180 



HERTZ .S RESEARCHES ON ELECTRICAL WAVES. 



away from P are marked — . The experiments were repeated with 

 diti'ereut lengths of wire m n, varying by steps of half a meter from 1 

 meter up to G meters. The first horizontal line in the table gives the 

 distances in meters of the center of the secondary circuit from the null 

 point, while the first vertical line gives the lengths of the wire on m, 

 also in meters : 



Table I. 



An inspection of this table shows, in the first place, that the changes 

 of sign take place at longer intervals than 2.8 meters ; and in the sec- 

 ond place that the change of phase is more rapid in the neighborhood 

 of the origin than at a distance from it. As a variation in the velocity 

 of propagation is very unlikely, this is probably due to the fact indi- 

 cated by theory that the electro- static e. m. f., which is more powerful 

 than the electro-dynamic B. m.f. in the neighborhood of the primary oscil- 

 lation, has a greater velocity of propagation than the latter. 



In order to obtain a definite proof of the existence of similar phenora^ 

 ena at greater distances. Dr. Hertz continued the observations in the 

 case of three of the lengths m n up to a distance of 12 meters, and the 

 result is given in the table below : 



Table II. 



If we make the assumption that at the greater distance it is only the 

 E. M. F. of induction which produces any effect, the experiments would 

 show that the interference of the waves excited by the e. m. F. of induc- 

 tion with the original waves in the wire changes its sign only at inter- 

 vals of about 7 meters. 



In order to investigate the E. m.f. of induction close to the primary 

 oscillation, where the results are of special importance, Dr. Hertz made 

 use of the interferences which were obtained when the secondary circuit 

 was in the third principal position, and the air space was rotated through 



