Dr. Oliver Lodge on Opacity. 397 



the wire as its lines slowly collapse. But observe that the 

 electric energy of the field remains unchanged by thi3 

 process : if the wire were electrostatically charged it would 

 remain charged, its average potential can remain constant. 

 Let the wire for instance be perfectly conducting, then the 

 current needs no maintenance, the potential might be 

 uniform (though in general there would be waves running to 

 and fro), and both the electric and magnetic fields continue 

 for ever, unless there is some dissipative property in space. 



Two kinds of dissipative property may be imagined in 

 matter filling space : first, and most ordinary, an electric con- 

 ductivity or simple leakage, the result of which will be to 

 equalize the potential throughout space and destroy the electric 

 field, without necessarily affecting the magnetic Held, and so 

 without stopping the steady circulation of the current mani- 

 fested by that field. The other dissipativo property in space 

 that could be imagined would be magnetic conductivity ; the 

 result of which would be to shrink all the circular lines of 

 magnetic force slowly upon the wire, thus destroying the 

 magnetic field, and with it (by the circuital relation) the 

 current ; but leaving the electrostatic potential and the electric 

 field unchanged. And this imaginary effect of the medium 

 in surrounding space is exactly the real effect caused by what 

 is called electric resistance in the wire *. 



Now for a simply progressive undistorted wave, i. e. one 

 with no character of diffusion about it, but all frequencies 

 travelling at the same quite definite speed l/\/yu,K,it is essential 

 that the electric and magnetic energies shall be equal. If 

 both are weakened in the same proportion, the wave-energy 

 is diminished, and the pulse is said to be " attenuated," but it 

 continues otherwise uninjured and arrives " undistorted/' that 

 is, with all its features intact and at the same speed as before, 

 but on a reduced scale in point of size. 



This is the case of Mr. Heaviside's "distortionless circuit" 

 spoken of above, and its practical realization in cables, though 

 it would not at once mean Atlantic telephony, would mean 

 greatly improved signalling, and probably telephony through 

 shorter cables. In a cable the length of the Atlantic the 

 attenuation would be excessive, unless the absence of distortion 

 were secured by increasing rather the wire- conductance than 

 the dielectric leakage ; but, unless excessive, simple attenua- 



* There is this difference, that in the real case the heat of dissipation 

 appears locally in the wire, whereas in the imaginary case it appears 

 throughout the magnetically conducting medium ; but I apprehend that 

 in the imaginary case the lines would still shrink, by reason of molecular 

 loops being pinched off them. 



Phil. Mag. S. 5. Vol. 47. No. 287. April 1899. 2 E 



