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Page:Popular Science Monthly Volume 89.djvu/287

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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-

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