NEGATIVE IMPEDANCES AND THE TWIN 21 -TYPE REPEATER 493 



condenser C is l)ridged across the arc as shown and the resistance is 

 made large, nothing occurs, but if the resistance is reduced to a certain 

 critical value a state of oscillation is established. This oscillation 

 causes an alternating current to flow through the resonant circuit and 

 the arc. If the oscillation is of audible frequency the arc will emit a 



BALLAST 



<nnnrinrb — °^wamv° 



ARC 



SOURCE OF 

 POWER 



Fig. 6 — -Arc as a negative resistance. 



singing or whistling sound. This property of the arc has found useful 

 application as a generator of high-frequency oscillations in the Poulsen 

 arc used in radiotelegraphy. The negative resistance of the arc has 

 series characteristics as oscillations will not occur if there is an excess 

 of positive resistance in the oscillating circuit. 



The dynatron,^ on the other hand, has shunt characteristics as it is 

 unstable when the external resistance is made large. 



Negative Resistances of the Ideal 21-Type Circuit 



Fig. 7 shows the ideal one-way amplifier of Fig. 1 connected with an 

 ideal hybrid coil to form a 21-type repeater circuit. The ideal hybrid 



reversing 

 switch 



jisumsm. 



Ifty* 



o-^7Rnnr- 



-nr^wp- 



o— nnnsT' — l — i^tpsw^ 



li — 



Fig. 7 — Ideal 21-type circuit. 



coil is assumed to have windings of zero resistance, no leakage react- 

 ance, no capacitance in or between the windings, no core loss and neg- 

 ligible exciting current. 



^See "The Dynatron," by A. \V. Hull, Proc. I. R. E., February, 1918. 



