hertz's researches on electrical waves. 201 



from the contral circle, and iu consequeuce is half a period behiud the 

 l^hase which would be at the poiut if an inliuiresiinal portion of the 

 center alone acted. For the effect of each ring can be considered as 

 destroyed by half the effect of its two neighbors, and thus half the 



Fig. 7. 



effect of the central circle is left uncompensated. But the distance of 

 the edge of this circle is half a wave-length farther from the point 

 than its center is, so that the resultant phase at tbe point will be be- 

 hind that due to the center, but in front of that due to the edge, which 

 effect would be half a period behind that arising from the center. 

 Taking the mean between them, the resultant phase then at the poiut 

 is a quarter of a period* behind what it would be if the center alone 

 acted. Thus it was that the reflection from the small sheet of zinc 

 differed from what I had expected it to be. 



Experiment showing pliasc of secondary irares. — To experimentally 

 test this, the small sheet of zinc was used as reflector in the Hertzian 

 exjjeriment of looi)s and nodes. Employing the circular resonator, the 

 position of interference was found to have shifted out from 17 to over 

 24 centimeters, which nearly corresponds to an acceleration of phase 

 of a quarter of a period, the wave going in all nearly a quarter of a 

 wave-length farther, and nevertheless being still only half a period 

 behind the phase on starting. The farthest out the loop could be is 

 25.5 centimeters : to obtain this would require an indefinitely small re- 

 flector. Of course, when the resonator was close in to the sheet, no 

 cliange of phase was found to occur, the sheet being then i)ractically 

 infinite. 



Another interesting observation was made. A long sheet of zinc, 

 30 centimeters wide, was found to act similarly to the sheet 30 

 centimeters square, provided it was i)laced with its breadth parallel to 

 the electric displacement. When thus placed at 24 centimeters from 

 the circular " resonator," there was interference, but on rotating the 

 reflector so as its length was i)arallel to the electric displacement, 

 sparking occurred, and now the " resonator" had to be brought back 

 to 17 centimeters in order to again obtain interference. This experi- 



*That it aided tboback ratlior than tlio front was probably diic! to tbeir phase not 

 being an exact period or half period dillerent from each other. 



