286 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1953 



(g) It would appear from the above that by gearing or other means 

 of increasing the mechanical motion that governs the inductance change 

 it should be possible to improve the precision. This is true up to the 

 point where consideration (c) is violated, to the point where play in the 

 mechanical parts permits variations of the same order as the nominal 

 precision. 



An obvious form of permeability adjustment would consist of a means 

 for varying the distance between two plane magnetic surfaces, as shown 

 schematically in Fig. 11 (a). One disadvantage of this, or of any other 

 means that involves a change in air gap length, is in the nonlinearity of 

 adjustment. The effective permeabiHty of a coil is given in (44). In most 

 voice and carrier frequency applications of ferrite the magnitudes of g 

 and a mil be such that, approximately 



M = - (46) 



and, if the cross section of the air gap is the same as that of the core 



1 



9 



A typical adjustment curve resulting from this inverse relationship is 

 shown in Fig. 11 (b). 



In addition to its nonlinearity the simple butted gap has a short- 

 coming in that the mechanical motions involved are of the order of only 

 a few hundredths of an inch, which requires that parts be very accurately 

 fitted. This can be somewhat alleviated by using cone or wedge shaped 



Fig. 10 — Inductance adjustment by addition or removal of turns. 



