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BELL SYSTEM TECHNICAL JOURNAL 



four per cent of the total ampere turns required to saturate the core 

 while on a relay which controls five grams the same assumptions 

 show over oO per cent of the total ampere-turns required to saturate 

 the core. Where small forces such as fi\e grams arc in\olved, we 

 are almost in\arialjK- concerned in niaintainint; a high efficiency 



I'i«. 4 Curves of Core Section and Pole Face Klux Densitv vs. Pull 



whereas in designs for the hea\icr forces a few additional ampere- 

 turns required in the core are relatively unimportant. 



The work done by an electromagnet is IT = 980 F L ergs where 

 F is expressed in grams and L in centimeters. The energy in ergs 



required to magnetize the core is II"=^_- where NI represents the 



ampere-turns required to force the flux <^ through the core. The 

 ratio of the core energy and the useful work may be taken as a criterion 

 of the efficiency of the core design. .Applying this reasoning to 



