24. STEINMETZ’S EXPERIMENT. LUMINOUS EFFECTS BY ALTERNATING CURRENT AND SOLID DIELECTRICS. _Trans. Amer. Inst. Elec. Eng._, Feb. 21, ’93.—In carrying on experiments in the accurate measurement of dielectric strength, he noticed that upon placing mica between the electrodes, as is hereinafter set forth, a spark did not at first form, but that which he called a corona. He attributed the appearances to a condenser phenomenon, or at least he suggested this as an explanation. § 3. As soon as the corona reached the edge of the plate, the disruptive discharge took place, by means of the sparks passing over the edge of the dielectric. § 38. He employed an alternating current dynamo of about 50 volts and 1 h.p., frequency of 150 complete periods per second. The E. M. F. of the alternator was varied, by changing the exciting current, up to 90 volts. Step-up transformers were employed. With a difference of potential in the secondary of 830 volts, and a thickness of mica of 1.8 mm. and when the experiment was performed in a dark room a faint bluish glow appeared between the mica and the electrodes. At 970 volts the glow was brighter, while at 1560 volts the luminosity was visible in broad day-light, and kept on increasing with the increase of E. M. F. He modified the experiment by using mica of a thickness of 2.3 mcm. The difference of potential was 4.5 kilo-volts. In addition to the bluish glow, violet streams or creepers broke out and increased in number and length as the E. M. F. became greater, forming a kind of aurora around the electrodes and on both sides of the mica sheet. A loud hissing noise occurred. § 9. As soon as the corona reached the edges of the mica, the disruptive discharge occurred in the form of intensely white sparks and it was noticeable that the length of these sparks was 10 fold greater than could be obtained in the air at 17 kilo-volts. These sparks were so hot as to oxidize the mica, as apparent from the white marks remaining. The electrodes also became very hot, and the mica was contorted and finally broke down.