268 PROCEEDINGS OF THE AMERICAN ACADEMY 



XV. 



ON TELEPHONING OVER LONG DISTANCES OR 

 THROUGH CABLES. 



By N. D. C. Hodges. 



Presented May 10, 1882. 



The first point I wish to bring up is, that within any conductor con- 

 nected with the earth the only electrical forces against which work 

 has to be done during the movement of electrified bodies are those 

 due to the mutual actions between the charges in these bodies, and not 

 to the charges which may exist outside the conducting surface. So 

 that in causing a movement of electricity from A to B, the work is 

 the same when A and B are inside a conducting surface as when they 

 are outside ; and to cause a current along any course from A to B, 

 the same amount of energy will be required as if the system A B 

 were in open space. 



Hence in the case of a double-wire cable of no great length com- 

 pared with its section, so that the resistance of the wire should not be 

 sufficient to cause it to act like a succession of short pieces, the source 

 of the electromotive force being contained in a conducting surface 

 continuous with the outside of the cable, a current could be produced 

 as easily as in an air-line. 



In the next place, in the case of a cable we have a condenser to 

 deal with, the circuit wire being the inner, and the water outside the 

 outer surface. In order to cause a current to flow through a conductor 

 situated in this way, a quantity of electricity must be supplied suffi- 

 cient to raise the potential along the conductor to such a degree that 

 the required current may flow. 



To raise the charge of a conductor, the work to be done is expressed 

 by ^ £ F, where e is the final charge of the conductor and V its poten- 

 tial ; or, in terms of the capacity and potential, ^ q V^. 



