FERRITE CORE INDUCTORS 289 



the other the area of registration between the semicircular faces of the 

 center posts is reduced or increased. A simple arrangement such as this 

 provides good linearity but has two limitations: (1) The total mechanical 

 motion available for adjustment is only 1/2 revolution. (2) As the cores 

 are rotated to reduce the air gap area and the inductance, the magnetic 

 flux in the cores tends to concentrate more and more in the vicinity of 

 this reduced area, and may under some conditions give rise to high 

 hysteresis loss. 



The method of adjustment used in the 1509 type inductor, Fig. 4, 

 is essentially a variable area method but it is designed in such a way as 

 to overcome these two limitations.^ The air gap arrangement, visible in 

 the cutaway view, consists of two gaps in parallel. The main annular 

 gap is fixed and is large enough in area to insure that under no antici- 

 pated conditions of operation will the flux concentration be too high. 

 The screw adjustment moves a cylindrical ferrite part into a depression 

 in the center post, as shown. The effective cross section of this adjustable 

 gap is approximately determined by the amount of surface of the cyl- 

 inder within the depression. The total useful adjusting range corresponds 

 to about six full turns of the adjusting screw. 



5. Adjustment of Inductance by Change in Coupling 



The overall inductance of two coils of equal inductance connected in 

 series is 



L = 2(1 + k)Li (48) 



where L = series inductance 



Li = inductance of either coil 



k = coupling coefficient 



k may have any value between —1 and +1, these values correspond- 

 ing to complete coupling and the windings connected in series opposing 

 and series aiding, respectively. It is practicable to make inductors whose 

 coupHng can be continuously adjusted from very high positive values 

 through zero to equally high negative values. This type of design is 

 especially useful where a very wide range of inductance variation is 

 desired. It will be seen from (48) that with couplings of plus and minus 

 90 per cent, respectively, in the extreme positions a range of 19 to 1 in 

 inductance variation would result. A coil of this type is shown in Fig. 

 14. 



