232 VARIABLE STATE. 



Without, for the present, dwelling further on this important 

 question, it will be seen what is the nature of the problem, and 

 what methods may be utilised for accelerating the transmission of 

 signals in electrical wires. 



240. USE OF CONDENSERS. We may add that in practice it 

 has been found very useful to keep the cable constantly insulated by 

 joining each of its ends with a condenser. The battery electrifies 

 one of the coatings of the condenser at the sending station ; the 

 other coating, which is connected with the cable, is electrified with 

 the opposite kind, and a flow of the same kind as that which the 

 battery would have given passes to the first coating of the condenser 

 at the other end. The second coating of this condenser is con- 

 nected with an electrometer, or is in communication with the earth 

 by a galvanometer. 



If the contact at the origin is continuous, the electrometer tends 

 towards a maximum deviation ; the galvanometer gives a deviation 

 which increases at first and then reverts to zero, so that even for a 

 permanent contact, the phenomenon appears as an electrical wave. 

 It is easily understood from this, that momentary alternate con- 

 tacts suitably chosen, may produce waves which are materially 

 shorter than if the wire had been directly charged by the 

 battery. 



241. PROPAGATION IN DIELECTRICS. The conclusions from 

 Fourier's formula applied to the variable state, are verified, at any 

 rate approximately, for good conductors in the phenomena presented 

 by transatlantic cables, and, for imperfect semi-conductors, by the 

 experiments of Gaugain. The formula appears general therefore, 

 and we are led to apply it to dielectrics, which are never absolutely 

 destitute of conductivity. 



A dielectric submitted to the action of an electromotive force, 

 may be considered as being at once the seat of a phenomenon of 

 polarisation, and of a phenomenon of conduction subject to the 

 ordinary laws. 



Let us suppose that the dielectric is isotropic and let /* be its 

 specific inductive capacity, and c its coefficient of conductivity. 

 The general equation of induction (116) applied to a volume 

 element dv situate at a point in which the density is p, gives 



/xAV + 4717) = 0. 



On the other hand, the variation of the charge ckdvdt of the 

 element, during the time dt, produces a corresponding increase of 



