

From "Fifty Years of Electricity" (The Wireless Press, Ltd.). 



FIG. 3. A PHOTOGRAPH OF AN OSCILLATORY ELECTRIC SPARK FROM A LEYDEN JAR TAKEN ON A 



REVOLVING PHOTOGRAPHIC PLATE 



A spark is said to be oscillating when it is produced by a current which passes first in one direction and then 

 in the other. Each little band of light in the photograph corresponds to one leap of the current, as it were. 

 Between the light produced by each leap and the return leap we see there are dark spaces. 





A B 



FIG. 4. DAMPED WAVES 



Waves are said to be damped when they get smaller 

 and smaller. If they get smaller rapidly they are 

 highly damped (B); if slowly, they are feebly damped 

 (A). 



WWIAA/WVW1 



FIG. 5. UNDAMPED WAVES 



Undamped waves, on the other hand, remain of 

 the same strength all the time. They are much more 

 difficult, to produce than damped waves, but they are 

 necessary in wireless telephony. For wireless tele- 

 graphy, however, damped waves can be used. 



From "Fifty Years of Electricity" (The Wireless Press, Ltd.}. 



FIG. 6. LINES OF ELECTRIC FORCE (LEFT-HAND DIAGRAM) AND OF MAGNETIC 



FORCE (RIGHT-HAND DIAGRAM) ROUND A PLAIN AER i AL WIRE. THESE 



SETS OF LINES ARE ACTUALLY SUPERPOSED, BUT ARE DRAWN SEPARATELY 

 FOR THE SAKE OF CLEARNESS 



The magnetic lines spread out like expanding ripples in a pool. The electric lines are 

 perpendicular to these and also to the earth's surface. The double system of lines con- 

 stitutes what is called an electromagnetic wave. 



