hertz's researches on electrical waves. 193 



The circle in the iuterfercnce expeiiinent was held in the horizontal 

 plane containing- the axis of the " vibrator," the ends of the circle of 

 wire being in such a })ositiou that a linejoiuiug the knobs was at right 

 angles to the " vibrator." In this position only the magnetic part of 



-oo- 



FiG. 2. 



the disturbance could affect the circle, the " magnetic lines of force/' 

 which are concentric circles about the axis of the " vibrator," passing 

 through the "resonator" circles. 



When the knobs of the circle are brought round through OO^*, so as 

 to be parallel to the " vibrator," the electric part of the disturbance 

 comes into play, the electric lines of force being, on the whole, parallel 

 to the axis of the " vibrator." The electric action alone can cause a 

 forced vibration in the knobs, even when the connecting wire is re- 

 moved, if placed fairly close to the " vibrator." 



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

 through 9(P, 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 dis- 

 turbance can be analyzed into its magnetic and electric constituents. 



Lastly, if the knobs be in the first position, while tbe 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 experiment 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 in- 

 tense as the resonator is carried away from the " vibrator," but by 

 screwing the knobs nearer together it was possible to get sparks at 

 and 7 meters away. On bringing a large sheet of metal (3 meters 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 meters further back, the spark- 

 H. Mis. 224 13 



