304 



length used be somewhat longer' than the 

 fundamental of the aerial, which is the 

 usual condition of practical wireless 

 telegraphy. 



In all the discussions up to this point 

 the use of sustained or undamped radio 

 frequency current has been assumed. 

 The generators indicated by the symbol 

 E in the diagrams have been supposed 

 to be radio frequency alternators of the 

 Fessenden type, which produce continu- 

 ous alternating current of a definite ra- 

 dio frequency depending only upon the 

 speed of the machine. Such an 

 alternator forces any attached 

 circuit to oscillate at the ma- 

 chine's generating frequency, 

 but the amount of the current 

 set up in the circuit depends 

 strictly upon the dynamo's 

 voltage and the circuit's im- 

 pedance to that frequency. 

 Transmitters of this general 

 type are coming into wider 

 use day by day, as is seen 

 from the work of the Gold- 

 schmidt, Fessenden and Tele- 

 funken companies. The cir- 

 cuit effects described are 

 substantially identical with 

 alternating current circuits 

 at commercial 



Popular Science Monthly 



Fig. 5 



those in 

 operating 

 power-distribution fre- 

 quencies of 25 or 60 per second ; in the 

 radio work, however, resonant or zero- 

 reactance effects are made useful, and 

 condensers are used directly in the cir- 

 cuits. In low-frequency practice, reso- 

 nance is usually carefully avoided and 

 series condensers are almost never used. 

 By far the- greatest number of radio 

 telegraph transmitters in use today are of 

 the spark condenser-discharge type. The 

 circuit behavior in these senders is 

 somewhat different from that in the sus- 

 tained wave alternator transmitters, but 

 most of the basic principles already ex- 

 plained hold true. The main difference 

 arises from the fact that with the alter- 

 nator the frequency of the oscillations 

 developed depends entirely upon the 

 speed of dynamo and is independent of 

 the circuits connected to it, while in the 

 spark transmitter the frequency depends 

 mainly upon the capacity and inductance 

 of the discharging circuit. 



Consider for a moment the arrange- 



ment of Fig. 4. Here an antenna A, 

 which possesses inductance, capacity 

 and resistance, has connected between it 

 and the earth E a spark gap 5". Across 

 the spark gap, by means of terminals 

 TT, a high voltage transformer, induct- 

 ance coil or other charging source is 

 connected. If the potential of this 

 charging source gradually increases, a 

 current flows into the antenna and, be- 

 cause of its electrostatic capacity, this 

 aerial system takes a charge. If the 

 voltage continues to rise until the elec- 

 trical pressure is so great that 

 the air between the spark gap 

 terminals at 6^ breaks down, 

 a spark will pass and the elec- 

 tric charge previously im- 

 pressed upon the aerial will 

 rush to earth. In an ordinary 

 antenna this discharge to 

 earth will be such that the 

 electrical inertia of the system 

 will cause the charge to "over- 

 shoot," in a sense, and the an- 

 tenna will take on a polarity 

 opposite to that which it had 

 originally but somewhat weak- 

 er. The insulating properties 

 of the air gap 5" are not regained in the 

 brief time of the charge's passage, and 

 so the current rushes up to the antenna 

 once more; at each swing or partial elec- 

 trical oscillation the electromagnetic in- 

 ertia due to inductance causes the effect 

 of "overshooting,'' and the oscillations 

 continue until the energy of the original 

 charge is used up. The electrical phe- 

 nomenon is in many ways similar to the 

 mechanical effects which may be ob- 

 served when a weight at the top of a 

 springy rod (which has its lower end 

 clamped in a vise) is swung back and 

 forth. 



Consider such a mechanical system, as 

 shown in Fig. 5. If the weighted end 

 A is pulled to the right by drawing on 

 the light thread B, the spring C will be 

 more and more strained until at last a 

 point is reached at which the thread 

 snaps. This is a fairly close analogy to 

 the straining of the air in the spark gap 

 S, Fig. 4, as the charging voltage grad- 

 ually increases to the breaking point. 

 Referring again to Fig. 5, as soon as the 

 "charge" of mechanical energy placed in 



