MAGNETIC DRUM TRANSLATOR FOR TOLL SWITCHING OFFICES 713 



tiiused to flow through the windings of the head in a direction opposite 

 to that taken by the preconditioning current. This pulse lasts for only 

 t wo or three microseconds, and movement of the drum surface is negli- 

 gibly small while the current persists. The peak value of the current pulse 

 is sufficient to magnetize to saturation in the opposite direction that por- 

 tion of the track which lies directly under the pole-tips at that instant. 

 Areas of the track far-removed in each direction from the pole-tips of 

 the head are, of course, unaffected by this operation, and remain at sat- 

 uration in the original polarity. A region of transition in magnetization 

 1 herefore e.xtends in each direction along the track from the area directly 

 under the pole-tips. 



Fig. 3 illustrates some of the wave forms resulting from writing into 

 and reading from four adjacent cells on one track of the dnun. Line A 



' shows the pulses of writing current which were applied to the windings 

 on the head. These were caused to appear at precisely spaced distances 

 ;ilong the track by the combined operation of the synchronizing system 

 and an "administration" circuit. In cells 1 and 3 the writing current po- 



I larity is chosen so as to write "I's." Cell 2 remains in its original precon- 

 ditioned state. In cell 4 a 1" was previously written but is now altered 

 lo a "O" by a writing current pulse of the same polarity as that chosen 



1 for the preconditioning operation. 



Line B in Fig. 3 illustrates the resultant magnetic state of the drum 

 .surface as \'iewed by the reading head. The polarization portrayed as re- 

 sulting from writing a "7" is a bell-shaped curve. When a " 1" is selec- 

 \We\y altered to a "0" the area of track directly under the pole-tips will 

 be carried to saturation in the original preconditioned polarity. The whole 

 I ell area, however, cannot be affected so strongly, owing to the hysteresis 

 properties of the coating material, and there will remain traces of the 

 ' 1" type of magnetization near the cell edges, as indicated by the solid 



: line in cell 4. 



i There is no difficulty in rewriting a "7" in a cell which has been sub- 



I jected to the above described treatment. The procedure is that outlined 

 tor the original writing of a "i" and the results are practically indistin- 



I guishable from those obtained by writing in a virgin cell. 



Heading Operations 



On subsequent revolutions of the drum, the passage, under the pole- 

 tips, of the magnetic irregularities created by writing "i's" will induce 

 a change of flux through the windings of the head. The change 

 is, of course, a function of distance along the drum surface but since tl.ri 

 drum is rotating continuously at a substantially uniform speed the change 



