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.