Discharges in Vacuo.
In a simple glass tube having two wires carrying balls inserted through its ends, from which the air has been partially exhausted, the study of the changes shown by the passage of the spark is extremely interesting. Before the commencement of exhaustion no luminous effect can be discerned; at a low degree of exhaustion a luminosity appears between the ends of the wires, the negative pole being surrounded by a violet glow and a larger pear-shaped red discharge from the positive. An interval near the negative electrode is in darkness, widening as the exhaustion progresses. When the degree of exhaustion is very high, a series of arches concentric with the positive ball appear and become broader and more distinct as the rarefaction progresses. The arches or bands are called striæ, and are most distinct when the tube is made in the form of a narrow cylinder, with a bulb at each end. Carbonic acid gas vacua give the best results. If the finger be placed on the bulb at either end a luminous spot appears, and by using a very rapid contact breaker in the primary circuit, the luminous discharges become highly sensitive, being diverted from their regular path on the approach of the hand, a magnet, or a grounded wire. An extended treatment of these phenomena would be out of place here, but can be found in nearly all comprehensive works on electricity.
If an incandescent-lamp bulb be held in the hand and one end be brought near to a terminal of the coil, a beautiful bluish light appears.[2] The carbon filament, if long, and not held by its loop, becomes electrified and oscillates, often giving out a clear, high, bell-like sound as it strikes the glass. Particles of carbon deposited on the glass during the burning of the lamp, shown in daylight as a blackening deposit, generally show little sparks, like stars scattered over the inside of the globe.
[2] This depends on the degree of exhaustion.
A vacuum tube will phosphoresce if held in the hand near a secondary terminal, or even if laid on the table near the coil, and will light quite brilliantly if one end be held against a terminal. This latter method, however, is generally inconvenient, as a certain amount of physical pain ensues from the discharge into the skin.
Different gases in the tubes give characteristic colors. In carbonic acid gas the whitish green hue prevails; in hydrogen, white and red; in nitrogen, orange yellow. The characteristic spectra are given by the gases in the tubes, and can be readily examined in the spectroscope. But there is sometimes a slight variation in these colors, dependent upon changes in the current.
In many Geissler tubes, a portion of the bulbs is made of uranium glass. On the passage of the spark in the tube this glass glows with a magnificent emerald green hue. Other tubes are constructed with an outside enveloping glass tube fitted with a corked orifice into which can be poured different solutions.
Fig. 45.
Fig. 46.
Fig. 47.
Fig. 45 shows a solution tube to be filled with solution of sulphate of quinine, etc.
Fig. 46 shows three exhausted tubes arranged in series.
A is of uranium glass, and glows dark green; B of English glass, showing a blue hue, and C of soft German glass, glowing with a bright apple-green tint.
Fig. 48.
Crystals of nitrate of calcium, nitrate of silver, benzoic acid, tungstate of calcium, lithia benzoate, sodium salicylate, zinc sulphide, and acetate of zinc fluoresce.
Fig. 47 is a highly exhausted tube, having at its lowest part a few pieces of ruby. When the secondary current is turned on at P and N the rubies shine with a brilliant rich red, as if they were glowing hot.
Fig. 48 shows the tube to exhibit the effect resulting from focussing the electric rays on a piece of iridio-platinum at B.
The cup A forms the negative pole; the metal disk C, the positive.
On increasing the intensity of the spark, the metal at B glows with extreme brilliancy, and melts if the intensity be carried too far.
CHAPTER VIII.
ROTATING EFFECTS.
Although the luminous discharges in the exhausted tubes are extremely beautiful, yet the effect is indescribably enhanced when the tubes are rotated. Gassiot's star was the name given to the earliest exhibit of a rotating tube carrying a luminous discharge, owing to the curious phenomenon ensuing from the interruptions of the spark. As the human retina is only capable of retaining an impression for a fraction of a second, and as the tube is only momentarily luminous during the passage of the spark, the effect of the revolving tube is that of a series of such arranged as the radii of a circle, the number apparent, being governed by the rapidity of rotation and the rate of interruption of the current.
Fig. 49.
Fig. 49 represents a form of rotating wheel which is easily made, and yet susceptible of many novel and attractive effects. Such a wheel, placed in a store window, would undoubtedly attract many persons by the beautiful variations of colored figures which it presents while in motion. And once a crowd is collected and its attention attracted to one spot, the capabilities of advertising the goods on sale are apparent.
A pasteboard or light wooden disk D, 3 feet in diameter or over, is mounted on a shaft, S, operated by an electric motor or such power as may be attainable. Upon its surface are mounted the tube-holders T T T T, connected, as shown, by wires leading from the secondary of the Ruhmkorff coil. Starting at the shaft S, the circuit runs to the first tube-holder, where the continuity of the wire is broken to allow of the attachment of the vacuum tube. From the first tube-holder the wire runs in turn to each of the other three tube-holders, terminating at R, where it passes through a hole to a metal ring on the back of the disk shown by the dotted circle. This ring and the shaft are in connection with the secondary coil, by reason of its electrodes being attached to two brushes or strips of metal pressing, one on the ring, the other on the shaft; or the bearing in which the shaft turns may displace one of the brushes. W W are two counterbalance weights, that the wheel may run smoothly and be not affected by the irregular distribution of the tubes or its surface. E E are elastic bands, looped over the wire and through rings in the disk, that the wires may not be liable to touch or short circuit.
At Fig. 50 is an enlarged view of a tube-holder, although, as it is meant only as a diagram, considerable variation of design is permissible. The springs at H H, to which the wires run, being bent back, the metal pins P P may be thrust through the rings on the ends of the tube, and the elasticity and pressure of the spring will hold it in place and make the necessary contact. A wooden block, B, secured to the face of the disk, is provided with a thumb-screw, S, securing the tube-holder to it, by means of which the tube-holders may be turned a trifle upon their axes and so vary the effect of the wheel.
Fig. 50. Fig. 51.
Fig. 51 is a side view of the wheel, showing one manner of mounting the disk and its connections. The same figures apply to the parts as in the preceding figure. M N are the wires leading to the coil, P is a pulley on the shaft whereby the rotary power may be applied. The wires on the face of the disk are not shown, as they would impair the clearness of the diagram unnecessarily.
The greatest danger in the operation of such a piece of apparatus will be the tendency of the high tension spark to wander where it is not wanted, and to take short but forbidden paths back to the coil. However, care and perhaps experiment will prove the remedy. It will be noticed by reference to Fig. 49 that a circle has been drawn almost bisecting two of the tube-holders. This circle represents a circle of danger, and where a thin material has been used for the disk, the disk may very well be reinforced by a piece of stouter card cemented on its face.
The disk, whether of wood or of pasteboard, must have a liberal coating of insulation, either shellac varnish, paraffin, or beeswax, and be absolutely free from unnecessary holes. Moreover, the ring R must be of such a distance from the support F, if the latter be metal, as will preclude any jumping of the spark. A Ruhmkorff coil giving upward of three quarters of an inch of spark will be large enough to operate a wheel carrying four 8-inch tubes.
The wheel may be set back in a window and surrounded by dark fabrics, or built in, as it were, in a cave of such. The judicious use of pieces of looking-glass scattered on the sides of the cave, in such manner as to reflect the light of the tubes, will enhance the effect. There is no danger of fire where ordinary care is used, as the long spark necessary to the production of the luminosity will hardly ignite anything but gas, unless specially arranged to do so.
Fig. 52 is a triangle formed of three Geissler tubes, and intended for rotation as a whole. M M are two pieces of mica or glass, to prevent any possibility of the spark jumping and short circuiting, in which event the tubes would fail to light.
Fig. 52.
This triangle is shown diagrammatically at A B C, Fig. 53, mounted on an insulated rotating disk. Before commencement of rotation, and upon the current being turned on to the tubes, a simple triangle will result, but at a certain stage of rotation the Maltese cross shown is formed. A still higher rate of rotation will produce the double star, Fig. 54, and as the rotation and rate of vibration of the coil contact-breaker is varied, an apparently endless succession of stars or triangles appears to grow out into view.
Fig. 53.
Fig. 54.
Although Figs. 53 and 54 serve to illustrate a triangle of tubes and its variations, a very pretty and simple effect can be obtained with it as follows: Three strips of looking-glass are cut and scratched across their silvered surface, as described for the luminous pane, Fig. 37. The current then being allowed to pass, and the wheel being rotated, the triangle acts as in the preceding paragraphs, multiplying and forming figures, which are extremely interesting to watch.
While treating on the subject of store-window attractions, a few suggestions on a display of stationary Geissler tubes may be made. Starting with the assumption that the platform on which the goods would be displayed is of wood, a very small amount of preparation is necessary. The platform is covered with a dark material free from gloss, such as canton flannel, on which the tubes are laid in any fancy pattern, or may be scattered haphazard. Fine bare wire (No. 36 B. & S. is not any too small) is run from tube to tube, using care that it does not touch itself in such manner as to short circuit the current. There is not much necessity to cover the wires, unless the rate of vibration of the contact be so rapid as to show the brush discharge from the wire strands. In a jewelry store the cylindrical portions of the tubes may be covered with strips of dark cloth, concealing all but the bulbs. The Uranium bulbs will resemble emeralds; the yellow bulbs, topaz; and the blue, turquoise—certainly a very striking collection of gems. A few diamond-shaped pieces of the foil-coated glass scratched across, by the whiteness of the tiny sparks will aid to set off the whole. The outfit is not expensive: a coil giving a one half inch spark will light from four to six tubes to great brilliancy. Cloths with metallic threads woven in them must not be used, nor any of the metallic powders known in the trade as "glitters."
CHAPTER IX.
GAS LIGHTING.
When it is desired to light clusters of gas jets situated in inaccessible places, or a number of them simultaneously, this method finds ready application. It operates in the division of a long spark among a number of burners, the gas being turned on at the main and the primary circuit of a Ruhmkorff coil closed and opened until the succession of sparks ignites the gas, Fig. 55. There are various commercial forms of these burners, prominent among which is the "Smith jump spark" burner.
Fig. 55.
A lava tip is provided with a mica or isinglass flange midway between the tip and the lower end of the burner. This flange isolates the electrodes from any possibility of the spark straying away to the metallic pillar in which the burner is inserted. The multiple lava tip burner is intended for use where a very short burner is needed, also for flash rings multiple lights. Here the tips are placed close enough together to ignite by contagion. In this case one of the common tips is removed from the ring and a multiple lava tip substituted. It is customary to allow sixteen burners to one inch of spark. Any number of series can be operated alternately by means of a suitable switch.
The wire used to connect the burners is generally bare copper, and as small in diameter as will sustain its own weight without injury, the amount of the current being infinitesimal. It is supported on porcelain or glass knobs screwed to the wall or ceiling, being carefully planned to avoid any metallic substances to which the spark might be tempted to escape. In wiring chandeliers, the wire is run through glass tubes wherever there is any liability of its coming near the metal pipes. There is a very great danger of this jumping of the spark where it is not wanted, and the utmost care must be taken in planning the course the wires shall take. Even a damp wall will cause trouble or a gilt cornice, although the latter may be entirely insulated from the ground. The switch bases for the groups of circuits must be of hard rubber, and the switch points and levers be placed so far apart that there is no liability of the spark jumping, which it certainly will do if it gets a chance. Ordinary insulated wires are ineffectually protected by the rubber compounds used. Glass, mica, and better still, a large air gap are the only insulations that will serve, for the tremendous potential or voltage of the current must be carefully considered whenever insulation is necessary. The coil is better provided with a spring key in the primary circuit than a vibrator, it gives better control of the circuit and probably a larger and better spark.