648 BELL SYSTEM TECHNICAL JOURNAL 



and communication facilities are in proximity, electromagnetic induc- 

 tion from the power system may cause disturbances in the communi- 

 cation system. The avoidance or minimizing of such disturbances, 

 with due regard to the service and other needs of both systems, is a 

 problem of coordination, which is conveniently divided into two parts, 

 one dealing with low-frequency inductive coordination and the other 

 with noise-frequency coordination. 



The present paper undertakes a general examination of the problem 

 of low-frequency inductive coordination in the light of developments 

 during the past decade. The situation as it existed at the beginning 

 of the decade is to be found well set forth in a paper presented in 1931 

 at the A.I.E.E. winter convention by R. N. Conwell and H. S. Warren. 

 The present paper, like its predecessor, derives from the work of the 

 Joint Subcommittee on Development and Research of the Edison 

 Electric Institute and the Bell System. It is largely concerned with 

 induction from currents due to power system ground faults and the 

 transients which accompany such faults. It gives relatively little 

 attention to continuous low-frequency effects since, up to the present 

 at least, such effects have not been a primary concern in the low- 

 frequency coordination of commercial power circuits and Bell System 

 communication circuits. 



A further object of the paper is to outline the various factors that 

 require consideration in practical situations and to discuss their 

 significance under present-day conditions. To provide necessary 

 background for this, recapitulations of fundamentals are included at 

 appropriate points. Detailed discussions necessarily omitted from the 

 paper itself are to be found in the papers listed in the bibliography, 

 many of which, particularly the Con well-Warren paper, contain further 

 references. 



Insulating Paper in the Telephone Industry.'^ J. M. Finch. This 

 article discusses briefly a few of the more important types of paper 

 insulations used by the telephone industry, and shows the relation the 

 manufacturing procedures bear to the initial properties, the perma- 

 nence, and the uses of the product. Special emphasis is placed on 

 chemical properties as criteria of permanence. The specification 

 control of paper is discussed with emphasis on the simplification of 

 chemical test methods and on minimizing the number of such tests. 

 Finally, mention is made of some of the modified forms of cellulose, 

 which possess insulating characteristics superior to paper and which are 

 already replacing it for some uses. 



* Indus, and Engg. Chemistry, August 1940. 



