314 Mr. G. A. Campbell on Loaded 



applications will be given ; a second paper will present an 

 engineering study and an account of experimental methods 

 and results. 



Vaschy*, Heavisidef, and others have either suggested or 

 unsuccessfully tested the insertion of self-induction coils on 

 actual lines. " Heaviside stated in 1893 that there was "no 

 direct evidence of the beneficial action of inductance brought 

 in in this way," and no progress was made till 1899, when 

 the subject was investigated independently by Professor M, 

 I. Pupini and myself. It has been shown that the loaded 

 line affords a practical method of improving the transmission 

 efficiency of long lines employed for telephonic, telegraphic, 

 or other electrical purposes. 



An interesting contribution to the general properties of 

 this structure has been made by Mr. Charles Godfrey § in a 

 paper on wave propagation along a periodically loaded string 

 and I am indebted to that article for equation (18) which 

 furnishes a complete solution of the propagation. 



This study has been made with special reference to tele- 

 phonic applications, and I have limited the mathematical 

 treatment to the forced harmonic steady state, as that fur- 

 nishes all the theoretical information which we are in 

 position to use in telephony, and practical applications 

 generally, provided only a sufficient frequency range is con- 

 sidered. The range which it is necessary to consider in 

 telephony might be determined by constructing a network 

 which would transmit uniformly all frequencies between 

 certain limits, and then experimentally determining the in- 

 terval which is just sufficient to preserve the full character 

 of speech. Practical cable transmission shows that speech 

 remains intelligible even w T hen the superior limit is compara- 

 tively low. Cable quality is, however, not desirable, and for 

 unimpaired articulation it appears from the tests w T bich have 

 been made that the limit lies well above two thousand cycles 

 per second. Efficient, clear transmission requires a low and 

 constant attenuation, and constant velocity, throughout the 

 telephonic frequency interval, and constant line impedance 

 of negligible reactance is desirable. With an open wire line 

 of heavy copper wire (resistance R, inductance L, capacity 

 C) this is approximately attained, the attenuation-coefficient, 



~JlJ * La Lumiere Electrique, January 12, 1889. 

 t Electromagnetic Theory, i. p. 445 (1893). 



t Trans. Am. Math. Soc. p. 259, July 1900 ; Trans. Am. Inst. Elec. 

 Eng. xvii. May 1900. 



§ Phil. Mag. xvi. p. 356 (1898). 



