1914] on Improvements in Long-Distance Telephony 97 



second. The speech hmit on this table is about -i? miles, and there 

 is considerable distortion of the waves. If, however, we insert in 

 the run of this cable, every 2 miles or so, inductance coils consisting 

 of rings of iron wire wound over with many turns of insulated 

 copper wire, and include this copper wire in the telephone circuit, 

 we shall give to the cable much more inductance per mile. (See 

 Fig. 4.) 



These coils are called loading coils, and they are usually made to 

 have an inductance of about 100 or 200 millihenrys. Suppose that 

 such coils are inserted every mile in the run of a standard cable. 

 Then without increasing much the capacity or resistance of the line, 

 the inductance per mile can be made 100 times greater. 



The result of this is to reduce the attenuation constant to about 

 one-third of that of the unloaded cable, the wave length to about 

 18 miles, and the velocity to about 13,280 miles per second for the 

 frequency of 800. 



Also, this loading reduces the inequality in the attenuation and 

 velocity of waves of different wave lengths, and hence prevents that 



Fig. 4. — Diagram showing Insertion of Loading or Pupin Coils 

 fiNTO A Telephone Circuit. 



extinguishing of the higher bannonics relatively to the lower, which 

 is the chief cause of the distortion of the wave form. If these 

 loading coils are not put farther apart in this cable than 2 miles or 

 so, a wave length of 18 miles will still extend over 9 or 10 coils, 

 and the loss of wave amphtude by reflection at each coil will not be 

 great. This proposal of Pupin to locate an added inductance in 

 separate coils, placed at intervals in the run of the cable and at such 

 a distance that 9 or 10 coils are covered by one wave length, proved 

 to be a very effective method of carrying into effect Heaviside's 

 recommendations. 



This loading has resulted in a great improvement in those long 

 distance telephone lines to which it has been applied. You may 

 ask for some common-sense explanation of this improvement. It is 

 really due to the greater energy which the added inductance imparts 

 •to the current waves. If, for instance, two ships of equal size and 

 shape, but unequal weight, were being launched down ways of equal 

 inclination into the water and allowed to run out freely as far as 

 they would go until their energv was exhausted, the heavier ship 



Vol. XXI. (Xo. 108) "^ h 



