ELECTRIC WAVE FILTERS 



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angle of approximately 60° about its horizontal axis. On this rotor 

 was wound a large number of turns of small gauge wire, and the winding 

 was short-circuited. A coil of this type is shown in Fig. 7. The 

 coupling between the rotor in its horizontal position and the main 

 coil is very small, and therefore the effective inductance between 

 terminals is practically that of the main coil. However, if the rotor 

 is rotated in such a way that its plane becomes more nearly parallel 



Fig. 7 — Photograph of "AA" coil. 



with that of the main coil the coupling between it and the main coil 

 is increased, and the effective inductance of the whole is reduced. It is 

 possible to vary continuously the inductance of a coil of this design 

 over a range of about 3-2 per cent. 



When coils of this type and their associated condensers are connected 

 together as they would appear in a filter, their resonant frequency or 

 frequencies are located by an adjustment of the coil or coils in the 

 mesh. If the mesh contains more than one adjustable coil, more than 



