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Popular Science Monthly 



ance and resistance, but this time in the direc- 

 tion of arrow Y. The process is repeated in- 

 definitely, each charge growing somewhat 

 smaller than the one preceding it, but 

 each producing a half-cycle of alternating 

 current, until the energy of the original 

 charge is all used up in heat or use- 

 ful work. The successive half- 

 cycles in opposite directions unite 

 to produce an alternating current 

 which gradually dies away in 

 strength or amplitude, but whose 



Fig. 32: An arrangement of transmitter depend- 

 ing on condenser discharge in closed circuit 



frequency remains constant. In such a con- 

 denser-discharge circuit, the frequency of 

 the current produced depends upon the ef- 

 fective capacity and inductance, and, to a 

 limited extent, upon the resistance. The 

 rate at which the current dies away depends 

 upon the effective values of these same 

 three electrical quantities. The intensity 

 of the current in the first half-cycle of the 

 discharge depends upon the voltage to 

 which the condenser was charged, as well as 

 its size and on the circuit inductance and 

 resistance. 



A Sinnple Radio Transmitter 



The condenser-discharge method of gen- 

 erating alternating currents is practical for 

 frequencies from a few thousand per second 

 up to millions per second, and is the basis of 

 all radio telegraph transmitters of the spark 

 type. One of the simple ways in which the 

 principle is applied is shown in Fig. 31. A 

 generator G of audio frequency alternating 

 current, of say 500 cycles per second, is con- 

 nected in series with a telegraph key K and 

 the primary coil of a 8tef)-up transformer 

 T. The secondary of this transformer, 

 v/hich produces a high voltage (of perhaps 

 20,000) at the applied frequency of 500 per 

 second, is connected across a spark gap 

 .S which lies between the aerial wires A and 

 the ground E, an inductance coil L being in 

 Beriea in this antenna circuit. Each voltage 



pulse from the secondary of the trans- 

 former puts a charge upon the aerial-ground 

 system, since the wires in the air act toward 

 the surface of the earth much as one plate 

 of a condenser acts toward the other. 

 When approximately the highest point of 

 voltage in each charging pulse is reached, 

 the spark-gap S breaks down and the 

 voltage established between antenna and 

 earth just before the rupture causes a rush 

 of current through the coil L and across the 

 gap S. By choosing suitable values for the 

 inductance capacity and resistance, the 

 discharge can be made to overshoot and to 

 recharge the antenna capacity in the oppo- 

 site direction, just as in the closed circuit 

 of Fig. 4. Thus an alternating current is 

 produced in the antenna-to-earth system. 

 Its frequency is controllable by changing 

 the inductance and capacity of the antenna 

 circuit, and it may be stopped and started 

 by opening and closing the telegraph key. 

 This ofifers one solution to the first problem 

 of radio telegraphy. 



Two- Circuit Spark Transmitter 



A type of transmitter which is preferred 

 to that of Fig. 31 is shown in Fig. 32. It also 

 depends upon the condenser discharge for 

 generation of radio frequency current in the 

 antenna. Here a condenser C receives the 

 voltage impulses from the transformer, and 

 discharges across the spark-gap S. The 

 radio frequency currents thus generated in 

 the closed circuit S, C, LI induce similar 

 voltages and currents in the antenna circuit 

 A, L3, L2, E by means of the transformer 

 action of the magnetically coupled coils 

 LI and L2. This is perhaps the most 

 effective form of simple spark transmitter 

 which has been used in radio, and forms the 

 basis of by far the greatest number of radio 

 stations in the world. It produces in the 

 aerial wires a controllable radio frequency 

 alternating current, and so satisfies the 

 primary condition of the radio transmitter 

 problem. 



This article has necessarily been in the 

 nature of a review of essentials, and could 

 not offer constructive assistance to the 

 student except in so far as it will help to 

 clarify his ideas as to the underlying prin- 

 ciples and relations which exist in the 

 sending apparatus. In the succeeding ar- 

 ticles these principles will be explained in 

 further detail. 



(To Be Continued) 



