A STUDY OF TELEPHONE LINE INSULATORS 723 



than qualitative. However, the accuracy of the assumptions is not 

 sufficiently high to expect a much better agreement. 



In accordance with this theory, it is evident that G could be produced 

 on a dry insulator by coating the surface with a thin metallic film of 

 high resistance. This has been checked experimentally. The dry- 

 weather conductance of an insulator of given shape and material can 

 readily be increased ten or more times by such a coating. In its general 

 magnitude this increase corresponds to that of the uncoated insulator 

 between dry and wet weather. 



Referring again to Fig. 21, G will be seen to be zero when the resis- 

 tivity of the circuit is zero. Thus, if a metal coating of low resistance 

 be placed on the outside surface, the value of G would be made negli- 

 gible for that surface. However, such a coating causes the losses on 

 the inner surfaces to increase. The net result is usually a substan- 

 tial reduction in leakage, as shown by many tests comparing coated 

 with uncoated insulators. 



2. Influence of Insulator Design. From the foregoing discussion, 

 the importance of insulator capacitance is quite apparent. For G to be 

 small, it is not only desirable that the total capacitance shall be small, 

 but particularly that any capacitance remote from the wire groove be 

 small, unless the resistance of the path from wire to that capacitance 

 can be maintained at a very high value. 



Insulator design is important from this standpoint. It is also impor- 

 tant because of its relation to the resistivity of the surface film, as was 

 discussed in some detail under B. 



3. Influence of Insulator Material. In general, the influence of 

 insulator material has been less than that of design, especially after the 

 insulator has aged for a couple of years. The influence of material on 

 G is determined both by the dielectric constant of the material and its 

 surface resistivity. 



While the dielectric constant of the materials tested varies about two 

 to one, the corresponding range of G is very much less, on account of 

 the low dielectric constant of the air which occupies a part of the 

 dielectric path. 



The factor of surface resistivity is probably the more important one. 

 For example, its effect on G probably accounts for most of the increase 

 in leakage at high frequencies that accompanies aging, which is a very 

 substantial one. 



As to the relative importance of these two factors, tests have been 

 made on only one design. At this writing the tests register a slight 

 balance in the favor of a good surface over a good (low) dielectric con- 

 stant. 



