COMMON BATTERY ANTI-SIDETONE SUBSCRIBER SET 253 



But in Figs. 9A and 95, due to neutralizing balance, as was shown in 

 discussing receiving efficiencies, 



r(i) _ n\) 3n^ ni) _ r(i) 



Finally, continuing from Fig. 95 upward to Fig. 75, it is seen that 



J- 1.S I -'2S ~ -'IS • 



Hence, 



ni2) _ ni) I /-(i) _ ni) _i_ rd) _ 7'(i2) 



-^lA — -'lA T^ -'2.4 — -'is T^ -'25 — -* IS • 



Primary Purposes of Winding C and of Neutralizing Balance 



The above relations between the efficiencies of the anti-sidetone 

 circuit and those of its sidetone complement are, however, merely 

 incidental to the primary purposes of winding C and of the neutralizing 

 balance which it provides. The major purpose of winding C is that, 

 entirely apart from its neutralizing action, the voltages induced in it 

 through its couplings make it possible to obtain sidetone balance by 

 adjusting Zn] i.e. — referring to Fig. 1A at the bottom of Group IV^ — 

 given any value of Zl, it is theoretically possible so to adjust Zn that 

 P^^ = — I^^^^K The current through the receiver under the trans- 

 mitting condition, i.e., sidetone, will then be zero. Neutralizing 

 balance permits this adjustment of Zn to be made without affecting 

 the circuit efficiencies. 



Sidetone Balance 



The following discussion of sidetone balance will proceed on the 

 assumption that the couplings of winding C with windings A and B 

 have already been adjusted for neutralizing balance, since this con- 

 dition is required to maintain the circuit efficiencies. This approach 

 is merely a matter of convenience, however, for it will be indicated that 

 the impedance of TV required to effect sidetone balance is the same 

 whether the neutralizing balance is taken into account or ignored. 

 Although sidetone balance is made possible by the couplings of winding 

 C, and the impedance of N needed to reduce sidetone to zero does de- 

 pend upon the values to which the self and mutual impedances of this 

 winding have been adjusted, neither the attainment of sidetone balance 

 nor the value of Z^ required to provide it depends upon the existence 

 of neutralizing balance. 



If sidetone is to be zero, the voltage across the receiver under the 

 transmitting condition, i.e., the sum of the voltages across C and N, 

 must be made zero. Expressed in terms of the voltages in Fig. 7 A 

 at the bottom of Group IV, this requires that 



n^rzAc - i'h'Zbc - nTiZc + z^) = o. (3) 



