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BELL SYSTEM TECHNICAL JOURNAL 



which varies with frequency as a compHcated function of the ratio 

 of the frequency to the cut-off frequency. With mid-coil termina- 

 tion the impedance-frequency characteristic droops with rising 

 frequency, approaching zero at the cut-of¥ frequency. On the other 

 hand, the mid-section termination has a rising characteristic, ap- 

 proaching infinity at the cut-off frequency. 



Early Standard Loading Systems. One of the fundamental questions 

 involved in the early commercial development work was that of 

 determining what range of frequencies should be transmitted in order 

 to furnish a satisfactory grade of speech transmission. The investi- 

 gation of this point resulted in the adoption of a standard cut-ofT 

 frequency of about 2,300 cycles. Table II lists the other important 

 transmission characteristics of the first loading systems standardized 

 about 1904 for use on cables : — 



TABLE II 

 First Standard Cable Loading Systems 



Note. These data apply to cables having a mutual capacitance of 0.070 m/- per 

 mile and assume loading coils, the electrical characteristics of whi ch a re given in 

 Table IV. The nominal impedance is defined by the expression Vi/C The new 

 unit of transmission loss (TU) is described in a recent Institute paper. "^ 



For open w^ire loading, only one loading system, known as "Heavy" 

 loading, was standardized. This involved the use of coils having 

 an inductance of 0.265 henry at a spacing of approximately 8 miles. 

 This loading had approximately the same cut-oft" frequency as the 

 cable loading standards described in Table II. The other important 

 line and transmission characteristics are summarized in Table III. 



Loading Coils. The loading coils developed for use in the loading 

 systems described in Tables II and III were of the toroidal type; i.e., 

 they had ring-shaped cores formed by winding up a bundle of insu- 



6 "The Transmission Unit and Telephone Transmission Reference Systems," W. H. 

 Martin, Trans. A. I. E. E., Vol. 43, 1924, p. 797; Bell System Technical Journal, 

 July, 1924. 



