428 



BELL SYSTEM TECHNICAL JOURNAL 



With a small air core coil and an infinite cylinder the axis of which is 

 perpendicular to the axis of the winding and to the direction of the 

 disturbing magnetic field the two definitions are equivalent. 



The circuit used in making measurements consists of a field coil 

 producing a magnetic field, a pick-up coil and a vacuum tube voltmeter. 

 The field coil is a loop two feet in diameter consisting of 500 turns of 

 wire. The magnetic field at the center of this coil is uniform over a 

 space sufficiently large for the purpose. The pick-up or search coil 

 when placed in the magnetic field will have a voltage produced across 

 its terminals. This voltage is measured with the vacuum tube 

 voltmeter and Eg and Ei in equation (21) are thus obtained. The 

 size of the shell type core on which the pick-up coil is wound is 

 3" X 1 29/32" X 1" where the 1 29/32" dimension is parallel to the 

 axis of the winding, the 1" dimensions being the pile-up of laminations. 

 Unless otherwise mentioned this coil was used in all of the following 

 measurements. 



Permalloy having an initial permeability of the order of 5000 at 

 low frequencies was employed for both the core and the shield through- 

 out this investigation. 



Permalloy Cases 



In Fig. 7 is shown the shielding efficiency vs. frequency of a rec- 

 tangular permalloy case which consists of five contiguous layers of 



500 1000 



FREQUENCY IN CYCLES PER SECOND 



Fig. 7 — Observed shielding efficiency of laminated permalloy case. 



.014" thick permalloy sheet. The size of this case is approximately 

 2 3/8" X 2 1/2" X 3 1/4" and the position of the coil inside the case 

 is such that the axis of the winding is parallel to the 2 1/2" dimension. 

 The relative size of the coil and case is such that there is approximately 



