HERTZES rp:searches on electrical waves. 183 



while the electro-dyiianiic e. M. F. is the ji^reater at f;re;iter distances. 

 There must therefore be some point at which tlie.v are equal, aud since 

 the}' do not balance, they must take different times to reach this point. 

 In the second place, the existence of points at which the direction of 

 the.resultant e. m. f. becomes indeterminate does not seem capable of 

 explanation, except on the supposition that the electrostatic and electro- 

 dynamic components perpendicular to each other are in appreciably 

 different phases, and therefore do not comi)ound into a rectilinear os- 

 cillation in a lixed direction. The fact that the two components of the 

 resultant are propagated with different velocities is of considerable im- 

 portance, in that it gives an independent proof that one of thenl at any 

 rate must ha\'e a finite velocity" of pro))agation. 



The latest researclies of Dr. Hertz on electrical oscillations of which 

 accounts have been published at present, are described in a paper "On 

 Electro-Dynamic Waves in Air, and their lietlections," in Wiedemann's 

 AnnaJen, 1888, vol. xxxiv, p. 609. The author bad been endeavoring 

 to find a more striking and direct proof of the finite velocity of propaga- 

 tion of electro-dynamic waves than those which he had hitherto given, 

 for though these are quite sufficient to establish the fact, they can only 

 be properly ai)preciated by one who has obtained a grasp of the results 

 of the entire series of researches. 



In many of the experiments which have been described, Dr. Hertz 

 had noticed the appearance of sparks at points in the secondary con- 

 ductor, where it was clear from geometrical considerations that they 

 could not be due to direct action, and it was observed that this occurred 

 chiefly in the neighborhood of solid obstacles. It was found moreover, 

 that in most positions of the secondary conductor the feeble sparks pro- 

 duced at a great distance from the primary became considerably 

 stronger in the vicinity of a solid wall, but disappeared with consider- 

 able suddenness quite close to the wall. The most obvious explanation 

 of these experiments was that the waves of inductive action were re- 

 flected from the wall and interfered with the direct waves, especially 

 as it was found tliat the phenomena became more distinct when the 

 circumstances were such as to favor reflection to the greatest possible 

 extent. Dr. Hertz therefore determined upon a thorough investigation 

 of the phenomena. 



The experiments were made in the Physical Lecture Theatre, which 

 is 15 meters in length, 14 meters in width, and G meters in height. 

 Two rows of iron columns, running parallel to the sides of the room, 

 would collectively act almost like a solid wall towards electro-dynamic 

 action, so that the available width of the room was only 8.5 meters. All 

 pendent gas-fittings were removed, and the room left empty, with the 

 exception of wooden tables and forms, which would not exert any ap- 



