AN IMPROVED TELEPHONE SET 



263 



tendency to "stick", the armature is mounted on a spring reed or hinge, 

 the stiffness of which is intended to balance the negative stiffness of the 

 magnetic field. This balance must be quite accurately maintained in spite 

 of variations in strength of magnet and stiffness of the reed spring. One or 

 the other of these constants must therefore be adjusted in each ringer. Since 

 it is easier to operate with precision on the strength of the permanent mag- 

 net than on the stiffness of the reed spring, the magnet of the ringer is 

 demagnetized in successive steps until balance is reached. In addition to this 

 balancing adjustment, it is also necessary to adjust properly the bias forces 

 to meet the required operate and non-operate current values; and this is 

 accomplished by the bending of the biasing spring. 



< 10 



-J 

 HI 8 



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——0500 TYPE SET 

 -A 302 TYPE SET 



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COMPONENT FREQUENCY IN KILOCYCLES PER SECOND 



Fig. 18— Ringer output spectrum. 



\ Since the magnetic field also affects the operate current for the ringer, 

 ^the adjustment of the permanent magnet and of the biasing spring are 

 ^ interdependent and must be coordinated to produce a satisfactory ringer. To 

 do this manually would be a slow, tedious and costly process. It is not too 

 much to say that large scale production with uniform adjustment of the 

 new ringer by conventional methods would have involved prohibitive manu- 

 facturing costs. The solution here described— automatic adjustment of ring- 

 ers—provides another example of the close cooperation between develop- 

 ment and manufacturing organizations to assure performance with economy. 

 Figure 19 illustrates the automatic adjusting process schematically. The 

 ringer, fully magnetized and with an overtensioned biasing spring, is held 

 in a fixture between the poles of an electromagnet. A shaft with a forked 



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