This increase in the efficiency of our sources of artificial illumination is only likely to be brought about when we abandon the process of heating a solid substance to make it give out light, and adopt some other means of setting the electrons in vibration. It is almost impossible to discuss the subject of æther waves without some reference to the most modern utilization of them in the so-called wireless telegraphy. Without entering upon the vexed questions of priority, or on the historical development of the art, we shall simply confine our attention here to a consideration of the methods employed by Mr. Marconi, who has accomplished such wonderful feats in this department of invention. We have already seen that when two insulated conductors are placed with their ends very near together, and are then electrified, one positively and the other negatively, and then allowed to be suddenly connected by an electric spark, they constitute an arrangement called an electrical oscillator. If the conductors take the form of two long rods placed in one line, and if their contiguous ends are provided with spark-balls separated by a small gap, we have seen that we have shown that, under the above-mentioned conditions, electric currents of very high frequency are set up in these rods. For creating these oscillations, an instrument called an induction coil or spark-coil is generally employed. You will understand the arrangements better if a brief description is given first of the spark-coil itself as used in wireless telegraphy. [Illustration: FIG. 81.—A 10-inch induction coil for wireless telegraphy (Newton).]