THE DESIGN OF ELECTROMAGNETS 69 



Fig. 22, and since the cross-section and length of each part of the 

 magnetic circuit are now known, the component of the total 

 ampere-turns necessary to overcome the reluctance of the iron 

 or steel casting can be calculated in the usual way. The re- 

 luctance of the steel sphere which constitutes the armature 

 would be considered negligible in these calculations. 



In a well-designed magnet of this type, the reluctance of the 

 iron portions of the circuit is but a small percentage of the air- 

 gap reluctance, unless the specified air gap is very small. When 

 the armature is in contact with the pole faces, the total flux will 

 be greater than the amount necessary to produce the required 

 initial pull. It is interesting and instructive to calculate the 

 pull between magnet and armature when the air gap is practically 

 negligible. The limit is reached when all the exciting ampere- 

 turns are required to overcome the reluctance of the iron, and 

 the calculation has to be made by assuming probable values of the 

 flux density, and then calculating the loss of magnetic potential 

 across each portion of the circuit. 



With reference to the important matter of cost; air-gap den- 

 sities other than the assumed density of 7,240 gausses may be 

 tried with a view to obtaining the design of lowest first cost. A 

 saving in copper may be effected by allowing the temperature 

 rise to approach as nearly as possible the specified limit; but in 

 this as in all economical designs of apparatus in which a saving 

 in first cost is accompanied by a loss of efficiency in working, the 

 interests of the user demand that proper attention be paid to the 

 cost of operation (in this case of the PR losses) when considering 

 the expediency of lowering the manufacturing cost by econo- 

 mizing in materials. 



