396 Dr. Oliver Lodge on Opacity. 



that extent correspond. The screening effect of space- 

 conductivity involves the very same dielectric property as that 

 which causes leakage or imperfect insulation of the cable 

 core. 



Returning to the imaginary magnetic conductivity, let 

 us trace what its effects would be if it existed, and try to 

 grasp it. It effect would be to kill out the magnetism of 

 permanent magnets in time, and generally to waste away the 

 energy of a static magnetic field, just as resistance in w T ires 

 wastes the energy of an unmaintained current and so kills 

 out the magnetism of its field. I spoke above as if it were 

 conceivable that such magnetic conductivity could actually 

 in some degree exist, likening it to a kind of hysteresis ; but 

 hysteresis — the enclosure of a loop between a to and fro path 

 — is a phenomenon essentially associated with fluctuations, and 

 cannot exist in a steady field with everything stationary. 

 Admitted : but then the molecules are not stationary, and the 

 behaviour of molecules in the Zeeman and Righi phenomena, 

 or still more strikingly in the gratuitous radiations discovered 

 by Edmond Becquerel, and more widely recognized by others, 

 especially by Monsieur et Madame Curie, (not really gratuitous 

 but effected probably by conversion into high-pitched radiation 

 of energy supplied from low-pitched sources), — the way 

 molecules of absorbent substances behave, seems to render 

 possible, or at least conceivable, something like a minute 

 magnetic conductivity in radiative or absorptive substances. 

 Mr. Heaviside, however, never introduced it as a physical 

 fact for which there was any experimental evidence, but as a 

 physical possibility and especially as a mathematical auxiliary 

 and unifier of treatment, and that is all that we need here 

 consider it to be ; but we may trace in rather more detail its 

 effect if it did exist. 



Suppose the magnetism of a magnet decayed, what would 

 happen to its lines of force ? They would gradually shrink 

 into smaller loops and ultimately into molecular ones. The 

 generation of a magnetic field is always the opening out of 

 previously existing molecular magnetic loops ; there is no 

 such thing as the creation of a magnetic field, except in the 

 sense of moving it into a fresh place or expanding it over a 

 wider region *. So also the destruction of a magnetic field 

 merely means the shrinkage of its lines of force (or lines of 

 induction, I am not here discriminating between them). 

 Now consider an electric current in a wire : — a cylindrical 

 magnetic field surrounds it, and if the current gradually de- 

 creases in strength the magnetic energy gradually sinks into 

 * This may be disagreed with. 



