520 BELL SYSTEM TECHNICAL JOURNAL 



Therefore, 





(21) 



and solving this for A^, we gel 



^,= ^ (22) 



The equivalence of the air-cliamber to a transformer shunted by a 

 compliance is shown earlier in the paper. 



In applying the foregoing method of design to a practical structure, 

 a number of design problems had to be solved. The construction 

 of the diaphragm and the method by which it is actuated have been 

 already described, except for the tangential corrugations constituting 

 the series compliance. The use of these corrugations results in the 



Fig. 17 — Photograph of mechanical reproducing system without the horn 



value of the series compliance being practically independent of the 

 nature of the clamping, and has eliminated a tendency to "rattle" 

 introduced by unevenness in the clamping surfaces. 



Another feature in connection with the sound box is the needle- 

 arm bearing shown in Figs. 17 and 18. Ordinary knife-edge bearings 

 are not sufficiently rigid as fulcrums and the rotational reactance as 

 well as the rotational resistance is undesirably large. A construction 

 which has been found to meet the necessary requirements is the ball 

 bearing type with the steel balls held in position by magnetic pull. 

 By making the ball-containing case of soft steel and magnetizing the 

 shaft, it has been possible to manufacture this bearing reliably and 

 cheaply. 



