11)14] on Improvements in Long-Distance Telephony 95 



In the years 1899 and 1900, Professor Pupin, of Columbia College, 

 New York, published two very important papers, in which he dis- 

 cussed the result of inserting into the cable at intervals loading coils 

 of insulated wire wound on an iron wire core so as to give them high 

 inductance. 



The important contribution he made to the subject was to show 

 that if these coils were at such a distance apart that there were 8 or 

 10 loading coils in the span of a single electric wave length on that 

 cable, corresponding to the mean speech frequency (8o0 per second), 

 they acted just as if their inductance were evenly distributed along 

 the cable. 



Under these conditions they do not cause mucli loss of wave 

 amplitude by reflection. If, however, the loads are farther apart so 

 that only two or three loads are comprised in a wave length, there 

 would be great loss of wave amplitude by reflections at the loading 

 coils. 



The mechanical equivalent of this can be very well shown with 

 the vibrating string apparatus 1 have just exhibited to you. If we 

 place on a stiing a hea\'y glass bead somewhere about the middle and 

 then set the string vibrating, we shall see quite easily that the wave 

 is reflected at the bead and only a small portion of it transmitted. 

 In the flrst section of the cord there are vigorous vibrations, but on 

 the other side of the load they are very feeble. (See III, Fig. 1.) 



If we put on the string several beads, and adjust the tension and 

 frequency of the vibrations so that the half wave length extends 

 rather more than the distance which separates two adjacent beads, 

 we shall notice that a reflection of the wave takes place at each bead 

 which rapidly attenuates the wave (see Y, Fig. 1). Precisely the 

 same thing takes place in the electrical case. Hence in practice it 

 is found that the wave length on the cable must extend over 8 or 9 

 loading coils at least, these coils being placed at equal distances. 



The principal result of this loading when so carried out is to 

 make a remarkable reduction in the attenuation of the waves, and to 

 render less unequal the attenuation of waves of different wave 

 lengths. 



Ill the case of an unloaded telephone cable the shortest waves 

 travel fastest and attenuate most rapidly. Hence, if we start a 

 complex oscillation on the cable, the shorter harmonics in the wave 

 form are weeded out soonest and die away fastest. It is as if a 

 crowd of men, youths and boys stare on a race. The youngsters, 

 with nimble limbs, race ahead at first, but they have not the staying 

 power of the men and are soon played out, whilst the strong men 

 stay in and survive to the end. 



Two very important characteristics of a cable are its attenuation 

 constant and ivave length constant. 



The reciprocal of the attenuation constant is the distance in which 

 the wave amplitude diminishes to 0*367 of its amplitude at the 



