 earth ; but as I fear my remarks may have been misunderstood, 

 I shall here enter more into detail upon that subject. 



In relation to the existing Atlantic Cable, the only assistance 

 which it could possibly afford, would be in very slightly im- 

 pi'oving the character of the earth discharge. I here avoid 

 'the term earth-circuit, because I do not believe that, except 

 under very peculiar circumstances and within very limited 

 bounds, any appreciable return current takes place through the 



