HERMANN VON HELMHOLTZ 



Helmholtz, by his pendulum methods, determined 

 the velocity of the propagation of electro-magnetic 

 induction at 314,400 metres per second. 



Faraday had shown that the passage of electrical 

 action involved time ; but, what was of even greater 

 importance, he also demonstrated that electrical 

 phenomena are brought about by changes in in- 

 tervening non-conductors, or dielectric substances. 

 The seat of electrical action was to be sought in the 

 tensions and strains that occur in the dielectric 

 medium. Upon this basis Clerk Maxwell founded his 

 theory of electrodynamics. This theory, carried out 

 to its logical conclusion, required that any electric 

 disturbance should be propagated through what had 

 been till then called the ' luminiferous ' ether. 

 Suppose a current passing in a metallic conductor in 

 which there is a minute break or gap filled up by a 

 non-conductor, such as air, the current, if sufficiently 

 strong, will pass across the gap as a spark. If 

 this action cause a disturbance in the dielectric, 

 this disturbance should be propagated into space by 

 the ether ? An imperfect analogy may help the 

 mind at this point. What occurs at the gap 

 may be like the effect of a stone dropped into still 

 water, when a wave will be started and propagated 

 from the centre of disturbance. Another disturbance 

 will cause another wave, another a third wave, and 

 so on. The shorter the interval of time between 

 successive disturbances the shorter will be the waves. 

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