400 Dr. Oliver Lodge on Opacity. 



graphy have been elaborately and thoroughly laid down by 

 Mr. Heaviside. 



There is a paragraph in Maxwell, concerning the way 

 a current rises in a conductor and affects the surrounding 

 space, which is by no means satisfactory : it is Art. 804. 

 He takes the current as starting all along the wire, setting up 

 a sheath of opposition induced currents in the surrounding 

 imperfectly insulating dielectric, which gradually diffuse out- 

 wards and die away, leaving at last the full inductive effect 

 of the core-current to be felt at a distance. Thus there is 

 supposed to be a diffusion of energy outwards from the wire, 

 which he likens to the diffusion of heat. 



But, as Mr. Heaviside has shown, the true phenomenon is 

 the transmission of a wave in the space surrounding the 

 wire — a plane wave if the wire is perfectly conducting, 

 a slightly coned wave if it resists, — a wave-front perpendicular 

 to the wire and travelling along it, — a sort of beam of dark 

 light with the wire as its core. 



Telegraphic signalling and optical signalling are similar ; 

 but whereas the beam of the heliograph is abandoned to 

 space and must go straight except for reflexion and refraction, 

 the telegraphic beam can follow the sinuosities of the wire and 

 be guided to its destination. 



If the medium conducts slightly it will be dissipated 

 in situ; but if the wire conducts imperfectly, a minute trickle 

 of energy is constantly directed inwards radially towards the 

 wire core, there to be dissipated as heat. Parallel to the 

 wire flows the main energy stream, but there is a small 

 amount of tangential grazing and inward flow. The initial 

 phenomenon does not occur in the wire, gradually to spread 

 outwards, but it occurs in the surrounding medium, and a 

 fraction of it gradually converges inwards. The advancing 

 waves are not cylindrical but plane weaves, and though the 

 diffusing waves are cylindrical they advance inwards, not 

 outwards. 



I will quote from a letter of Mr. Heaviside's: — " The easiest 

 way to make people understand is, perhaps, to start with a 

 conducting dielectric with plane waves in it without wires 

 [thus getting] one kind of attenuation and distortion. Then 

 introduce wires of no resistance ; there is no difference except 

 in the v 7 ay the lines of force distribute [enabling the wires to 

 guide the plane waves]. Then introduce magnetic con- 

 ductivity in the medium, [thereby getting] the other kind of 

 attenuation and distortion. Transfer it to the wires, makino- 

 it electrical resistance. Then abolish the first electric con- 

 ductivity, and you have the usual electric telegraph," 



