166 HEETZ'S RESEARCHES ON ELECTRICAL WAVES. 



may therefore serve to eliminate some of them as uuteuablc. iu mak- 

 ing these experiments however, an unexpected difficulty was encoun- 

 tered, as it was found that at distances of from 1 to 1.5 meters from the 

 primary the maximum and minimum, except in certain i30sitions, be- 

 came indistinctly defined ; but when the distance was increased to up- 

 wards of 2 meters, though the sparks were then very small, the maximum 

 and minimum were found to be very sharply marked when the sparks 

 were observ^ed in the dark. The jiositions of maximum and minimum 

 were found to occur with the circle in planes at right angles to each 

 other. At considerable distances the sparking diminished very slowly 

 as the distance was increased. Dr. Hertz was not able to determine an 

 upper limit to the distance at which sensible effects took place, but in 

 a room 14 meters by 12, sparks were distinctly observed when the pri 

 mary was i)laced in one corner of the room, wherever the secondary 

 was placed. When however the primary was slightly displaced, no 

 effects could be observed, even when the secondary was brought con- 

 siderably nearer. The interposition of solid screens between the two 

 circuits greatly diminished the effect. 



Dr. Hertz mapped out the distribution of force throughout the room 

 by means of chalk lines on the floor, putting stars at the points where 

 the direction of the E. m, f. became indeterminate. A portion of the dia- 

 gram obtained in this manner is shown on a reduced scale iu Fig. 9, 





Fig. 9. 



with respect to which the following points are note- worthy: 



1. At distances beyond 3 meters the E. M. F. is everywhere parallel 

 to the primary oscillation. Within this region, therefore, the electro- 

 static E. 31. F. is negligible in comparison with the e. m. f. of induction. 

 Now all the theories of the mutual action of current elements agree iu 

 giving an E. m. p. of induction inversely proportional to the distance, 

 while the electro-static e. m. f., being due to the dilferential action of 

 the two extremities of the primary, is approximately inversely propor- 

 tional to the cube of the distance. Some of these theories however 

 are not in accordance with the experimental result that the effect dimin- 

 ishes much more rapidly in the direction of the primary oscillation than 

 iu a direction at right angles to it, induced sparks being observed at a 

 distance exceeding 12 meters in the latter direction, while they disap- 

 peared at a distance of about 4 meters in the former direction. 



