5 io THE POPULAR SCIENCE MONTHLY. 



If, now, the Ruhmkorff be excited, the following phenomena 

 result : If the point of contact between the conducting wire and 

 the rectangle be moved along the latter, it will be found that, 

 for most places, a spark passes between the balls of the rect- 

 angle, which varies in intensity, and at one place entirely disap- 

 pears. This place, if we suppose the opening in the rectangle to 

 be in the middle of one end and both balls to be of the same 

 size, is in the middle of the other end. If, now, while no spark 

 is passing in the rectangle, an insulated conductor be brought 

 into connection with either ball, the sparks again appear. These, 

 again, may be caused to disappear by moving the point of 

 contact toward the manipulated terminal. The same effect 

 would also be produced if, instead of changing the point of 

 contact, an equal insulated conductor were touched to the other 

 ball. 



The length, resistance, and quality of the conducting wire have 

 no influence upon the sparks ; neither does the resistance or mate- 

 rial of the rectangle affect it noticeably : e. g., one half of the 

 rectangle being made of thick copper wire and the other of very 

 fine German-silver wire did not alter the phenomena. Another 

 conductor being brought in contact with the joint between the 

 conducting wire and the rectangle has no influence. 



The size of the rectangle has a great influence upon the size 

 and length of the spark between its terminals ; the larger giving, 

 within certain limits, always the longer spark. 



The air distance of the Ruhmkorff discharger is of great im- 

 portance ; under five and more than fifteen millimetres proved to 

 be infelicitous. 



Hertz's explanation of these phenomena is the following : At 

 the moment when a discharge takes place between the terminals 

 of a Ruhmkorff coil, in the whole circuit, and in all conductors in 

 contact with it, powerful wave disturbances are agitated, which 

 follow each other in such infinitesimal portions of time that the 

 time which is required to travel with enormous velocity even a 

 short wire is appreciable. These waves, arriving through the 

 conducting wire at the rectangle, divide and traverse simultane- 

 ously both branches. If both sides are electrically symmetrical, 

 the two wave-branches arrive at the balls of the rectangle in ex- 

 actly the same phase, but oppositely directed, and interfere ; there 

 can then be, of course, no spark. If, however, they are not sym- 

 metrical, as when the contact is not in the middle, they do not in- 

 terfere totally, but a spark passes. As the contact moves around 

 the rectangle, the spark at its terminals will be less or more pow- 

 erful as the interference is more or less total. 



The electrical symmetry depends not alone upon the length of 

 the wire, but upon its self-induction coefficient and its capacity. 



