Experimental Electricity 



Practical Hints 

 for the Amateur 



Wireless 

 Communication 



A Wireless Telegraph Transmitter 

 with Two Spark Frequencies 





ORDINARILY a radio transmitter 

 using a rotary gap sends out its 

 signals on practically a constant 

 spark frequency, 

 and at the receiv- 

 ing station it is 

 often possible to 

 recognize a numljer 

 of different sending 

 stations by this 

 characteristic 

 alone. It is entirely 

 feasible, ho\ve\er, 

 to change the group 

 frequency of a 

 spark transmitter. 

 One way of doing 

 this by m e r e 1 \' 

 pressing a key or 

 closing a switch is 

 shown in U. S. 

 patent 1,175,418 

 which was issued 

 during the current 

 year to R. A. I'es- 

 senden. With a 

 device of this sort 

 the transmitting 

 operator can send 

 messages on 

 approximately half 

 power, by using the 

 lower spark fre- 

 q u e n c y when 

 atmospheric and 

 interference condi- 

 d i t i o n s permit. 



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Diagram of the complete transmitter 



apparatus showing the relations of the 



various parts to one another 



Should it be necessary to signal through 

 strong disturbances on high frequency 

 of spark tone, the full power is im- 

 mediately avail- 

 able. The method 

 of variable group 

 frequency may also 

 be applied to two- 

 tone sending, by 

 using one rate of 

 sparking to signal 

 dots and the other 

 for dashes. In this 

 plan of telegraph- 

 ing, the length of 

 impulse for a dash 

 is no longer than 

 that for a dot, and 

 the pitch of tone is 

 the onl>- distin- 

 guishing feature. 

 Thus messages may 

 be transmitted at a 

 somewhat higher 

 speed. Since the 

 rotary- synchronous 

 gap gives absolute- 

 ly pure tones of 

 spark, the arrange- 

 ment of this patent 

 should be especially 

 useful for the two- 

 tone signaling sys- 

 tem. 



The figure is re- 

 produced from the 

 patent specifica- 



