hertz's researches on electrical waves. 179 



to stretch the wire loosely, aud to fix it by means of au insulated clamp 

 on each side of C alternately. It was found that when the wire was on 

 the same side as the plate P the waves in it diminished the previous 

 sparking-, and when on the opposite side the sparking- was increased, 

 both results being unaffected by the position of the air space in the 

 secondary circuit. Now it has been already pointed out that at the 

 moment when the plate A has its maximum positive charge, and at 

 which therefore the primary current begins to flow from A, the cur- 

 rent at the first node of the rectilinear wire begins to flow away from 

 the origin. The two currents therefore flow around (- in the same di- 

 rection when G lies between the rectilinear wire and A and in oppo- 

 site directions when the wire and A are on the same side of C. The 

 fact that the position of tlie air space is indiflerent confirms the con- 

 clusion formerly arrived at, that the ilirection of oscillation is that due 

 to the electro-dynamic E. m, f. These interferences are also changed 

 in direction when the wire m w, 1 meter in length, is replaced by a wire 

 •4 meters in length. 



Dr. Hertz also succeeded in obtaining interference phenomena when 

 the center of the secondary circuit was not in the base-line, but these 

 results were of no special importance, except that they confirmed the 

 previous conclusions. 



Interference phenomena at various distances. — Interference may be 

 produced with the secondary at greater distances than that of the null 

 point, but care must then be taken that the action of the waves in the 

 wire is of about the same magnitude as the direct action of the primary 

 circuit through the air. This can be effected by increasing the distance 

 between P aud A. 



Now if the velocity of propagation of the electro-dynamic disturbances 

 through the air is infinite, the interference will change its sign at every 

 lialf wave length in the wire — that is to say, at intervals of about 2.8 

 meters. If the velocities of propagation through the air and through 

 the wire are equal, the interference will be in the same direction at all 

 distances. Finally, if the velocity of propagation through the air is 

 finite, but different from the velocity in the wire, the interference will 

 change in sign at intervals greater than 2.8 meters. 



The interferences first investigated were those which occurred when 

 the secondary circuit was rotated from the first into the second prin- 

 cipal position, the air space being at the highest point. The distance 

 of the secondary from the null point was increased by half-meter stages 

 from up to 8 meters, and at each of these positions an observation 

 was made of the effects of directing the normal towards and away from 

 P respectively. The points at which no difference in the sparking was 

 observed in the two positions of the normal are marked in the table 

 below. Those in which the sparking was least, showing the existence 

 of interference, when the normal was directed towards P are marked +, 

 and those in which the sparking was least when the normal was directetl 



