Bulletin . e spot does showthat there is still considerable dielectric strength after the firstpuncture. This dielectric strength is not, however, due to theprotective efiect of the oil. The current which follows the arcfuses the porcelain and when the circuit is broken the fused porce- DIELECTRIC STRENGTH OF PORCELAINS. 43 lain solidifies in place, forming a glassy spot. Due, perhaps,to flaws and to the presence of foreign material such as carbonizedoil, which probably prevents the formation of a homogeneousstructure, the strength on second puncture is less than on thefirst. The voltage at wh
Bulletin . e spot does showthat there is still considerable dielectric strength after the firstpuncture. This dielectric strength is not, however, due to theprotective efiect of the oil. The current which follows the arcfuses the porcelain and when the circuit is broken the fused porce- DIELECTRIC STRENGTH OF PORCELAINS. 43 lain solidifies in place, forming a glassy spot. Due, perhaps,to flaws and to the presence of foreign material such as carbonizedoil, which probably prevents the formation of a homogeneousstructure, the strength on second puncture is less than on thefirst. The voltage at which a sample will puncture the secondtime has, I believe, no relation to the real dielectric strength,i. e., on first puncture, of the porcelain sample. I believe thatthe only serious efJect of testing under oil would be with verypoorly vitrified porcelains such as are obviously not fit for hightension insulators. In testing these porous porcelains comparableresults possibly would not be obtained under
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