168 BELL SYSTEM TECHNICAL JOURNAL 



It will be assumed that the recording medium has been previously 

 magnetized by drawing it past a pole-piece so that the residual mag- 

 netization in it has a direction as indicated by the upper arrow at the 

 left.^ In this method of recording the magnetization is principally 

 parallel to the axis of the medium but in order to simplify the drawing, 

 the direction of magnetization in Fig. 1 is shown at a considerable 

 angle. If the pole-piece P carries a steady flux in the direction indi- 

 cated by the heavy lines, this flux will spread in the medium. At the 

 point 2 in the middle of the pole-face, the flux will be substantially 

 perpendicular to the axis of the medium. On either side it will be 

 approximately parallel to the axis of the medium but of opposite 

 directions. As the elements of the recording medium approach the 

 pole-piece P, they will first be subjected to the flux 1 which is in ap- 

 proximately the same direction as the residual magnetization in the 

 medium so that no appreciable change will take place. When the 

 elements are directly opposite the face of the pole-piece they will be 

 acted on by a flux 2 which is nearly perpendicular to the residual 

 magnetization of the medium. When the elements reach the position 

 3, the flux will be in opposition to the original magnetization within 

 the medium. If it were not for these changes in the direction of the 

 flux while the elements are passing from the position 1 to the position 

 3 a signal record without appreciable distortion could be left on the 

 medium at the point 3 by superimposing a signal flux on the steady 

 flux in the pole-piece P. It will be realized that the positions 1, 2 and 3 

 are not discrete points but that they cover an appreciable distance. 

 The spreading of the flux at 3 will be considerable so that it will be 

 necessary for the medium to travel at high speed in order to get the 

 recorded signals away from the recording flux before the record is 

 distorted by subsequent signals. 



Figure 2 shows a similar diagram for two pole-piece recording. 

 Where two pole-pieces are relatively close to each other, the flux 

 will not spread so much in the medium and the direction of magnetiza- 

 tion will be approximately the same as that of the recording flux. 

 It is again assumed that the residual magnetization within the medium 

 is mainly in the opposite direction to the motion of the medium as 

 indicated by the upper arrow at the left. The flux 1 will have no 

 appreciable effect on the residual magnetization. The flux 3 is in the 

 opposite direction to the residual magnetization and were it not for 



2 In Figs. 1, 2, 3, 4 and 6, the heavy lines passing through the pole-pieces represent 

 the instantaneous recording flux. The density of the fine lines in the recording 

 medium represents the intensity of magnetization. The arrows above and below 

 the medium show the direction of this magnetization. The curve below represents 

 the nature of the signal that has been recorded on the tape. 



