36 



BELL SYSTEM TECHNICAL JOURNAL 



receiver winding and the opposing force X exerted by the connecting 

 rod. 



k = 300 



Fig. 7 



The amplitude of motion of the recei\-er diaphragm is then given by 



Zi-X 



y = 



Sr 



(13) 



If the lever is rigid and weightless and has an arm ratio c, then 



Ft = cFr (14) 



and due to the restraint 



(15) 



c'X 



y = cz = 



St 



Using these equations together with equation (11) it is seen that 



IhZ 



M = 



cSr + ~St 



S = Sr + -^St, 

 c~ 



The relation between / and T is given by 



E 



1 = 



T+Roc+k 



(16) 



(17) 

 (18) 



(19) 



where Rdc is the direct current resistance of the receiver winding 

 and k is the line resistance. The condition (3) for howling then 

 becomes 



IhZ = {Zd-\-k + T){cSR+^ST) ^jicZ'c. 



(20) 



