FERRITE CORE INDUCTORS 



285 



(b) The slope should not be negative at any position in the adjusting 

 range as this would introduce the possibility of false or double balance 

 when the coil is being adjusted by null or peaking methods. 



(c) The slope should not be so small that play in the mechanical parts 

 will cause changes in slope of the same magnitude as the slope itself. 



(d) Within the above limitation the smaller the slope the more pre- 

 cisely the adjustment can be made. 



(e) Conversely, the greater the slope the greater the range of ad- 

 justment for a given amount of mechanical motion. 



(f ) It follows from (d) and (e) that the amount of mechanical motion 

 available determines the product of range and precision. Where the 

 mechanical motion is rotary, such as with adjustment by turning a 

 screw, it is possible to adjust a coil to a precision of about 1/400 of a 

 revolution without undue difficulty. If the range is covered by N re- 

 volutions of the adjusting screw, and the over-all range is =b i? per cent 

 of the mean value : 



2R 



R 



400i\r 200N 



where P = the precision of adjustment in per cent of the nominal value. 

 In the coil shown in Fig. 4, six turns of the adjusting screw are effec- 

 tive in producing an over-all change of ±15 per cent in the inductance. 

 The precision with which the adjustment can be made is, therefore, 

 very close to ±0.01 per cent of the value desired. 



AIR GAP-' 



Fig. 9 — Inductance adjustment by constriction in magnetic cross section. 



