DEVELOPMENTS IN TELEPHONY AND TELEGKAPHY JEWETT. 493 



have the circuits and terminal apparatus balanced merely with re- 

 gard to their ohmic resistance. They must also be balanced with 

 regard to mutual capacities and inductances, not for a single fre- 

 quency alone but for a multiplicity of frequencies, if the necessary 

 freedom from cross interference is to be obtained. 



With the conditions which existed at the beginning of the present 

 century, neither the lines themselves nor the terminal apparatus were 

 suitably balanced for the introduction of the phantom principle. 

 All of the cables then in service were constructed with a view to se- 

 curing freedom from interference between simple metallic circuits. 

 So also with open-wire lines, which, although transposed according 

 to definite rules, were designed merely to provide freedom from 

 cross interference between circuits operated on an ordinary metallic 

 basis. The terminal apparatus in general use at that time, although 

 reasonably satisfactory for the services originally intended, was even 

 more hopeless than the lines from the standpoint of phantom 

 operation. 



During the years between 1900 and 1906 or 1907 great strides were 

 made in the application of phantom operation. A .transposition 

 system, permitting the use of phantom circuits on open-wire lines, 

 was developed and the necessary mechanical details for interchanging 

 the wires were worked out. Phantom repeating coils were designed 

 and the art of their manufacture developed. Little or no progress, 

 however, was made in the matter of successfully adapting cable con- 

 struction to the requirements of phantom operation. 



The condition at about the beginning of 1907 was, therefore, one 

 in which there was a fairly large and successful application of the 

 phantom principle on nonloaded open-wire pole lines which termi- 

 nated directly at central offices or terminated through relatively short 

 lengths of cable. Where the toll lines had to be brought into the 

 central offices through long lengths of cable, it was necessary either 

 to i^lace the phantom repeating coils at the junction of the cable with 

 the open wire toll line or to endeavor, by a process of experimental 

 selection, to find a sufficient number of interference-free pair combi- 

 nations in the cable. The first of these alternatives was highly objec- 

 tionable, both from the standpoint of maintenance on the coils and 

 more particularly because it practically eliminated half of the wires 

 from the possibility of use for superimposed telegraphy. The second 

 alternative was almost equally objectionable because every repair to 

 the cable required a reselection of pairs. 



At this time (1907) there were no means known for combining the 

 phantom principle with the benefits from loading, now to be de- 

 scribed. 



