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THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1953 



3. Adjustment of Inductance by Change in Number of Turns , N 



An adaptation of turns adjustment for use in a shell type structure 

 is shown in Fig. 10.^ The center post with the winding on it can revolve 

 and turns can be removed by pulling on the outer lead, or added by 

 rotating the knob at the end of the shaft. Continuous adjustment is 

 possible since it is not necessary to add or remove integral numbers of 

 turns. Although an inductor of this sort involves some mechanical com- 

 plexity it has the advantage that the core parts do not have to be pre- 

 cisely machined. The turns adjustment can be used to compensate for 

 sizable variations in the dimensions of the core parts and the resulting 

 air gaps. 



4. Adjustment of Inductance by Change in Permeability , n 



Adjustment by change in permeability, that is, change in length or 

 area of air gaps, can be accomplished in a variety of ways to meet dif- 

 fering design needs and can be mechanically simple and economical. 

 For most purposes permeability adjustment will offer more advantages 

 than any of the other methods. 



Whatever the method of adjustment used its effectiveness will de- 

 pend on proper correlation between the mechanical motion that pro- 

 duces the change and the inductance itself. For most filter and network 

 apphcations the following considerations will apply: 



(a) The slope of the line showing inductance plotted against the dis- 

 placement that produces the adjustment should be reasonably con- 

 stant over the adjusting range. 



Fig. 8 — Inductance adjustment by decrease in magnetic path length and in- 

 crease in air gap area. 



