hertz's researches on electrical waves. 



185 



when the air sj)aco was directly away from the wall. These phenonieua 

 were reversed at a distance of 3 meters, and recurred, as in the first 

 case, at a distance of 5.5 meters. At a distance of 8 meters the sparks 

 were stronger when the air si)ace was turned away from the wall, as at 

 the distance of 3 meters, but the difference was not so well marked. 

 When the distance was increased beyond 8 meters no further reversal 

 took place, owing to the increase in the direct effect of the primary 

 oscillation and the complicated distribution of the K. M. F. in its neigh- 

 borhood. 



Fig. 12 



The positions I, II, and IV, (Fig. 12) of the secondary circle are 

 those in which the si>arks were strongest, the distance from the wall 

 being shown by the horizontal scale at the foot. When the secondary 

 circle was in positions V, VI, and VII, the sparks were equally 

 strong in both positions of the air space, and quite close to the wall the 

 difference between the sparking in the two positions again diminished. 

 Therefore the i^oiuts A, B, C, D in the diagram may in a certain sense 

 be regarded as nodes. The distance between two of these i>oints must 

 not however be taken as the wave half length, for if all the electrical 

 motions changed their directions on passing through one of these i)oints 

 the phenomena observed in the secondary circuit would be repeated 

 without variation, since the direction of oscillation in the air space is 

 indifferent. 



The conclusion to be drawn from the experiments is that in passing 

 any one of these points part of the action is reversed, while another 

 partis not. The experimental results however warrant the assumption 

 that tsvice the distance between two of these points is equal to the half 

 wave-length, and when this assumption is made the phenomena can be 

 fully explained. 



For suppose a wave of e. m. f., with oscillations in a vertical direc- 

 tion, to impinge upon the wall, and to be reflected with only slightly 

 diminished intensity, thus giving rise to stationary waves. If the wall 

 were a perfect conductor, a node would necessarily be formed in its sur- 

 face, for at the boundary and in the interior of a perfect conductor the 

 E. m. f. must be infinitely smail. The wall c;innot however be con- 

 sidered as a perfect conductor, for it was not metallic throughout, and the 



