By some artifice or other, it is easy to concentrate the streams upon small surfaces and to produce very strong light effects.  Two thin wires may thus be rendered intensely luminous.

In order to intensify the streams the wires should be very thin and short; but as in this case their capacity would be generally too small for the coil—­at least, for such a one as the present—­it is necessary to augment the capacity to the required value, while, at the same time, the surface of the wires remains very small.  This may be done in many ways.

Here, for instance, I have two plates, RR, of hard rubber (Fig. 9), upon which I have glued two very thin wires ww, so as to form a name.  The wires may be bare or covered with the best insulation—­it is immaterial for the success of the experiment.  Well insulated wires, if anything, are preferable.  On the back of each plate, indicated by the shaded portion, is a tinfoil coating tt.  The plates are placed in line at a sufficient distance to prevent a spark passing from one to the other wire.  The two tinfoil coatings I have joined by a conductor C, and the two wires I presently connect to the terminals of the coil.  It is now easy, by varying the strength and frequency of the currents through the primary, to find a point at which, the capacity of the system is best suited to the conditions, and the wires become so strongly luminous that, when the light in the room is turned off the name formed by them appears in brilliant letters.

[Illustration:  Fig. 9.—­Wires rendered intensely luminous.]

It is perhaps preferable to perform this experiment with a coil operated from an alternator of high frequency, as then, owing to the harmonic rise and fall, the streams are very uniform, though they are less abundant then when produced with such a coil as the present.  This experiment, however, may be performed with low frequencies, but much less satisfactorily.

[Illustration:  Fig. 10.—­Luminous discs.]

When two wires, attached to the terminals of the coil, are set at the proper distance, the streams between them may be so intense as to produce a continuous luminous sheet.  To show this phenomenon I have here two circles, C and c (Fig. 10), of rather stout wire, one being about 80 centimetres and the other 30 centimetres in diameter.  To each of the terminals of the coil I attach one of the circles.  The supporting wires are so bent that the circles may be placed in the same plane, coinciding as nearly as possible.  When the light in the room is turned off and the coil set to work, you see the whole space between the wires uniformly filled with streams, forming a luminous disc, which could be seen from a considerable distance, such is the intensity of the streams.  The outer circle could have been much larger than the present one; in fact, with this coil I have used much larger circles, and I have been able to produce a strongly luminous sheet, covering an area of more than one square metre, which is a remarkable effect with this very small coil.  To avoid uncertainty, the circle has been taken smaller, and the area is now about 0.43 square metre.