872 



SCIENCE. 



[N. S. Vol. n. No. 52. 



other, it is evident that in the case of the 

 oscillatory discharge of the Leyden jar, 

 which we have just considered, the oscil- 

 latory current in the conducting wire must 

 be accompanied by oscillatory variations 

 of the electric and magnetic force at every 

 point of the field, and that these oscillatory 

 variations are propagated ivith a finite velocity 

 and in complete accordance iviili the latvs of prop- 

 agation of waves through an elastic solid. 



This is the neiv element which Maxwell intro- 

 duced into our view of the oscillatory phenomena 

 of electroinagnetism, and it is the very heart and 

 soul of the modern electromagnetic theory. Sup- 

 pose now that these variations are verj^ 

 rapid, and that by a suitable detector of 

 the electric or of the magnetic force we 

 actually detect these waves and measure 

 their length, then the ratio between this 

 length and the period of oscillation will 

 give us the velocity of propagation. Max- 

 well predicted that this velocitj^ is the same 

 as that of light of the same wave length, 

 but he never told us how to produce these 

 waves nor how to measure their length. 

 In fact, he never mentioned a word about 

 the oscillatory discharges of a Leyden jar, 

 and without a complete understanding of 

 these there seemed to be no way of getting 

 at Maxwell's full meaning. 



Referring to the theory of these oscilla- 

 tions Mr. O. Heaviside remarks: " It had 

 been given by Sir W. Thompson in 1853, 

 but it is a singular circumstance that this 

 very remarkable and instructive phenome- 

 non should not be so much as mentioned in 

 the whole of Maxwell's treatise, though it is 

 scarcely possible that he was unacquainted 

 with it; if, for no other reason, because it is 

 so simple a deduction from his equations. I 

 lay stress on the word simple, because it is 

 not to be supposed that Maxwell was fully 

 acquainted with the whole of the conse- 

 quences of his imfiortant scheme." (Electr. 

 Papers, Vol. II., p. 83). The omission is 

 certainly puzzling, but it can hardlj' be as- 



sumed to furnish any evidence, as Mr. 

 Heaviside seems to infer, that Maxwell was 

 not fully acquainted with the whole of the 

 consequences of his theory. For when one 

 sees as clearly as Maxwell certainly did 

 that the waves of light are the same thing 

 as the electric waves, accompanying the os- 

 cillations of a Leyden jar discharge, he can 

 well afford to ignore these and pass on 

 without delay to the discussion of the lumi- 

 nous waves considered as electric waves. 

 This was the ultimate aim of what Mr. Heav- 

 iside calls Maxwell's 'important scheme.' 

 From a practical, and what one might call a 

 business point of view, it must, of course, be 

 admitted that Maxwell would have pro- 

 moted much more rapidly his ' important 

 scheme ' if he had elucidated it first by the 

 oscillations of a Leyden jar discharge, and 

 this omission is, in a sense, a mark of incom- 

 pleteness in Maxwell's presentation of Far- 

 adaj''s view of electromagnetic phenomena. 



This unfinished pai't of Maxwell's monu- 

 mental work remained practically just as 

 Maxwell left it for over twenty years until, 

 in 1887, the genius of Hertz, of Karlsruhe, 

 completed the magnificent structiire in a 

 manner quite worthy its original designer. 

 The existence of electric waves accomiDany- 

 ing a Leyden jar discharge and their finite 

 velocity of propagation, equal to the veloc- 

 ity of light, was demonstrated bj' Hertz in 

 a series of brilliant experiments whose par- 

 allel one would seek in vain outside of Far- 

 aday's ' Experimental Eesearches.' They 

 revealed to us for the first time the whole 

 view of the electro-magnetic phenomena as 

 thej' appeared to Faraday and Maxwell ; 

 they convinced iis that the doctrine of di- 

 rect action at a distance has no place in 

 these phenomena ; and they also inspired 

 us with a hope that our view of the phe- 

 nomena of gravitation maj', perhaj^s, some 

 daj' be liberated from the narrow prison 

 walls of this per.sistent doctrine. 



Hertz's contribution to Maxwell's work 



