In the case of an exhausted tube having no electrodes, the wires from the coil may be made into a little helix and placed at each end of the tube, and the induced currents within will show themselves in flashes and streams of light, varying in colour and tint according to the gaseous or other contents of the tube.
Tube excited by friction.
In some cases the ordinary forms of galvanic or electrical machine for supplying the current of electricity may be dispensed with. A long straight tube exhausted and closed at each end, and without electrodes, Plate X. fig. 6, being slightly warmed and then excited by friction with the dry hand or a piece of flannel, silk handkerchief, or the like, is soon filled with the most brilliant flashes of light playing in the interior, and when once thoroughly charged needs but little further excitation to keep up the effect.
Geissler’s mercury tube.
Geissler has introduced a form of tube in which electricity in its form of flashes and glow of light is produced by the friction of mercury. The outer tube is strong, and contains within it a smaller tube of uranium glass with balls blown upon it (Plate X. fig. 7). The tubes are exhausted and a small quantity of mercury is introduced which has access to both surfaces of the inner tube, as well as to the inner surface of the outer tube. Upon the tube being reversed end for end or shaken, the mercury runs up or down the tube and causes a very considerable display of whitish light.
The before-described tubes are also referred to, and their spectra described, in the section “On the comparison of some tube and other Spectra with the Aurora” (Part II.).
The aura or brush from the electrical machine has been considered as resembling the Aurora, while the hissing and crackling accompanying it has been supposed to corroborate the reports of similar noises having been heard during an auroral display.
Prof. Lemström’s instrument to demonstrate the nature of Auroræ.
Prof. Lemström, of the University of Helsingfors, has devised an instrument for the purpose of demonstrating that Auroræ are produced by electrical currents passing through the atmosphere. An illustration of this instrument (for which I am indebted to the Editor of ‘Nature’) is introduced (fig. 1).
The instrument was exhibited at the recent Scientific Loan Collection at South Kensington, and a full description of it, together with an essay by Prof. Lemström, “On the Theory of the Polar Light,” will be found in the third edition of the Official Catalogue, p. 386. no. 1751. The apparatus is intended to show that an electric current passing from an insulated body does not produce light in air of normal pressure; but as it rises to the rarefied air in the Geissler tubes a phenomenon very like the real Polar Light is produced.