392 



NA TURE 



{Feb. 2 1, 1889 



Again, if the knobs be kept in this position, but the 

 circle be turned through 90", so that its plane is vertical, 

 only the electric part can act, the magnetic lines of force 

 just grazing the circle. In this way the disturbance can 

 be analyzed into its magnetic and electric constituents. 



Lastly, if the knobs be in the first position, while the 

 circle is vertical, there will be no action. 



To exhibit these alone forms an interesting set of 

 experiments. It also makes a very simple and beautiful 

 experiments to take a wire twice as long and fix it instead 

 of the first, but with two turns instead of one ; no 

 sparking is then found to occur. This is, of course, quite 

 opposed to all ordinary notions, double the number of 

 turns being always expected to give double the electro- 

 motive force. In this way the reality of the resonance is 

 easily shown. 



Interference Experiment. — The sparking, of course, 

 becomes less intense as the resonator is carried away 

 from the " vibrator," but by screwing the knobs nearer 

 together it was possible to get sparks at 6 and 7 metres 

 away. On bringing a large sheet of metal (3 metres 

 square, consisting of sheet zinc) immediately behind the 

 "resonator," when in sparking position, the sparking 

 increased in brightness, and allowed the knobs to be 

 taken further apart without the sparking ceasing ; but 

 when the sheet was placed at about 2 '5 metres further 

 back, the sparking ceased, and could not be obtained 

 again by screwing up the knobs. On the other hand, 

 when the sheet was placed at double this distance (about 

 5 metres), the sparking was slightly greater than without 

 the sheet. 



Now these three observations can only be explained by 

 the interference and reinforcement of a direct action of 

 the "vibrator" with one reflected 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 interferes with it. 



If it were possible to tell the direction of the current in 

 a " resonator " 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 direc- 

 tions, 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 dis- 

 tance apart, although unable to tell more than whether 

 there be a current or not. 



The distance from the position of interference to the 

 sheet is a quarter of the wave-length, being half the 

 distance between these simultaneous 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 cur- 

 rents are in the same direction their effects reinforce 

 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 th^ direct action in the 

 "resonator" everywhere, and by also supposing that, in 

 the immediate neighbourhood of the sheet, the direct 

 action is overmastered by that from the sheet, while at 

 2"5 metres away the two just neutralize each other. 



On this explanation, at all distances further the direct 

 action should be opposed by that from the sheet, so that 

 the fact of being increased at 5 metres upsets this ex- 

 planation. 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 pait of the disturbance had effect. Hertz 

 has also used the other positions of the resonating 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. 



Ordinary masonry walls were found to be transparent to 

 radiation of this wave-length — that is, of about 10 metres — 

 and some visitors to the opening meeting of the Dublin 

 University Experimental Society, last November, were 

 much astonished by seeing the sparking of the resonating 

 circle out in the College Park, while the vibrator was in 

 the laboratory. 



Attempts were first made last December to obtain 

 reflection from the surface of a non-conductor, with the 

 hope of deciding by direct experiment whether the mag- 

 netic or electric disturbance was in the plane of polarization; 

 that is, to find out whether the " axis of the vibrator " 

 should be at right angles to the plane of reflection or in 

 it, when at the polarizing angle, for obtaining a reflected 

 radiation. It is to be observed that in these radiations the 

 electric vibration is parallel to the " axis of the vibrator " 

 while the magnetic is perpendicular to it, and that they 

 are consequently polarized in the same sense as light is 

 said to be polarized. 



Two large glass doors were taken down and used for 

 this purpose, but without success ; and until lately, when 

 reflection from a wall was tried, the experiment seemed 

 unlikely to be successful. 



In working with the glass plate, the resonator circle 

 was first placed so that the "vibrator" had no effect on 

 it. Then the glass plate was carried into position for 

 reflection, but without result, though even the reflection 

 from the attendants moving it was amply sufficient to be 

 easily detected. 



To obviate a difficulty arising from the fact that the 

 wave was divergent, we decided to try Hertz's cylindrical 

 parabolic mirrors, for concentrating the radiation. Two 

 of these were made with sheets of zinc nailed to wooden 

 frames, cut to the parabolic shape required. 



In the " focal line " (which was made 12*5 centimetres 

 from the vertex) of one of these, a " vibrator " was 



< 70 c. > 



Fig. 3.— Plan. 



placed, consisting of two brass cylinders in line, each 

 about 1 2 centimetres long and 3 centimetres in diameter, 

 rounded at the sparking ends. 



In order that the "resonator" wire may lie in the 

 " focal line " of the receiving mirror, it has to be straight ; 



