FERRITE CORE INDUCTORS 283 



contrary, it would probably be more advantageous as well as mechani- 

 cally simpler to have the air gap length constant so that the increase in 

 inductance due to shortening the magnetic path would be augmented 

 by an increase in permeability. In a device such as shown schematically 

 in Fig. 8, as the two windings approach each other the over-all magnetic 

 path decreases and at the same time the area of the air gap increases. 

 Actually, the preponderant effect is due to the air gap change and the 

 effect of variation in length of path becomes of secondary importance. 

 This is hkely to be the case generally, when both path length and air 

 gap change simultaneously, since most of the reluctance is in the air 

 gap. 



2. Adjustment of Inductance by Change in Magnetic Cross Section, A 



To provide for change of inductance by constriction of a part of the 

 magnetic cross section is apt to be undesirable since it forces a concen- 

 tration of flux in the constricted part of the magnetic circuit and may 

 introduce unduly high hysteresis losses. Even if the levels are low enough 

 so that this is not a consideration the amount of inductance variation 

 that can be achieved even by a large constriction in part of the core is 

 relatively small. To illustrate this we will consider a structure such as 

 shown schematically in Fig. 9, in which one sector of the core can be 

 varied in effective cross section. The reluctance of the structure is equal 

 to the sum of the reluctances of the fixed part of the core, the sector of 

 length nC, whose cross section aA can be varied, and the air gap: 



^ ^ (1 - n)i _^ _rU_ ^ g{ 



^rnA fJ-maA A 



(45) 



= -^\l + n(-- 1) + guJ. 

 y,mA \_ \a J 



Let us assume the arbitrary but reasonable values of 2000 for jijn, 

 0.01 for g, and 0.1 for n. Then approximately 



R= ' 



2000A 



(--^)- 



It will be seen that as a is varied from 1.0, corresponding to the full 

 cross section of the main core, to one-tenth of that, the change in in- 

 ductance, which is inversely proportional to reluctance, will only amount 

 to about 5 per cent. 



