562 



BELL SYSTEM TECHNICAL JOURNAL 



There is always a loss in efficiency due to the losses in the shield, 

 and this loss is greater, the closer the shield is placed to the coil, that is, 

 the stronger the field in which the shield is placed. However, even 

 with solenoidal air-core coils very effective shielding may be attained 

 by moderately thick copper shields spaced about the distance of a 

 diameter from the coil. 



Principles of Electrostatic Shielding 

 In both theory and practice all measurements assume that between 

 different terminals or junction points of the system there are im- 

 pedances having values known to a degree of definiteness consistent 



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Fig. 1 — Methods of Shielding Series Impedances. 



with the accuracies sought in the test. In an unshielded circuit it 

 will generally be the case that the elements connected by the various 

 terminals or junction points will not provide impedances so definitely 

 known or in other words will not carry all of the current flowing 

 between the points in question. 



Resistors 

 In the case of the simple resistor such as pictured schematically in 

 Fig. lo there are admittances from different parts of the conductor 



