200 hertz's researches on electrical waves. 



passing from a rare to adense mediiim) that the " chanoe of phase" 

 occurs, aud not at the back (at a reflection from a dense to a rare med- 

 ium), as is ordinarily supposed. For Hertz's experiment of loops and 

 nodes showed that there was no change of phase on metallic reflection, 

 that is, of the magnetic displacement ; there is a change of phase of the 

 electric displacement. It is important to bear in mind that the electric 

 loop and the magnetic node occurred at the same place, and of course 

 so too the electric node and the magnetic loop. 



In order to investigate this, attempts were made to obtain Hertz's 

 loops and nodes off a parafQne wall as reflector, but no reflection could 

 be discovered, the intensity of the vertically reflected rays being in- 

 sufficient. However, by inclining the incident radiation to an angle of 

 57°, the intensity of the reflection was found to be amply sufficient. 



Fig. 6. 



With a circular resonator, which is for these waves about 10 centime- 

 tres in diameter, sparks were obtained close to the reflector, the circle 

 being held at right angles to the wall so as to be equally inclined to 

 both direct and reflected radiation, and this was confirmed b}^ a straight 

 resonator giving none there. At 30 centimeters from the wall* there 

 was interference with the circle, and vigorous sparking with the 

 straight resonator. This being about the right distance for the loop to 

 be from the reflector at an incident angle of 57°, 



^X = I -^ V = p sec i{i + cos 2/) = 2p cos i. 



Thus there is no doubt that it is on the second reflection that the 

 change of i)hase occurs. 



Here then was a difficulty ; the small sheet of zinc at the back of 

 the paraffin undoubtedly reflected with a change of phase, while, ac- 

 cording to the Hertzian experiment, metallic reflection is unaccompa- 

 nied by change of i)hase. On mentioning this to Professor Fitzgerald, he 

 pointed out to me its complete agreement with wave theory. For by 

 considering the secondary waves produced by dividing up a primary 

 wave with reference to any point into half-period zones, it can be 

 seen that the effect of the primary is equivalent to half of that arising 



* It would occur ;it about 17 centimeters on vertical roflectiou. Thia experiiueut 

 was also tried with a metallic reliector. 



