250 ELECTRO-TELEGRAPHY 



where it will be available as a motive force. But, if the coil-wires are increased in 

 length, BO as to oppose a resistance equal to 40 miles of the cablo, an amount of mag- 

 netism equal to one-fifth of that developed in the cablo, instead of one-tenth, as in the 

 former instance, will now bo developed in the telegraph instrument, if the same quan- 

 tity of electricity as in the former case is forced through. Hence, the reason is ap- 

 parent, why such fine wire, and so great a length of it, is found to answer best for the 

 coils of telegraphic apparatus. In practice, there is of course a limit, to which tho 

 coil-wire may be increased in length, or bo diminished in sectional area, with ;ui van- 

 tage. 



' These are not new facts, for the bearing of tho resistance in tho coils, to that 

 opposed by the cablo itself, is pretty well understood and appreciated by many 

 electricians ; but it is believed that attention has not been publicly directed to it 

 before. Tho remarks which are constantly made show that it is not so well compre- 

 hended as it should be. When it was urged upon the projectors of tho Atlantic cable 

 that attenuation at any one portion of tho cable would seriously decrease tho conducting 

 power of the whole throughout, it was put to tho test of a practical experiment. 

 Finding it did not perceptibly affect tho working, it was accounted for, not from tho 

 reasons here given, but from its being at the attenuated portion, a Loydon arrange- 

 ment of smaller dimensions. This shows that tho idea, that reduction at one spot is 

 tantamount to reduction throughout, is entertained by some, whoso practical experience 

 deservedly gives weight to their opinions. In tho work before alluded to it is said, 

 " The fact is, that this fear of tho conducting wire of tho cablo being telegraphically 

 disabled by attenuation, is based upon the notion that tho insulated strand is to act 

 as a simple conductor, and not as an inductively charged Lcyden jar. It may be, and 

 probably is true, that a simple conductor transmits an electrical current with a facility 

 which is in a measure proportioned to its capacity at the smallest part of its length ; 

 the entire conductor being virtually reduced in its dimensions to tho standard of this 

 smallest part. But it is not true that the transmitting power of an insulated sub- 

 marine wire, which is virtually a Leydon jar, and open to inductive charges, is molli- 

 fied, from the same cause, to anything like the same extent. Here tho induction, which 

 has taken a mischievous part in producing retardation of electrical movement in coated 

 wires, turns round and makes amends for its interference, by causing small and easily- 

 laid wires to be more manageable for submarine telegraphy than large ones could lie." 



' The next point for consideration is, what will be the action of a galvanic battery 

 when connected to send a current on a long submarine circuit ? The wire and the 

 earth being only separated by tho thin layer of gutta-percha, induction can readily 

 take place between them ; whilst the wire itself opposes a very great resistance to the 

 flowing of the large quantity of fluid which the battery can generate, and wants to 

 force through. As the wire offers but little resistance to a small quantity of electricity, 

 a small amount will travel through almost instantly, tho wire being statically charged 

 at the same time ; and as the wire becomes more and more highly charged, so will 

 the stream flow out, in larger and larger quantities, at tho further extremity. 



'It has been already shown that induction manifests itself in proportion to tho resis- 

 tance offered by the wire, other circumstances being tho same, and that a wire offers 

 comparatively speaking, no resistance to very small amounts of electricity. This, at 

 first sight, might lead to the idea, that if very sensitive instruments could be employed, 

 supposing the wire to bo perfectly insulated, as tho induction would be less, tho signals 

 would be passed through more quickly, or, in other words, that increasing tho delicacy 

 of the instrument would bo the same thing as magnifying the wire. A little reflec- 

 tion will, however, show tho fallacy of such a supposition, for in proportion as tho 

 wire opposes less resistance, so will tho tension of tho current bo lowered, and there- 

 fore it will possess a proportionately smaller amount of energy to urge forward tho 

 current. If tho diameter of tho cable is increased to double tho si/.e, it will then 

 possess four times tho sectional area, and consequently have four times tho conducting 

 power of a wire of half thin diameter. But tho outside surface, on which induction 

 depends, will, at the same time, bo doubled, and hence there will bo twice tho induc- 

 tive force. Nevertheless, thoro will 1m a pain ; for if, in the original wire, the resistance 

 was one, and tho induction ono also, in tho wire of twico the diameter, although tho 

 inductive force would bo two, the resistance of tho wire would bo brought down to one 

 fourth, tho relative balance of tho forces beinir reduced one half. Th"ivf. n-, tin TO 

 would be a proportionate gain in favour of rapidity of conduction, and as doubling tho 

 diameter of the wire has reduced tho retarding induction, one-half signals might l.o 

 expected in this case to travel twico as fast. Supposing four cables to bo range! 1 ndo 

 by side, it is evident they will oppose one-fourth less resistance than one would do ; 

 and, consequently, tho same tension of current would force four times tho dynamic 

 quantity of electricity through as would bo forced through ono alone. Signals would 



