. The Street railway journal . isrelative strain test is shown herewith. No copper line canwithstand such strains to an insulator as these tests strain of the copper conductor is a limiting mechanicalfactor beyond which it is not profitable to increase thestrength of the pole line. The composite insulator, which is formed of a metallicsaddle for the feeder and a metallic bushing for the pin, isa form of insulator largely used. These two metallic partsare held together and insulated from each other by insulat-ing material which is generally molded into place underheat and great press
. The Street railway journal . isrelative strain test is shown herewith. No copper line canwithstand such strains to an insulator as these tests strain of the copper conductor is a limiting mechanicalfactor beyond which it is not profitable to increase thestrength of the pole line. The composite insulator, which is formed of a metallicsaddle for the feeder and a metallic bushing for the pin, isa form of insulator largely used. These two metallic partsare held together and insulated from each other by insulat-ing material which is generally molded into place underheat and great pressure. The desirable qualities in thecomposition used are that it have sufficient mechanicalstrength and that the strength be maintained when thetemperature of the insulator has been raised to 150 degs. worst fault of these composite insulators is that theybecome plastic in warm weather and yield to conductorstrains. This composite insulating material should not beaffected by rain so as to dissolve any of the substances in. FORMS OF FEEDER INSULATORS DEVICE FOR TESTING STRENGTH OF FEEDER INSULATORS of mica and shellac, rubber and asbestos. The mechanicalproperties of this insulation should be such that it willstand strains without fracture, and that the surface will notabsorb moisture. Glass or porcelain is generally used forNo. 000 feeders or smaller, but for larger feeders with longspans, exceedingly strong insulators should be selected,especially where it is also necessary to use an iron has been greatly improved in strength duringrecent years, and the present method of firing it in the kilnhas thoroughly vitrified the body of the insulator, still leav-ing it tough. The method of testing a porcelain insulator,to determine whether vitrification has taken place, is tobreak it and apply analine ink to the fractured surface ofthe broken portions of the body. If, after washing, a gen-eral discoloration of the porcelain is found, it has not beenproperly fired. In
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectstreetr, bookyear1884
