304 ELEMENTS OF ELECTRICAL ENGINEERING. 



the watts lost by discharge between two bare wires each 0.0126 inch in diameter and 

 1,040 feet long stretched side by side at a distance of 48 inches apart ; and curve B 

 shows the watts lost by discharge between two bare wires each 0.128 inch in diam- 

 eter, 1,040 feet long and 48 inches apart. The abscissas in Fig. 173 represent 

 effective values of the alternating electromotive forces used. An alternating electro- 

 motive force reaches a maximum value equal to about 1.4 times its effective value. 



122. Pole-line insulation. When the voltage between two line 

 wires is less than that required to cause a break-down of the in- 

 tervening air, the leakage of current through the air from wire to 

 wire is entirely negligible. The only perceptible leakage of cur- 

 rent is at the supporting insulators and through branches of trees 

 and other objects which happen to touch the wires. 



If the insulators are made of glass or thoroughly vitrified 

 porcelain the leakage of current through the material of the insu- 

 lator is always negligible, unless the insulator is ruptured, but the 

 leakage of current over the surface of the insulator may be con- 

 siderable. This leakage over the surface of the insulators is 

 reduced to a minimum by designing the insulators so that the 

 length of the leakage path measured along the surface may be as 

 great as possible, and so that a portion of the surface may be 

 shielded from rain and mist. This is accomplished by making a 

 series of deep grooves around the bottom of the insulator as 

 shown by the sectional view of the insulators in Fig. 160. This 

 type of insulator is called the petticoat type ; single, double or 

 triple petticoat as the case may be. An objection to the petticoat 

 insulator is that insects are apt to build their nests in the deep 

 recesses between the petticoats and thus impair the insulation. 

 This is especially the case when the insulators are opaque and the 

 grooves dark. 



The strength of an insulator to withstand a high voltage with- 

 out rupture has nothing directly to do with its insulation resist- 

 ance. Analogously, the strength of a porous earthenware jar to 

 withstand pressure without bursting has nothing directly to do 

 with the facility with which the porous walls of the jar permit the 

 water to flow through them. Insulators for high-voltage lines are 



