ARMSTRONG, OPERATING FEATURES OF THE AUDION 337 



to be the case. When the wing inductance is properly adjusted at the 

 resonance frequency, energy from the wing circuit is transferred freely 

 to the grid circuit, and the oscillations build up therein and are rectified 

 in the usual way. 



A curve showing the general relation between the signal strength and 

 value of wing inductance is shown in Fig. 18, the circuits used being 

 those of Fig. 17. As the capacity of the audion is the main means of 

 transferring energy from the wing to the grid circuit, best results are 

 obtained when the condenser C is very small. On account of the very 

 small capacity of the audion, the effectiveness of this method of tuning 

 is more pronounced at the higher frequencies, but by the use of a shunt 





Cz 



Fig. 21 



condenser across the inductance of the wing circuit very good amplifica- 

 tion is secured on frequencies as low as 30,000 cycles (10,000 meters 

 wave length). The best results, however, are obtained with some combi- 

 nation of coupling and wing circuit tuning, as illustrated in Fig. 19. 

 Other methods of coupling may be employed between the grid and wing 

 circuits, electrostatic and direct magnetic couplings being illustrated in 

 Figs. 20 and 21. The arrangement of Fig. 21 operates in the same way 

 as the system with the two-coil coupling; but the electrostatic coupling 

 of Fig. 20 works in an odd way. It is necessary, in this connection, to 

 complete the wing circuit for the continuous current of the battery, and 

 this is done by shunting the coupling condenser C4 by a coil of high in- 

 ductance. The continuous current of the wing circuit flows through this 

 coil and C ^ provides a path of low impedance around this coil for the 

 radio frequency oscillations of both the grid and wing circuits. When 



