68 POPULAR SCIENCE MONTHLY. 



of secret wireless telegraphy — a problem which it is most important 

 to solve if the S3'stem is to be made practical; for at present the 

 message spreads out from the sending spark in great circular rip- 

 ples in all directions, and may be received by any one. 



Several methods enable us to transform electrical energy so as 

 to obtain suitable quick and intense blows on the surrounding me- 

 dium. Is it possible that there is some mysterious vibration in the 

 spark which is instrumental in the effective transmission of elec- 

 trical energy across space? If the spark should vibrate or oscillate 

 to and fro faster than sixteen times a second the human eye could 

 not detect such oscillations; for an impression remains on the eye 

 one sixteenth of a second, and subsequent ones separated by inter- 

 vals shorter than a sixteenth would mingle together and could not 

 be separated. The only way to ascertain whether the spark is oscil- 

 latory, or whether it is not one spark, as it appears to the eye, 

 but a number of to-and-fro impulses, is to photograph it by a rapidly 

 revolving mirror. The principle is similar to that of the biograph 

 or the vitoscope, in which the quick to-and-fro motions of the spark 

 are received on a sensitive film, which is in rapid motion. One 

 terminal of the spark gap, the positive terminal so called, is always 

 brighter than the other. Hence, if the sensitive film is moved at 

 right angles to the path of the discharge, we shall get a row of 

 dots which are the images of the brighter terminal, and these dots 

 occur alternately first on one terminal and then on the other, show- 

 ing that the discharge oscillates — that is, leaps in one discharge 



Flo. 8. — I'hotograph of electrical pulses. The interval between tlie pulses is one uiilliouth 



of a second. 



(which seems but one to the eye) many times in a hundred thou- 

 sandth of a second. In practice it is found better to make an im- 

 age of the spark move across the sensitive film instead of moving 

 the film. This is accomplished by the same method that a boy uses 

 in flashing sunlight by means of a mirror. The faster the mirror 



