HERTZ, THE DISCOVERER OF ELECTRIC WAVES 331 



of the plane of polarization of light by a magnetic field. In his specula- 

 tions there was one question which was continually presenting itself 

 to him. Do electric and magnetic forces, like light, require time for 

 their propagation? Are there waves? But to this question he was 

 unable to find an answer. 



And then came Maxwell, building upon the foundation that Faraday 

 had laid, translating Faraday's ideas into the language of mathematics, 

 and making the grand generalization that light and electric waves are 

 one and the same phenomenon, propagated by the same medium, with 

 the same velocity, and differing only in wave-length. Like Faraday, 

 he considered the energy of the electromagnetic field to reside in the 

 dielectric. He conceived the medium to have properties analogous to 

 those of an elastic solid, which would spring back to its original state 

 upon the removal of the straining force. The alteration of the dis- 

 placement, or "polarization," in the medium was viewed by him as an 

 electric current, which he called the "displacement current," as dis- 

 tinguished from the "conduction current" existing in conductors. 

 From the general equations which he formulated it was shown that only 

 transverse vibrations (like light) could be propagated in such a medium 

 and that the velocity of propagation should be equal to the ratio of 

 the two systems of electrical units, that is, to the number of electro- 

 static units of electricity contained in one electromagnetic unit. This 

 ratio had been experimentally determined by Weber and Kohlrausch 

 (it was later redetermined by Maxwell himself, by a different method), 

 and the fact that it agreed so very closely with the measured velocity 

 of light was one of the strongest points in favor of the view that light 

 waves were identical with the hypothetical electromagnetic waves. 

 Maxwell's comprehensive theory was first enunciated in his 1865 paper 

 entitled "A Dynamical Theory of the Electromagnetic Field," and 

 afterwards elaborated in his great "Treatise on Electricity and Mag- 

 netism," published in 1873, but for a number of years it was regarded 

 by many as merely a speculation, by others as probably true, and by 

 none as conclusively proved. It remained for Hertz to add the cap- 

 stone to the theory by actually demonstrating for the first time the 

 existence of electromagnetic waves in space; and furthermore, to show 

 experimentally that they had all the physical properties of ordinary 

 light waves. 



In 1888, while he was teaching at the Technical High School in 

 Karlsruhe, Hertz carried out the brilliant experiments which have 

 made his name famous. These were actually a part of a long series of 

 experiments which began in 1886, and which came about partly by 

 accident, and yet were the result of his keen interest in everything 



