720 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1957 



Fig. 8 — Internal view of the J-7 valve body. 



very short gaps of considerable cross-sectional areas. It has been sho^\^l 

 elsewhere^ ■ ^ that if the distribution of reluctance of the magnetic 

 structure is symmetrical along its longitudinal axis, minimum leakage 

 flux and hence maximimi force would be developed if the working gap 

 were at the exact center of the coil. To a close approximation, the valve 

 solenoids are symmetrical in this manner. The references show that the 

 maximum pull is not sensitive to small changes from the optimum loca- 

 tion of the working gap. The gap in the J-7 valve is displaced toward 

 the center of the valve from the location of maximum pull. The slight 

 reduction in magnetic force was accepted as a suitable compromise for 

 the resulting reduction in the mass of the moving pole piece and the 

 corresponding increase in resonant freciuency. 



The gap between the solenoid pole faces is 0.01-i inch when there is 

 no signal and the valve is centered. The two fixed faces have 0.004 inch 

 non-magnetic shims attached so that the maximum motion of the arma- 

 ture is limited to ±0.010 inch. The shims prevent the armature from 

 sticking against the fixed faces l)y maintaining appreciable gaps. To 

 further compensate for the inverse square law of magnetic attraction, 

 the moving pole pieces have a reduced section that saturates under high 

 flux or large forces. This neck can be seen plainly in Fig. o. The saturating 



