On the Theory of Lightning- Conductors. 217 



figurations uniformly throughout the substance of a rigid 

 metal like steel, is analogous to that of dissolving molecules 

 of acid or salt in a non-conductor like pure water. These 

 added molecules are the unstable groups with which Clausius's 

 theory deals. In both cases the effect produced is propor- 

 tional to the number of unstable molecules distributed. If 

 the number be sufficiently increased, the medium will 

 ultimately be a viscous fluid in the one case and an electro- 

 lytic conductor in the other. At the outset, pure water 

 typifies the rigid solid. 



The applied stress imparts a permanent strain to the solid. 

 Viscous deformation is therefore accompanied by a residual 

 phenomenon, which manifests itself when the applied stress 

 is removed or reversed*. In liquids, when acted on by 

 electromotive forces, the analogous reaction is the reciproca- 

 ting force of galvanic polarization. 



Again, Clausius's and Maxwell's theories mutually sustain 

 each other. For if the conception that in a solid, molecular 

 configurations are present in all degrees of stability is 

 necessary to explain the behaviour of strained solid matter, it 

 follows that configurations of more pronounced instability 

 will be present in electrolytic systems. Conversely the fact 

 that many solids (?) can be electrolyzed, points to the occur- 

 rence in these of a very advanced state of molecular instability. 

 To take the concrete example of glass, the same molecular 

 mechanism which at 300° promotes electrolytic conduction, 

 when the solid is influenced by an electromotive force, mani- 

 fests itself at low temperatures as the viscosity of the solid 

 under stress, 



Laboratory, U.S. Geological Survey, 

 Washington, D.C., U.S.A. 



XXI. On the Theory of Lightning- Conductors. 

 By Prof. Oliver J. Lodge, D.Sc, LL.D., F.R.Sj 



THAT a condenser discharge is oscillatory has been known 

 ever since 1858, when Sir William Thomson's great 

 paper " On Transient Currents " appeared. Quite recently 

 it has been recognized, first quite explicitly perhaps, by 

 Mr. Heaviside in the ' Electrician ' for January 1885 J, that 

 rapidly alternating currents confine themselves to the exterior 

 of a conductor§ ; and Lord Rayleigh (Phil. Mag. May 1886) 



* Kohlrausch, Pogg. Ann. cxxviii. p. 419, 1866. 



t Communicated by the Author. 



X See also Phil. Mag. August 1886 et seq. 



§ It is not possible, I think, to give Mr. Heaviside the credit of the 



