oz 



THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY HJ54 



decay of an electromagnet when the core is saturated. A similar condition 

 applies to the leakage field of enclosed reed relays where the magnetic 

 material constitutes only a small part of the inside cross-section of the 

 coil. In such cases the reluctance of interest is that characterizing the 

 complete magnetic circuit of an air core solenoid. 



A number of simple relationships for various coil configurations follow 

 from the identities between reluctance and inductance. Inductance is 

 defined as the ratio of flux linkages to applied current: 



^ _ AV 47rA V 

 from which: 



(f{ = 



(19) 



showing the inverse relationship between (51 and the single-turn induc- 

 tance L/N'. Inductance for most of the usual coil configurations has 

 been studied by Rosa and Grover' who give relations from which reluc- 

 tance may be estimated. Most such expressions are quite involved; 

 among the simpler is that for the case where h and c in Fig. 19 are small 

 compared to a, given by: 



(R = 



0.2317a + 0.446 + 0.39c 



- a'_ MEAN--' 



-" -' , RADIUS-''' 



Fig. 19 — ]\IaKiif'1i<' fi<'l<l of a solpnoid. 



