384 Anniversary Meeting. [Nov. 30, 



mental demonstration of magnetic waves to which he was led that 

 he began his electric researches in a problem happily put before him 

 thirteen years ago by Professor von Helmholtz, of which the object 

 was to find by experiment some relation between electromagnetic 

 forces and dielectric polarisation of insulators, without, in the first 

 place, any idea of discovering a progressive propagation of those 

 forces through space. 



It was by sheer perseverance in philosophical experimenting that 

 Hertz was led to discover a finite velocity of propagation of electro- 

 magnetic action, and then to pass on to electromagnetic waves in air 

 and their reflection, and to be able to say, as he says in a short review- 

 ing sentence at the end of his eighth paper : " Certainly it is a fasci- 

 nating idea that the processes in air which we have been investigating, 

 represent to us on a million-fold larger scale the same processes 

 which go on in the neighbourhood of a Fresnel mirror, or between 

 the glass plates used for exhibiting Newton's rings." 



Professor Oliver Lodge has done well, in connection with Hertz's 

 work, to call attention* to old experiments, and ideas taken from 

 them, by Joseph Henry, which came more nearly to an experimental 

 demonstration of electromagnetic waves than anything that had been 

 done previously. Indeed Henry, after describing experiments show- 

 ing powerful enough induction due to a single spark from the prime 

 conductor of an electric machine to magnetise steel needles at a dis- 

 tance of 30 ft. in a cellar beneath with two floors and ceilings inter- 

 vening, says that he is " disposed to adopt the hypothesis of an 

 electrical plenum," and concludes with a short reviewing sentence : 

 " It may be further inferred that the diffusion of motion in this 

 case is almost comparable with that of a spark from a flint and steel 

 in the case of light." 



Professor Oliver Lodge himself did admirable work in his investi- 

 gations with reference to lightning rods,f coming very near to experi- 

 mental demonstrations of electromagnetic waves ; and he drew 

 important lessons regarding " electrical surgings "in. an insulated bar 

 of metal "induced by Maxwell's and Heaviside's electromagnetic 

 waves," and many other corresponding phenomena manifested both 

 in ingenious and excellent experiments devised by himself and in 

 natural effects of lightning. 



Of electrical surgings or waves in a short insulated wire, and of 

 interference between ordinary and reflected waves, and positive elec- 

 tricity appearing where negative might have been expected, we hear 

 first, it seems, in Herr von Bezold's " Researches on the Electric 

 Discharge" (1870), which Hertz gives as the third paper of his 



* * Modern Views of Electricity,' pp. 369372. 



f ' Lightning Conductors and Lightning Guards,' Oliver J. Lodge, D.Sc., F.K.S. 

 Whittaker and Co. 



