the wires underground. The eeirly experiments starting in 1878 took 

 advantage of the known advantageous properties of lead water pipes. 

 Insulated copper wires were drawn into lead pipes of this character. 

 By 1886, practical means had been developed whereby lead heated to 

 the point of plasticity could be extruded over a compacted group of 

 insulated conductors, thus forming the pipe tightly about the con- 

 ductors, and this general principle has been followed in telephone cables 

 to the present day. By 1890 there was a general development of 

 insulated telephone cables in the congested parts of the larger cities. 

 In 1891 Bell System engineers introduced the use of jxiper for in- 

 sulating cables, and this practice is still followed in cable manufacture. 



Early Toll Service 



The success of the early installations of hard-drawn copper wire for 

 short lines indicated that a type of conductor had been developed by 

 w^hich it might be possible to extend telephone service over consider- 

 able distances. This led to a very important experiment, the con- 

 struction in 1883 of an experimental toll line between New York and 

 Boston carrying two wires. 



Prior to the construction of the New York-Boston line, telephone 

 lines, following telegraph practice, were generally of one wire grounded 

 at both ends, the so-called "ground return circuit." However, based 

 upon experiments with iron wires over shorter distances, particularly 

 between Boston and Providence, J. J. Carty of the Bell System had 

 determined that the ground return circuit was so noisy, due to inter- 

 ference from telegraph lines and other causes, that such circuits could 

 not be used over long distances, and had also discovered that by using 

 two wires connected as a metallic circuit, the interference was very 

 greatly reduced. Carty's metallic circuit was used with success in the 

 New York-Boston line and was adopted for all the following construc- 

 tion of long toll circuits. The metallic circuit principle thus developed 

 first for toll lines extended back into local lines so that in highly devel- 

 oped areas all telephone circuits are now constructed on the metallic 

 circuit principle. 



This experiment successfully demonstrated the practicability of 

 "long distance" transmission and led to the determination to extend 

 long distance service as widely and as rapidly as the state of the art 

 permitted and to the incorporation in 1885 of the American Telephone 

 and Telegraph Company for this purpose. The first telephone line 

 constructed by this new company was the New York-Philadelphia 

 line, using hard-drawn copper wire on a metallic circuit basis. It was 

 found that wuth two or more metallic circuits on a pole line, speech 



