194 hertz's researches on electrical waves. 



ing ceased, and could uot be obtained again by screwing up the knobs. 

 On the other hand, when the sheet was placed at double this distance 

 (about 5 meters), the sparking was slightly greater than without the 

 sheet. 



Now these three observations can only be explained by the interference 

 and re-enforcement of a direct action of the " vibrator " with one re- 

 flected from the metallic sheet, and in addition by the supposition that 

 the action spreads out from the vibrator at a finite velocity. Accord- 

 ing to this explanation, in the first position the reflected part combines 

 with the direct and reinforces its effects. In the second position (that 

 of no sparking), the reflected effect in going to the sheet and returning 

 has taken half the time of a complete vibration of the " vibrator," and 

 so is in the phase opposite to the incident wave, and consequently inter- 

 feres with it. 



If it were possible to tell the direction of the current in a " resona- 

 tor" at any moment, then, by employing two of them, and placing one 

 just so much beyond the other that the currents induced in them were 

 always in opposite directions, we would obtain directly the half-way 

 length. Now by reflection, we virtually are put in possession of two 

 " resonators," which we are enabled to place at this distance apart, al- 

 though unable to tell more than whether there be a current or not. 



The distance from the position of interference to the sheet is a quar- 

 ter of the wave-length, being half the distance between these simulta- 

 neous positions of opposite effects. 



In the third position, the reflected wave meets the effect of the next 

 current but one, in the " vibrator," after the current it itself emanated 

 from, and since these two currents are in the same direction, their effects 

 re-enforce each other in the " resonator." This occurs at half the wave- 

 length from the sheet. 



The first two observations alone could be explained by action at a 

 distance, by supposing the currents induced in the metallic sheet to 

 oppose the direct action in the "resonator" everywhere, and by also 

 supposing that in the immediate neighborhood of the sheet, the direct 

 action is overmastered by that from the sheet, while at 2.5 meters away 

 the two just neutralize each other. 



On this explanation, at all distances further the direct action should 

 be oi)posed by that from the sheet, so that the fact of being increased 

 at 5 meters upsets this explanation. Again, behind the sheet, evidently 

 on this supposition, the two actions should combine so as to increase 

 the sparking, but instead of this the sparking was found to cease on 

 placing the sheet in front of the " resonator." 



In performing these experiments, the "resonator" circle was always 

 placed in the position in which only the magnetic part of the disturb- 

 ance had effect. Hertz has also used the other positions of tlie resonat- 

 ing circle, whereby he has observed the existence of an electric dis- 

 turbance coincident with the magnetic one, the two together forming 

 the complete electro-magnetic wave. • 



