MAGXKTIC DKSir.X oF RlsLAYS /O 



I)( s/'i/n Coiisidcralions 



The I'clal ions coiit i-()lliii<2; the sciisil i\it y and woi'k capacity jjioxidc a 

 direct, indication of the crt'cct of the conlii2,ui'ation and dimensions of an 

 electroma<i;net upon its performance. Hei'e tliese will lie considered only 

 in lieneral terms. Other things heinji ('([ual, the sensiti\ity r('(|uii'ement 

 determines the coil size, and hence the o\'er-all dimensions, while the 

 work capacity determines the cross-sec t ions of the maonetic path. 

 Within the fiamewoi'k of these generalizations, the dependence of ))er- 

 formance on size and shape can be most readily considered in terms of 

 the individual magnetic parameters. 



The core reluctance (Re is characterized primai'ily by its minimum 

 \-alue (Sic , and by the saturation flux (p" which sets the upper limit to 

 the work capacity. </?" varies with the core cross-section a, as does ip', the 

 flux le\'el for minimvun reluctance. (Re is given approximately by t/(n'a), 

 where ( is the core length, which varies with the coil size. For heavy duty 

 relays with a large cross-section a, (Re is a minor component of the total 

 reluctance except for ip very close to saturation. For highly sensitive 

 light duty relay's, ( is large and a small, making (Re large, and a must be 

 made large enough for <p to be close to <p', so that (Re may be close to 

 its minimum \'alue (Re . 



The leakage reluctance (Rl depends upon both the dimensions and the 

 shape of the magnetic path. The leakage per unit length depends upon 

 the ratio of the perimeter of the magnetic members to their separation, 

 and is a minimum for a scjuare magnet outline as in the two coil tele- 

 graph type relay. Space and accessibility considerations cause telephone 

 relays to have a long narrow single coil configuration, increasing the 

 leakage per unit length by a factor of from two to four over that of the 

 scjuare outline. Aside from this shape factor, the leakage reluctance 

 varies in\-ersely as the length. With respect to the static characteristics 

 the leakage flux (1) increases the core cross-section required to attain a 

 given work capacity, (2) decreases the equivalent pole face area, as 

 shown by ecjuations (25), (3) increases the equivalent closed gap re- 

 luctance (>to , as shown by the same eciuations, thus decreasing the sensi- 

 ti\-ity. In addition, as chscussed in the companion articles''^" it delays 

 operation and release by increasing the total field linked by the coil. In 

 all these effects, the controlling factor is the ratio (i{/,/(5to , rather than the 

 absolute value of (R/, , so these effcH'ts are mcjst readily minimized by 

 making 6U small. 



The closed gap reluctance (Ro is the sum of the reluctances of the return 

 meml)ers and of the closed gaps and joints. The reluctance of \hc retiu'ii 

 meml)ers is comparable with that of the c-oi-e but smallcM', and in most 



