330 BELL SYSTEM TECHNICAL JOURNAL 



eagerly read. Had it not been for this rather accidental circumstance 

 the importance of his subsequent work might have gone temporarily 

 unnoticed, as has happened with some of the greatest discoveries. 



The third volume in the series is his book, "Die Prinzipien der 

 Mechanik," completed with difficulty during his last illness and pub- 

 lished a few months after his death. A lengthy preface by the vener- 

 able Helmholtz gives an appreciative sketch of the author's life and 

 work — this time a sorrowful tribute from master to pupil. 



In order to understand and appraise the work of Hertz on electric 

 waves, it will be needful to review briefly the ideas about light and 

 electricity that prevailed at his time and before. With regard to light, 

 Newton's corpuscular theory had given way in the early 1800's to 

 the wave theory of Young and Fresnel, and long before Hertz's day 

 the idea of transverse vibrations in a hypothetical elastic-solid medium 

 called the luminiferous ether had become firmly established. Length 

 of wave and velocity had been measured many times. The wave 

 theory accounted satisfactorily for all the known phenomena in optics 

 and there was no doubt in anybody's mind about its essential correct- 

 ness, regardless of any difficulties encountered in explaining the nature 

 of the ether and its relation to matter. 



As to electricity and magnetism, the older theories of instantaneous 

 action at a distance were beginning to weaken with the discoveries, 

 in the early part of the nineteenth century, of the reactions between 

 electric currents and magnets, and the phenomenon of induction. 

 Hitherto there had been no postulation of an intervening medium to 

 explain the transmission of the force between two charged bodies, and 

 it was supposed that electricity and magnetism, like gravitation, acted 

 across empty space in straight lines and instantaneously. In some- 

 what different dress these ideas were given new life around the middle of 

 the century, particularly by some of the German physicists. To Fara- 

 day, however, such views were unacceptable. He wished to get rid of 

 the idea of action at a distance and in his mind pictured a medium, 

 along the contiguous molecules or particles of which the force was 

 propagated. In this medium he visualized "lines of force" emanating 

 from or terminating upon the electric charges or magnetic poles, acting 

 like stretched elastic threads, repelling each other sidewise as well as 

 tending to contract. Thus, in his thinking, attention was focused upon 

 the insulating medium surrounding a conductor, the "dielectric" as he 

 called it, for here, he thought, was the real seat of the action. He be- 

 lieved also that there existed some direct relation between light and 

 electricity or magnetism. He was ever seeking to find such a relation 

 and in the course of his many experiments he discovered the rotation 



