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BELL SYSTEM TECHNICAL JOURNAL 



core section these data may be used direciK-. If, linwcxer, tlie pole 

 face area differs from the core section, the air-gap used in looking 

 up thejeakage in Fig. 2 and Fig. 3 should lie reduced to a \alue which, 



Fig. 2 — Curves of IVrccntngc Useful Flux vs. Length of Air dap in Terms of Core 

 Diameter for Single Spool Relays 



with a pole face area ecjual to the core section, would gi\e the same 

 air-gap reluctance. 



The core flux density and the pole face (k'usity depend hugely U|)()n 

 the requirements of the particular design, but the considerations 

 outlined in the next four paragraphs arc of prime importance. 



In some cases the annual power charges are relaii\ el y iinini|iortant, 

 there being plenty of pow'cr available during the short inter\als of 

 time required for operation. Obviously in this case efiiciency of 

 operation can be sacrificed, and consequently power, .in order to 



