WIRELESS TELEGRAPHY. 69 



moves the faster moves the image of the light. In this way a speed 

 of a millionth of a second can be attained. In this case the dis- 

 tance between the dots on the film may be one tenth of an inch, 

 sufficient to separate them to the eye. The photograph of elec- 

 tric sparks (rig. 8) was taken in this manner. The distance be- 

 tween any two bright spots in the trail of the photographic im- 

 ages represents the time of the electric oscillation or the time of 



Fig. 9.— Photograph of a pilot spark, which is the principal factor in the method of wireless 



telegraphy. 



the magnetic pulse or w^ave which is sent out from the spark, and 

 which will cause a distant circuit to respond by a similar oscil- 

 lation. 



At present the shortest time that can, so to speak, be photo- 

 graphed in this manner is about one two-millionth of a second. 

 This is the time of propagation of a magnetic wave over four hun- 

 dred feet long. The waves used in wireless telegraphy are not 

 more than four feet in length — about one hundredth the length 

 of those we can photograph. The photographic method thus re- 

 veals a mechanism of the spark which is entirely hidden from the 

 eye and will always be concealed from human sight. It reveals, 

 however, a greater mystery which it seems incompetent to solve — 

 the mystery of Avhat is called the pilot spark, the first discharge 

 which we see on our photograph (Fig. 9) stretching intact from 

 terminal to terminal, having the prodigious velocity of one hun- 

 dred and eighty thousand miles a second. None of our experi- 

 mental devices suffice to penetrate the mystery of this discharge. 

 It is this pilot spark which is chiefly instrumental in sending out 

 the magnetic pulses or waves which are powerful enough to reach 

 forty or fifty miles. The preponderating influence of this pilot 

 spark — so called since it finds a way for the subsequent surgings 



