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BELL SYSTEM TECHNICAL JOURNAL 



give the requisite mechanical strength for winding or mounting. It 

 may, therefore, be necessary to use a very low llux density in these 

 instances in the core design. 



The best flux density and area for the pole face as regards electro- 

 mechanical cfificicnc\- is obtained by making the air-gap reluctance 



Fig. 6^Curvcs of Core .Section and Pole Face Flux Density vs. Pull. These Curves 

 Assume That the Pole F"ace Area is C.reater than the Core Section 



equal to the reluctance of the remainder of the magnetic circuit. 

 Here again it is found that practical considerations must be carefully 

 weighed, otherwise an unreasonable design results. If, for instance, 

 the pole face density on a 5 gram relay is taken equal to the cus- 

 tomary core density, a very small pole face area results. To make 

 the air-gap reluctance, then, eqtial to the reluctance of the remainder 

 of the magnetic circuit, it is found that an air-gap of possibly .001" 

 or less results. Such a small armature movement, of course, is gener- 



