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.