Dr. Oliver Lodge on Opacity. 395 



becomes exactly of the form (4) or (5) reckoned above for the 

 general screening-effect, or opacity, of conducting media in 

 space. 



For the number which takes the place of the quantity there 

 called the critical number, namely either R//>L or Q/pS, the 

 other being zero, we may write tan e ; in which case the 

 above is 



«»ori8»=J|>»L 1 S 1 (sece+l); (12') 



or, rewriting in a sufficiently obvious manner, with 2tt/\ for 

 p/v if we choose, 



_^sini6 pcos±e ^ ff 



v{qos e)*' v(cose)a 



Instead of attending to special cases, if we attend to the 

 general cable equation (11) as it stands, we see that it is 

 more general than the corresponding equation (3) to waves 

 in space, because it contains the extra possibility R of wire 

 resistance, which does not exist in free space. 



Mr. Heaviside, however, prefers to unify the whole by the 

 introduction of a hypothetical and as yet undiscovered dissipa- 

 tion-possibility in space, or in material bodies occupying 

 space, which he calls magnetic conductance, and which, 

 though supposed to be non-existent, may perhaps conceivably 

 represent the reciprocal of some kind of hysteresis, either the 

 electric or the magnetic variety. Calling this g, (#H 2 is to 

 be the dissipation term corresponding with RC 2 ) , the equation 

 to waves in space becomes 



V 2 F=(« ? + y. / a)(^ + ^K)F, . . . (13) 



just like the general cable case. And a curious kind of 

 transparency, attenuation without distortion, would belong to 

 a medium in which both conductivities coexisted in such 

 proportion that g : ii = Airk : K ; for g would destroy H just 

 as k destroys E. 



In the cable, F may be either current or potential, and 

 LSv 2 = l. In space, F may be either electric or magnetic 

 intensity, and /jlKv 2 =1 ; but observe that g takes the place 

 not of Q but of R, while it is Att/ct that takes the place of Q. 

 Resistance in the wire and electric conductivity in space 

 do not produce similar effects. If there is any analogue 

 in space to wire resistance it is magnetic not electric con- 

 ductivity. 



The important thing is of course that the wire does not 

 convey the energy but dissipates it, so that the dissipation by 

 wire-resistance and the dissipation by space-hysteresis to 



