166 BELi SYSTEM TECHNICAL JOURNAL 



producing pole-pieces may be placed directly opposite each other so 

 that the magnetic pattern consists of variations in the intensity of 

 magnetization, the direction of the magnetization being substantially 

 perpendicular to the surface of the tape. This type of magnetization 

 will be called perpendicular magnetization. There is another method 

 of recording in which the magnetization is in a direction perpendicular 

 to an edge and parallel to the surface of the tape which has been called 

 cross or transverse magnetization. 



In spite of the fact that the principle of magnetic recording has been 

 known for a long time, there has been very little literature on the sub- 

 ject until recently. Several papers ^ which deal almost entirely with 

 the longitudinal method of magnetization have been published abroad 

 during the past two years. Cross magnetization is discussed briefly 

 in one of the papers. Apparently, perpendicular magnetization has 

 not been seriously considered. This paper will treat mainly the 

 perpendicular method of magnetization with which a good frequency- 

 response characteristic may be obtained with a tape speed of only 16 

 inches per second. 



Forms of Recording Media 



Steel wire has been used as a recording medium in most of the 

 telegraphones. This was probably because it was easier to obtain. 

 When wire is used it is necessary to make the longitudinal separation 

 of the pole-pieces rather large. This is done in order to minimize the 

 distortion caused by the continual rotation of the wire about its axis. 

 Such rotations change the relation of the magnetic patterns in the wire 

 with respect to the reproducing pole-pieces from that which existed 

 at the time the record was made. «. 



When the pole-pieces have a wide separation, high linear speed must 

 be used in order to record and reproduce high frequencies. The high 

 speed required in this method of recording gives rise to a number of 

 mechanical difficulties. The contacting pole-pieces wear away rapidly 

 and it is difficult if not impossible to construct and hold them so that 

 they will ride smoothly against the wire. These variations in contact 

 with the wire change the magnetic reluctance of the flux path so 

 that the signal strength varies and an excessive amount of noise is 

 introduced. 



Recording on steel discs has been investigated from time to time 

 but no practical results have yet been reported. 



^ See list at end of this article of recently published papers dealing with magnetic 

 recording. 



