Wireless Work in Wartime 



IX. — The Primary Oscillation Circuits 

 of the Spark-gap Transmitter 



]5y John V. L. Hogaii 



THE March article of this series 

 discussed the power-supply circuits 

 of the spark-type radio transmitter, 

 showing how alternating current energy 

 was supplied to the power transformer 

 and converted to a higher voltage 

 suitable for charging the primary 

 condenser. The illustration 

 Fig. 34 is reproduced this month 

 so that the details of this assembly 



r-#'® — ^ 



Fig. 32: The coupled two-circuit transmitter 

 overcomes difficulties from the plain antenna 



of apparatus may be held clearly in mind, 

 and Fig. 32 is also shown again since it 

 gives the normal connections from the 

 primary condenser C to the spark-gap S 

 and the several tuning coils. This in- 

 stalment will take up the behavior of the 

 spark-gap and the primary oscillation 

 circuit, which is composed of the gap G, 

 the condenser C and the primary Li of 

 the oscillation transformer or inductive 

 coupler. 



In the first place, it must be understood 

 that the transmitter really has two pri- 

 mary circuits and two secondary circuits, 

 just as it has two transformers. With 

 reference to the low^ or audio frequency 

 power-supply, (which is usually of from 

 60 to 500 cycles per second frequency), 

 the primary circuit comprises the genera- 

 tor armature, the key and the primary of 

 the power transformer. The audio fre- 

 quency secondary circuit involves the 

 power transformer secondary S, and the 

 condenser, C. In regard to the radio 

 frequency oscillations which are produced 

 by the condenser (and which in turn pro- 

 duce the wireless waves), the other pri- 



mary circuit consists of the condenser C, 

 the spark-gap G, and the primary coil Li 

 of the oscillation transformer or inductive 

 coupler. The radio frequency secondary 

 is formed of the antenna A, the antenna 

 loading coil L3, the secondary L2, of 

 the inductive coupler, and the ground 

 connection E. 



The Voltage Applied to the 

 Condenser 



Considering Fig. 34 for a moment, it 

 should be evident that if an alternating 

 current of 500 cycles (complete reversals) 

 per second is developed by the generator, 

 corresponding voltages will be applied to 

 rhe plates of the condenser C. At inter- 

 vals of 1/TOOO second this condenser will 

 be charged in alternate directions, first 

 with the upper plate positive and the 

 lower negative, then with the upper 

 plate negative and the lower positive. 

 If the v.dres X and Y are left open 

 or disconnected, the condenser charges 

 wull merely flow back through the 

 secondary winding S as the potential 

 changes at each half cycle. The il- 

 lustration Fig. 35 should make this 

 action even more clear, since it shows the 

 potential of the upper plate of the con- 



Fig. 34: In addition to the generator itself 

 the circuits of the transmitter are shown 



denser (with respect to the lower plate), 

 at the various instants in a full cycle of 

 1 500 second. The wavy line repre- 

 sents the passage of time by its progress 

 toward the right (thousandths of one 

 second are marked off along the hori- 

 zontal line or axis of time) and the poten- 

 tial of the condenser by its vertical height 



635 



