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BELL SYSTEM TECHNICAL JOURNAL 



an alternating current Is flowing is enclosed in such a shield, one part of 

 the flux lines from the coil will encounter the shield; the remaining 

 part will not. Let every line A(^ of the flux which encounters the 

 shield be divided into components perpendicular to the shield and 

 parallel to it. If 9 is the angle between such a line and the plane of 

 the shield, the component perpendicular to the shield, which is effective 

 in setting up eddy currents in it is A^sin 6. The other component, 

 A^ cos 6, makes no coupling with this shield, and sets up no eddy cur- 

 rents. To every line A^ sin 6 the shield presents a certain inductance 



Fig. 9 — Photograph of filter in soldered shield. 



and resistance, and the average inductance and resistance presented 

 to all the linking lines may be taken as the inductance and resistance 

 of the shield. The equivalent circuit of a coil surrounded by a shield 

 may be indicated, then, as a coil Li of two sections coupled to a second 

 coil L2, in parallel with a resistance i?2- This parallel circuit is the 

 equivalent circuit of the shield. The coil Li has a total inductance 

 equal to that of the coil in the shield, and is considered to be divided 

 into two sections, the first of which, "A," makes no coupling at all 

 with L2, and the second, "B," which makes perfect coupling with L?. 

 If K is the coefficient of coupling between the coil Lx as a whole and L2, 



