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.
25. MORGAN’S EXPERIMENT. NO DISCHARGE IN HIGH VACUA. _Wiedemann_, vol. 2. _Phil. Trans._, 1875, vol. 75.—He was led to believe by an experiment, that when the vacuum is sufficiently perfect, no electromotive force could drive the spark from one terminal to the other, however close together they may be. § 18. Details of Morgan’s Experiments were as follows, given roughly in his own words:—A mercurial gauge about fifteen inches long, carefully and accurately boiled till every particle of air was expelled from the inside, was coated with tinfoil five inches down from its sealed end, and being inverted into mercury through a perforation in the brass cap which covered the mouth of the cistern, the whole was cemented together and the air was exhausted from the inside of the cistern, through a valve in the brass cap, which, producing a perfect vacuum in the gauge, formed an instrument peculiarly well adapted for experiments of this kind. Things being thus adjusted (a small wire having been previously fixed on the inside of the cistern, to form a communication between the brass cap and the mercury, into which the gauge was inverted), the coated end was applied to the conductor of an electrical machine, and notwithstanding every effort, neither the smallest ray of light nor the slightest charge could ever be procured in this exhausted gauge.