Magnetic Shielding of Transformers at Audio Frequencies 



By W. G. GUSTAFSON 



The first part of this article is a descriptive discussion of mag- 

 netic shielding in general. Formulae are then given for the 

 calculation of shielding efficiency of cylindrical shells for steady 

 and alternating magnetic fields. By means of these formulae 

 the shielding efficiency for various types of cylindrical shields has 

 been calculated for a steady magnetic field. 



The second part of the article contains experimental information 

 on various types of transformer shields. This information supple- 

 ments the theory in connection with factors which would be very 

 laborious to treat theoretically. 



The theory and the experimental data are coordinated in such 

 a manner that the shielding efficiency of a particular shield can 

 be calculated with an accuracy which is sufficient for practical 

 purposes. 



IN connection with the development of repeaters for long distance 

 telephone lines it is found that noise is introduced into the telephone 

 lines due to magnetic pick-up by transformers and coils in the repeaters. 

 This applies also to sound pictures equipment, public and private 

 address systems, etc., where high gain amplifiers are used. The 

 stray magnetic field causing this pick-up may be produced by neigh- 

 boring generators, transformers, rectifiers and other power equipment. 

 It may also be produced by other amplifier transformers and coils or 

 by relays located in the vicinity of the disturbed coil. The intensity 

 of the disturbing field may frequently be of the order of 0.1 oersted 

 at the point of pick-up. However, a field intensity of the order of 

 0.02 oersted often causes objectionable noise and under extreme 

 conditions values as low as 0.001 oersted may be undesirable. As the 

 gain of the amplifier increases and the demand for good quality 

 becomes greater, it becomes increasingly important to control magnetic 

 pick-up. The limiting of this pick-up is in fact today one of the 

 important problems to be considered in the design of high-gain 

 amplifiers. 



One method by which the magnetic pick-up can be decreased is by 

 arranging the core structure and winding distribution of the trans- 

 former in such a way that the voltages induced by an external field 

 are at least partially neutralized. In many cases, however, this 



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