466 



Popular Science Monthly 



of any circuit, either from the out- 

 side (by magnetic induction, for in- 

 stance) or internally by a high frequen- 

 cy alternator or other apparatus, a 

 forced alter?iating current of the gene- 

 rating frequency will flow. The fre- 

 quency of this forced current cannot be 

 changed by varying the constants of the 

 circuit, for it is determined by the gene- 

 rating source. The amount of current 

 which is set up for a certain voltage, 

 however, is governed very largely by the 

 circuit constants. As was shown in 



^ . .'. , 



T 



Fig. 4. Direct coupled ^=:/ 

 sender 



the January article, the greatest current 

 flows when the applied alternating fre- 

 quency is of the value for which the ca- 

 pacity and inductance of the driven cir- 

 cuit neutralize each other, or that for 

 which the impedance (alternating cur- 

 rent resistance) is therefore the smallest. 

 The other type of alternating cur- 

 rent, called "free," occurs when a con- 

 denser is charged and then allowed to 

 discharge through an inductance and re- 

 sistance (which must not be of too 

 high value). The frequency of this free 

 alternating current so produced is de- 

 pendent entirely upon the constants of 

 the circuit, and, for the same values of 

 capacity and inductance, is practically 

 identical with the resonant or minimum 

 impedance frequency. 



The critical value at which resistance 

 becomes too high for free oscillations to 

 exist m a condenser-and-inductance cir- 

 cuit, is almost never encountered in ra- 

 dio transmitters. It may be computed 

 from a simple rule, as follows: (1) Di- 

 vide the total circuit inductance, in hen- 

 rys, by the total capacity in farads, (2) 

 take the square root of this ratio, and 

 (3) multiply the result by 2. The re- 

 sult is the "critical resistance" in ohms. 

 For the antenna circuit of Fig. 1, this 

 is found to be (1) 0.000373 henrv di- 

 vided by 0.0000000012 farad = 3 10^000 ; 

 (2) the square root of this is 556; (3) 

 2 times 556=1112 ohms. Thus if the 



resistance is less than 1112 ohms, 

 the result of the condenser discharge 

 will be oscillations at the rate of 236,- 

 000 per second; of course no ordinary 

 sending circuit ever reaches so high 

 a resistance value, so oscillations are al- 

 ways to be expected. In receivers, how- 

 ever, when detectors may be placed di- 

 rectly in series within the circuit, the 

 direct-current resistance is often seve- 

 ral thousand ohms. Free oscillations 

 cannot exist in such circuits, but a defi- 

 nite tuning effect for forced oscillations 

 is present, since, by adjusting the ca- 

 pacity and inductance reactances to neu- 

 tralize, the greatest alternating current 

 can be made to flow. 



Referring to Fig. 2, it is obvious that 

 for a given charge in the condenser, the 

 greatest current will flow when the re- 

 sistance R is of the smallest value. It 

 is also true that the oscillations will per- 

 sist for the longest time when this re- 

 sistance is smallest. The actual resist- 

 ance in circuit may be made only that 

 of the wires and spark-gap, so that the 

 free oscillations may be made to vibrate 

 back and forth hundreds of times for 

 each spark. In an antenna like Fig. 1, 

 however, the effective resistance can- 

 not be reduced indefinitely, because in 

 addition to the spark-gap and wires 

 forming the inductance and leads, the ra- 

 diation of energy in electromagnetic 

 waves adds a few more ohms. Because 

 of this, and also because the capacity of 

 an antenna cannot be increased indefi- 

 nitely without great expense, the two 

 circuits of Figs. 1 and 2, are often com- 

 bined in the arrangement of Fig. 3. 

 Here the coil in the closed circuit, Lj, 

 forms the primary of a transformer 

 whose secondary is coil L2 in the open 

 or antenna circuit. 



When condenser C is charged and al- 

 lowed to discharge through the closed 

 circuit, free oscillations are produced of 

 the frequency determined by the effec- 

 tive capacity and inductance of the cir- 

 cuit. In passing through the primary 

 Lj, these free oscillations induce alter- 

 nating voltages of their own frequency 

 in the secondary coil L2 and the con- 

 nected antenna circuit A Li L2 E. By 

 adjusting the inductance of the secon- 

 dary and loading-coils, so as to neutral- 

 lize the capacity reactance of the an- 



