536 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1952 



over a period of several cycles. It is well known that a neutralization 

 erase results in a residual background noise which may be as much as an 

 order of magnitude below that produced by a saturation erase. The 

 neutralization erase is therefore widely used in tape recording, and is 

 obtained by energizing the erase head with alternating current of a 

 frequency several times the highest signal frequency passed by the 

 recorcUng equipment. 



With the impregnated recording medium, the recorded signal can be 

 successfully erased by using a conventional ring-type erase head energized 

 with high-frequency current. The field from this type of erasing effec- 

 tively neutralizes the surface layer which contains the recorded signal, 

 but does not penetrate appreciably beyond. Therefore, if precautions are 

 not observed, the lower layers of tliis medium beyond the reach of the 

 erase field may acquire a random cumulative magnetization from switch- 

 ing surges, accidental exposure to magnetized tools and strong fields, 

 and this will be evidenced by a gradual deterioration in the signal to 

 noise ratio at the low-frequency end of the transmission band. The qual- 

 ity, however, remains entirely adequate for commercial telephone use. 



The foregoing limitations are minimized by an erasing method which 

 has been developed at these Laboratories for applications where it is 

 convenient to erase the entire message in one revolution of the recording 

 cylinder, preparatory to recording a new message. Tliis method employs 

 an erasing structure in the form of an E-shaped stack of magnetic 

 laminations, carrying on the center leg a coil which is energized by low- 

 frequency (60 cycle) alternating current. The lamination stack is ap- 

 proximately the width of the recording medium, and the gaps between 

 the center leg and each side leg are about j inch T\dde. When this struc- 

 ture is spaced about y§- inch from the surface of the recording mediiun 

 traveling at 6 inches per second or less, and is energized by 60-cycle 

 power to produce a maximum field of about 2000 gauss, the entire 

 thickness of the recording medium is subjected to an alternating magnetic 

 field which reaches saturation intensity and over a period of several 

 cycles decreases progressively to zero. This effectively demagnetizes the 

 full tliickness of the recording medium. If the current is switched off 

 with the erase structure in operating position, those elements of mag- 

 netic material wdthin the field at that instant would be subjected to no 

 further reversals and would consequently behave substantiallj'' as if they 

 had been subjected to a direct-current magnetic field of the same in- 

 tensity as the alternating-current field at the time it was interrupted. 

 The section of record medium under the influence of the erase structure 

 at the time it was de-energized would exhibit excessive noise in com- 



