178 hertz's researches on electrical waves. 



same, the sparks disappeared when the normal was turned away from 

 P. Further variations of the experiment gave results in accordance 

 with thesfe. 



It is easily seen that these phenomena were exactly what were to be 

 expected. To fix the ideas, suppose the air space to be at the highest 

 point and the normal directed towards P, as in Fig. 11. Consider what 

 happens at the moment that the plate A has its greatest positive charge. 

 The electrostatic, and therefore the total E. M. F., is directed from A 

 towards A'. The oscillation to which this gives rise in B is determined 

 by the direction of the E. M. F. in the lower portion of B. Therefore 

 positive electricity will flow towards A' in the lower portion, and away 

 from A' in the upper j)ortion. 



Consider next the action of the waves. As long as A is positively 

 charged, positive electricity will flow from the plate F. This current 

 is, at the moment considered, at its maximum value at the middle 

 point of the first half wave-length. A quarter of a wave-length further 

 from the origin — that is to say, in the neighborhood of the null point — 

 it first changes its direction. The e. m. f. of induction will here there- 

 fore impel positive electricity towards the origin. A current will there- 

 fore flow round B towards A' in the upper portion and away from A' 

 in the lower portion. The electro-static and electro-dynamic e. m. f.'s 

 are therefore in opposite phases and oppose each other's action. If 

 the secondary circuit is rotated through 90 deg., through the first prin- 

 cipal position, the direct action changes its sign, but not so the action 

 of the waves, so that they now tend to strengthen each other. The 

 same reasoning holds when the air space is at the lowest point of B. 



Greater lengths of wire were then included between m and w, and it 

 was found that the interference became gradiially less marked, until 

 with a length of 2.5 meters it disappeared entirely, the sparks being of 

 equal length whether the normal were directed towards or away from 

 P. When the length of wire between m and n was further increased, 

 the distinction between the different quadrants re-appeared, and with a 

 length of 4 meters the disappearance of the sparks was fairly sharp. 

 The disappearance however then took place (with the air space at the 

 highest point) when the normal was directed away from P, the opposite 

 direction to that in which the disappearance previously took place. 

 With a still further increase in the length of the wire the interference 

 re-appeared and returned to its original direction with a length of 6 

 meters. These phenomena are clearly to be explained by the retarda- 

 tion of the waves in the wire, and show that here again the direction of 

 motion in the advancing waves changes its sign at intervals of about 

 2.8 meters. 



To obtain interference phenomena with the secondary circuit (7 in 

 the third principal position, the rectilinear wire must be removed from 

 its original position, and placed in the horizontal plane through C either 

 on the side of the plate A or of the plate A'. Practically it is sufficient 



