326 BELL SYSTEM TECHNICAL JOURNAL 



^"=F^ (1) 



and the current /12, received by the terminal or load impedance, 22, 

 is given by 



/12 = ^ ' (2) 



Here Z12 is the transfer impedance of the transducer for the specified 

 terminations. 



The power Pu" developed by the generator of e.m.f. Ex is 



The power Pji delivered to the transducer is 



P„ = K„|/„l= = ^;^£,= (4) 



and the power P12 delivered to the load impedance z-i is 



P,2 = r,\Ii2? =y^,E:~. (5) 



Now reverse the direction of transmission; that is insert an e.m.f. 

 E2 in series with the terminal impedance z^; corresponding to equa- 

 tions (3)-(5) we have then 



^'' -iz22 + 22r^' ^^ 



I Z22 + Z2 r 



P21=-^£2^ (8) 



As a consequence of the Reciprocal Theorem the transfer impedances 

 are equal ; that is 



Z'ii = Z12. (") 



From the preceding we get at once the following expressions for 

 the ratios of the powers delivered to the load impedances; 



Pi2^r2/£iY 

 P2X rAEi) 



