hertz's researches on electricat. waves. 167 



2. For distances less tluiii 1 meter (as already proved), Mie distribu- 

 tion of E. M. F. is practically tbat of the electro-static e. m. f. 



3. There are two straight lines, at all points of which the direction of 

 the E. M. F. is determinate, namely, the line in which the primary oscil- 

 lation takes place, and the perpendicular to the primary through its 

 middle point. Along the latter the e. m. f. does not vanish at any point, 

 the sparking diminishes gradually as the distance is increased. This 

 again is inconsistent with some of the theories of mutual action of cur- 

 rent elements, according to which it should vanish at a certain definite 

 distance. A very important result of the investigation is the demon- 

 stration of the existence of regions within which the direction of the 

 e. m. f. becomes indeterminate. These regions form two rings encir- 

 cling the primary circuit. Since the e. m. f. within them acts very 

 nearly equally in every direction, it must assume different directions in 

 succession, for of course it can not act in different directions simulta- 

 neously. 



The observations therefore lead to the conclusion that within these 

 regions the magnitude of the e. m. f. remains very nearly constant, 

 while its direction varies through all the points of the compass at each 

 oscillation. Dr. Hertz states that he has beeii unable to explain this 

 result, as also the existence of overtones, by means of the simplified 

 theorj^ in which the higher terms of the expansion of F are neglected, 

 and he considers that no theory of simple action at a distance is capa- 

 ble of explaining it. If however the electro-static E. M. F. and the 

 E. M. F. of induction are propagated through space with unequal veloc- 

 ities it admits of very simj)le explanation ; for within these annular 

 regions the two E. M. F.'s are at right angles and of the same order of 

 magnitude; they will therefore in consequence of the distance trav- 

 ersed, differ in phase, and the direction of the resultant will turn 

 through all the points of the compass at each oscillation. 



This X)henomenou appears to him to be the first indication which has 

 been observed of a finite rate of propagation through space of electrical 

 actions, for if there is a difference in the rate of propagation of the elec- 

 tro-static and electro-dynamic E. M. F. one at least of them must be 

 finite. 



At the end of the paper in which the preceding experiments are de- 

 scribed Dr. Hertz describes some observations which he has made on 

 the conditions at the primary sparking point which affect the produc- 

 tion of sparks in the secondary circuit. He finds that illuminating the 

 primary spark diminishes its power of exciting rapid oscillations, the 

 sparks in the secondary being observed to cease when a piece of mag- 

 nesium wire was burnt, or an arc lamp lighted, near the ])rimary spark- 

 ing point. The observed effect on the ])rimary sparks is that they are 

 no longer accomi)anied by a sharp crackling sound as before. The effect 

 of a second discharge is especially noteworthy, and it was found that the 

 secondary sparks could be made to disappear by bringing an insulated 



