436 BELL SYSTEM TECHNICAL JOURNAL 



are obtained with a spacing of 1/16". A comparison between the 

 curves C and E shows the improvement which is obtained in this 

 case with 1/32" copper plates. The effect is somewhat less than with 

 a spacing of 1/32", at least at frequencies below 1000 cycles. At 

 low frequencies copper plates, 1/16" thick, show a slight improvement 

 over the 1/32" copper plates as is shown by the curve D. 



Curve B on Fig. 10 shows that if the airspace between the core and 

 the box is small the shielding efficiency is very low. In a case like 

 this a copper spacer is very effective. For example, a 5 or 10-mil 

 copper plate replacing an airspace of the same thickness will greatly 

 improve the shielding efficiency. 



General 



The foregoing is a discussion of the magnetic shielding of trans- 

 formers from external magnetic fields. The reverse problem of 

 shielding a transformer or coil so as to prevent its magnetic field from 

 affecting other apparatus has not been considered. However, it is 

 safe to assume that approximately the same degree of shielding will 

 be obtained, provided the leakage field does not produce excessive 

 saturation in the shield. That is, assuming that a power transformer 

 is producing a disturbing magnetic field in the space occupied by an 

 input transformer, then a shield over the power transformer will 

 produce approximately the same effect as a shield over the input 

 transformer, where each shield has been constructed in accordance 

 with the information on the foregoing pages. This has been demon- 

 strated experimentally in an article by J. E. R. Constable in the 

 Wireless World of February 26, 1937. 



Although this paper has been restricted to the magnetic shielding 

 of transformers it is equally applicable to any apparatus which is 

 susceptible to inductive pick-up. This is because in any apparatus 

 where there is inductive pick-up there is in effect a coil. It may be 

 an actual coil and it may be only a loop of lead wires. 



The author wishes to thank Mr. E. T. Hoch for many helpful 

 suggestions. 



Bibliography 



1. J. Stefan, Wied. Annalen, Vol. 17, p. 928, 1882. 



2. A. W. Rucker, Phil. Mag., Vol. 37, p. 95, 1894. 



3. H. DuBois, Wied. Ann., Vol. 63, p. 348, 1897; Vol. 65, p. 1, 1898, also Electrician, 



Vol. 40, 1897, pp. 218, 316, 511, etc. cont. 



4. A. P. Wills, Phys. Rev., Vol. 9, p. 193, 1899; Vol. 24, p. 243, 1907. 



5. James Russell, Roy. Soc, Edinb., Trans., Vol. 40, p. 631, 1903. 



6. E. F. Nichols and S. R. Williams, Phys. Rev., Vol. 27, p. 250, 1908. 



7. W. Esmarch, Ann. der Phys., Vol. 39, p. 1540, 1912. 



8. W. W. Coblens, Bui. Bu. of Standards, Vol. 13, p. 423, 1916. 



