170 hertz's researches on electrical waves. 



posite to that of tbe electrostatic E. m. f. in the upper half of the circle, 

 and coincides with it iu the lower half where the electrostatic e. m. f. has 

 been assumed to be positive. Since the new oscillation so produced is 

 in the phase as the previously existing one, their amplitudes must be 

 added to give the resultant amplitude, which explains the phenomena. 



Effects of the Approach of Conductors. — In making these observations 

 it was found necessary to remove all conductors to a considerable dis- 

 tance from the apparatus, in order to obtain a complete disappearance of 

 sparking at the points a and a'. Even the neighborhood of the observer 

 was sufficient to set up sparking when the air space/ was in either of 

 these positions, and the sparks had therefore to be observed from a 

 distance. The conductor used for the experiments was of the form 

 shown at C (Fig. 10), and consisted of thin metal foil. The objects kept 

 iu view in selecting the material and dimensions were to obtain a con- 

 ductor which would give a moderately large effect, and having an os- 

 cillation period less than that of the primary. 



When the conductor C was brought near to A A', it was found that 

 the sparking distance decreased at ?> and increased at &', and the null 

 points were displaced upwards, — that is, iu the direction of G. 



From the results of experiments already described it is evident that 

 the effect of displacing A A' upwards would be the same, qualitatively, as 

 that of a current in the same direction as that in A A' directly above it. 

 The effect produced by the approach of G was the reverse of this, and 

 could be explained by an inductive action, supposing there were a cur- 

 rent in G in the opposite direction to that in A A', which is exactly 

 what must occur; for the electro-static e. m. f. would give rise to such 

 a current, and since the oscillations iu G are more rapid than those 

 of this E. M. F. the current must be in the same phase as the induc- 

 ing E. M. F. The truth of this exi)lanation was confirmed by the 

 following experiments. The horizontal plates of the conductor G being 

 left in the same position as before, the vertical plate was removed, 

 and successively replaced by wires of increasing length and fineness, 

 iu order to lengthen the oscillation period of C. The effect of this 

 was to displace the null points more and more in an upward direction, 

 while at the same time they became less sharply defined, a minimum 

 sparking taking the place of the i)revious absolute disappearance. The 

 sparking distance at the highest point had previously been much less 

 than at the lowest point, but after the disappearance of the null points 

 it began to increase. At a certain stage the sparking distance at the 

 two positions became equal, and then no definite minimum points 

 could be found, but sparking took place freely at all positions of/. 

 Beyond this stage the sparking distance at the lowest point diminished 

 and very soon two minimum points made their appearance close to it, not 

 clearly defined at first, but gradually becoming more distinct, and at 

 the same time approaching the points a a', with which they ultimately 



