370 BELL SYSTEM TECIIMCAL JOURNAL 



a real loaded cable, the iiKiiiciance of which \aries with both cuirent 

 and frequency and in which all the above noted resistance factors 

 are present, may give, and in general will give when operated 

 at its maximum speed, greater distortion of signals than a non- 

 loaded cable. 



To solve the question of distortion on a purely theoretical basis 

 required consideration of tiie transmission of a transient over the 

 loaded cable. This was made extremely difficult by the existence 

 of numerous possible causes of signal distortion, the effects of which 

 could only be approximated in the solution of the transient problem. 

 In addition to the distortion resulting from the rajjid increase of 

 attenuation with fre(]uency due to the various sources of alternating 

 current losses, distortion peculiar to the magnetic characteristics of 

 the loading material had also to bo taken into account. There art 

 several types of magnetic distortion to be concerne<l about. First, 

 there is the production of harmonics as a result of the non-linear 

 magnetization cur\e of the loading material; second, there is a pos- 

 sible asymmetrical distortion due to hysteresis, and third, there 

 is a possible modulation resulting from the superposition of signals 

 on each other, that is, in effect, a modulation of the head of the wa\e 

 of one impulse by the tail of the wave of a preceding impulse. The 

 first two of these are effective at the sending end of the cable and 

 the third near the receiving end. 



A compulation of distortion, inchuiing ilie i)eciiliar magnetic 

 effects, by a steady state a.c. nietliod based on measurements of short 

 loaded conrluctors indicated thai the cable should operate satis- 

 factorily with ordinary sending xoUagcs. I'lirtlier evidence that 

 none of these \arioiis types of distortion would be of serious con- 

 sequence and tlial llu- di>ioilion of a loadi-d lahli- could be corrected 

 by terminal apjiaralus, was obtained b\' exiK-riments with an arti- 

 I'uial lini' constructed to simulate closeK', with regard to electrical 

 characteristics, the t>lie of loaded conductor witli whicli we were then 

 experimenting. This artiticial line was loaded wiili iron dust core 

 coils which served the purpose admirabh'. not oiiU- as regards in- 

 ductance and alternating current resistance but also as regards 

 m.ignetic distortion. Iron dust is, of course, very different in its 

 magnetic characteristics from permalloy. However, owing to i In- 

 large number of turns on a coil, it is operated u mwU liighcr held 

 strengths and on a part of the magnetization cur\c corresponding 

 approximately to thai at which permalloy is operated on the cable. 

 The ca-ic for magnetic distortion was in fact .i little worse with the 



