728 BELL SYSTEM TECHNICAL JOURNAL 



Of the remaining factors, the direct surface leakage B contributes 

 about 4 per cent or less of the total at 30 kc. The losses in the glass C 

 are liberally estimated at 10 per cent or less, while the surface losses G 

 contribute about 85 per cent or more. 



In this design the absolute magnitude of B has been decreased some- 

 what, chiefly because the small diameter of the steel pin permits a 

 small diameter of insulator, the advantages of which were pointed 

 out in the detailed discussion of this item. 



The low capacitance made possible by the small steel pin has 

 helped to make both C and G relatively small, although most of the 

 improvement in C is due to the borosilicate glass. 



The improvement in the surface losses G over the D. P. design is 

 quite marked. 



For the new designs the two factors B and G are the controlling ones. 

 In this particular test B happens to be quite small in magnitude, and 

 would naturally lead one to conclude that B had been made unneces- 

 sarily small at the expense of G, especially since these two items in many 

 respects place conflicting requirements on insulator design. However, 

 B has been observed at times to reach a value as high as one third of the 

 total leakage at 50 kc. The necessity of engineering for such cases 

 makes the design more reasonable, especially when it is recalled that 

 the insulators must serve for direct current and low frequencies, as well 

 as for the carrier range. 



Of the new designs, the electrical superiority of the C. S. over the 

 C. W. design is apparent. This fact, together with economic consider- 

 ations, has led to the almost universal choice of the C. S. rather than 

 the C. \V. type for the field of application of the new insulators in the 

 telephone plant. 



The utility of the C. S. insulators in the telephone plant will be more 

 clearly apparent from a consideration of the reduction in attenuation 

 which their use brings about. 



The losses in transmission over a pair of wires at carrier frequencies 

 come chiefly from two sources: one, substantially fixed in magnitude, 

 depending mainly on the resistance of the wires ; the other, quite vari- 

 able in magnitude, depending on the leakage conductance between 

 the wires and, therefore, on the weather.^ 



In the case of 165-mil copper wires on 12-inch spacing these two 

 components of loss are approximately equal in wet weather at 30 kilo- 

 cycles when the older type of insulators are used. The C. S. type cuts 



^For a more detailed discussion of attenuation see a companion paper, "The 

 Transmission Characteristics of Open-Wire Telephone Lines," by E. I. Green, 

 presented at the Summer Convention of the A. I. E. E., Toronto, June, 1930 and 

 printed in this issue of the Bell System Technical Journal. 



