PRESIDENTIAL ADDRESS — SECTION A. 25 



another in unison with it and placed in its vicinity, and the 

 oscillations of the second jar may accumulate sufficient inten- 

 sity to enable them to burst across a short air-space arranged 

 between the coatings, and thus give a visible spark. If we vary 

 the capacity and thus the period of the second jar, it will no 

 longer be excited in sympathetic resonance by a discharge of 

 the first. 



Hertz, to whom we owe the complete experimental verifica- 

 tion of Maxwell's theory, used as a vibrator or originator of his 

 electric waves not a Ley den jar, but two equal and similar 

 pieces of brass cylinder wdth rounded ends. These he placed 

 in line with a small sparking-distance between them, and con- 

 nected each to one of the terminals of a small induction coil. 

 At every discharge of the coil the electricity surged up and 

 down these cylinders, oscillating at a regular calculable rate, 

 and disturbing the ether in a manner exactly similar to a diverg- 

 ing beam of plane polarised light. 



As a resonator or detector of the arrival and presence of the 

 ether disturbance. Hertz used a looped conductor with its ends 

 bi'ought close together, or a pair of cylinders like those used 

 as vibrator. When the detector is in unison with the vibrator, 

 and the spark-gap small, the presence of the electric disturbance 

 is manifested by a series of microscopic sparks across the 

 air-space between the ends of the resonator. 



Fitzgerald lately discovered that if the two ends of the 

 resonator be connected through a very high-resistance gal- 

 vanometer there will be a deflection when the resonator is 

 disturbed by ether vibrations. The reason of this phenomenon 

 is unknown, but it renders the performance of Hertz's experi- 

 ments very much easier, and enables one to show them all to a 

 large audience, as he has done lately at the Eoyal Institution, 

 London. 



With this apparatus one is fully equipped to prove that 

 electric oscillations and light are the same thing. The oscillator 

 corresponds to the source of light, the resonator or detector to 

 the eye. Fitzgerald's arrangement of the galvanometer added 

 to the resonator simply corresponds to rendering the eye more 

 sensitive. The only difference between the two phenomena is 

 that the wave-lengths of the ether disturbances we call light 

 lie between four ten-millionths (jooosooo) ^^^^ eight ten- 

 millionths (xo oonooo) ^^ '^^ metre approximately, wdiile we can get 

 electric waves of any length from a few feet up to ten thousand 

 miles. With Hertz's apparatus they are usually a few feet 

 long ; while those sent out from an ordinary alternating-current 

 dynamo, running at a speed corresponding to 100 periods per 

 second, are about 2,000 miles long. With the apparatus de- 

 scribed above it can be shown that everything that can be 

 done with light can be done with electric waves. They can be 



