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 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. 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.

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.

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."