156 BELL SYSTEM TECHNICAL JOURNAL 



factors. A factor directly proportional to frequency was adopted to 

 represent inductive coupling between power and telephone circuits 

 (Item 3), the work of the Joint Subcommittee on Development and Re- 

 search having indicated that, in general, coupling may be so repre- 

 sented. After studying data available on Item 4, it was concluded that 

 no type of frequency weighting could be adopted which would satis- 

 factorily represent all types of telephone circuit unbalances. Thus 

 T.I.F. as measured by the method described here is a correct index to 

 the influence of a power circuit voltage or current only for those cases 

 where unbalances are independent of frequency. This is usually the 

 case on open-wire toll circuits and open-wire exchange circuits em- 

 ploying bridged ringers. In other cases some empirical modification 

 may be necessary. 



Since a large amount of data has been obtained with the old T.I.F. 

 meter, it was considered desirable, if practicable, to adjust the scale 

 of the revised set so that readings made by this method would, in 

 general, be approximately the same as readings obtained by the old 

 meter. In this connection calculations using the old and new weight- 

 ings were made on a large number of machines and circuits of various 

 types on which harmonic analyses were available. These calculations 

 were supplemented by a considerable number of comparative meas- 

 urements in the factory and in the field, using meters employing 

 the old and new weightings. These calculations and tests indicated 

 that in the average case, reasonably satisfactory correlation between 

 the readings made by the two meters would result if a peak value 

 of 12,000 were assigned to the new weighting characteristic, as shown 

 in Fig. 2. 



Several experimental models of T.I.F. measuring sets were made 

 employing the new weighting characteristic and these have given ver>^ 

 satisfactory results. The adoption of the rule that coupling is to be 

 considered proportional to frequency also makes it possible to use a 

 circuit noise meter and a small amount of auxiliary apparatus to form 

 a T.I.F. meter. 



The development of the revised method of measuring T.I.F. has also 

 been conducted under the auspices of the Joint Subcommittee on 

 Development and Research of the Edison Electric Institute and the 

 Bell Telephone System, 



