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These frequency values are not only 

 the numbers of cycles per second in the 

 radio waves, but also the frequencies of 

 the oscillating currents which will set up 

 such waves. Referring to Figure i, if 

 E represents a radio frequency alter- 

 nator which generates current of 100,000 

 cycles per second in the antenna-to- 

 ground circuit A, I, B, E, G, the system 

 will radiate waves corresponding to that 

 frequency, or 3,000 meters in length. 

 The stronger the 100,000 cycle current 

 in the antenna, the more powerful will 

 be the radiated waves. It is therefore 

 desirable to do anything possible to in- 

 crease this antenna current. Also, the 

 higher the antenna the more powerful 

 will be the radiation of waves for a 

 given current. It is for this reason that 

 great heights are sought in erecting 

 sending antennas. 



\\'hen a battery or direct current gen- 

 erator applies a voltage or electrical 

 pressure across the terminals of a circuit 

 having resistance, an electric current 

 flows through that circuit. The strength 

 of the current is fixed by the amounts 

 of voltage and resistance, and, measured 

 in amperes, equals the number of volts 

 pressure divided by the number of ohms 

 resistance. This is simply Ohm's Law 

 in its elementary form, and the fact is 

 one of the first things learned in the 

 study of electricity. But its extension to 

 alternating current circuits is not so well 

 understood, though it is very little more 

 complicated. In fact, the same law in 

 the same form applies to alternating cur- 

 rents, if one uses instead of the simple 

 ohmic resistance its alternating current 

 eqviivalent, or impedance. 



Impedance, or effective alternating- 

 current resistance, is the property of cir- 

 cuits which determines how much cur- 

 rent will flow when a certain alternating 

 voltage is applied. The current in am- 

 peres is always equal to the applied 

 electro-motive force in volts divided by 

 the impedance in ohms. If, in Figure i, 



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the alternator E generates 100,000 cy- 

 cles and 100 volts, and if the total im- 

 pedance of the antenna-to-ground circuit 

 is 5 ohms for this frequency, a radio- 

 frequency current of 20 amperes will 

 flow through the ammeter I, and waves 

 of corresponding intensity will be radi- 

 ated. If the impedance were 10 ohms, 

 only 10 amperes would flow and the 

 waves would be very much weaker. Evi- 

 dently for powerful sending the antenna 

 circuit impedance must be kept as small 

 as possible, since then the current is 

 largest. 



How can the impedance be made 

 small? Before this question can be an- 

 swered it is necessary to find out what 

 impedance really is, and whether it is 

 always the same for any particular 

 circuit. 



Four quantities enter into the makeup 

 of impedance, and these are the resist- 

 ance, capacity and inductance of the cir- 

 cuit, and the frequency of the current 

 flowing in it. That portion which de- 

 pends upon the capacity and inductance 

 of the circuit is called the reactance, and 

 changes as the frequency changes. This 

 reactance is always added by a special 

 rule to the simple resistance to make up 

 the total impedance. The resistance it- 

 self remains practically constant for rea- 

 sonably small changes of frequency, but 

 the reactance may vary greatly if the 

 frequency is changed even slightly. The 

 effort to increase antenna current by 

 making impedance as small as possible 

 must therefore be confined almost en- 

 tirely to reducing the reactance portion, 

 since the simple resistance of coils, wires 

 and earth connection is always reduced 

 to the smallest feasible amount to begin 

 with. 



The computation of reactance in al- 

 ternating current circuits is not compli- 

 cated, and may be considered in two 

 parts. Referring to Fig. 2, a resistance 

 R is seen in series with an alternator E 

 and ammeter I. Since reactance de- 

 pends upon the presence of inductance 

 or capacity, and since the circuit of Fig. 

 I has no inductance or capacity, there is 

 zero reactance. The impedance is there- 

 fore made up of the resistance R only, 

 and the current I is found, in amperes, 

 by dividing the resistance in ohms (which 

 in this case equals the impedance in 



