MANUFACTURING LEAD-COVERED TELEPHONE CABLE 325 



The insulation withstands a potential test of 500 volts (maximum 

 instantaneous value). The increasing number of pairs per cable and 



Fig. 5 — Principal stages in the development of paper-insulated cable 



the corresponding decreasing cost per mile of circuit resulting from the 

 changes described is shown in Figs. 5 and 6. 



Fig. 6 — Cost of a mile of circuit in full-size cable 



With the growth of large office buildings and further increases in the 

 demand for telephones in the great cities, even 1,212 pairs of wire per 



