i>i:sn;x cii.iR.icir.NisTics or i-.i.i:cih'OM,iGxnrs 211 



ohl.iin a low first rost. Ki-frrrin>j to MaxwcH'-; formula for trarlion 

 or |)ull 



P = 



8ir980 



the pull P is proportional to the square of the armature air-pap flux 

 density B, conse(|uently the total flux required will be less the greater 



i 



Hg. J — Curves of Percentage L'scfui Flux vs. Length of Air Gap in Terms of Core 

 Diameter for Double Spool Relays 



•he gap density. A high core flux density and pole face density gives 

 .1 small core section and consequently a small and cheap magnet. 

 The limit to the decrease in size is the allowable temperature limit of 

 the winding. 



Of course, there is a limit to the sacrifice of efficiency to obtain a 

 low first cost. If the reasoning in the preceding paragraph is ap- 

 plied to a 5,000 gram electromagnet, the results will show three to 



