At this point, however, it will be best to withhold further discussion on points of theory until we have considered the facts to be brought before you in the next lecture, showing that the electric radiation manufactured by means of electric oscillations is only one variety of a vast range of æther waves, some forms of which are recognizable by us as light and radiant heat. CHAPTER VI. WAVES AND RIPPLES IN THE ÆTHER. Having in the last chapter explained the nature and mode of production of electric oscillations, and shown that when these take place in an open electric circuit or long straight rod they give rise to certain actions at a distance, rendered evident by the changes taking place in the conductivity of metallic powders, we have now to present the outlines of a proof that these actions are really due to a wave-motion of some description set up in the æther, which in nature is essentially the same as that which constitutes the agency we call light. We shall begin by studying a few of the epoch-making discoveries we owe to the celebrated Heinrich Hertz, announced in a series of famous researches with which he surprised and delighted the scientific world in the years 1887 and 1888. These investigations opened a new and remarkable field of experimental work. The precise form of apparatus used by Hertz in these researches is, however, unsuited for lecture demonstration, and I shall use on this occasion some arrangements of my own, which are only convenient modifications of appliances previously employed by other experimentalists. The devices here shown are, however, very convenient for public demonstrations. This apparatus consists of two parts, a part for generating electric waves, which we shall call the radiator, and a part for detecting them, which is called the receiver. The radiator consists of a zinc box, A (see Fig. 73), which is provided with hollow trunnions, and can be fixed to a suitable stand and turned in any direction. The box has an open end to it, and in its interior there are two brass rods about 4 inches long, each terminating in brass balls, S, 1 inch in diameter. These rods are thrust through corks fixed in the end of two ebonite tubes, which pass through the hollow trunnions of the box. The rods have their ends attached to very closely wound spirals of gutta-percha-covered wire contained in the ebonite tubes. These spirals are called _choking coils_. When the balls are arranged in the interior of the box in their proper position, they are about ¹⁄₁₆ inch apart, and the rods to which they are attached are in line with each other. [Illustration: FIG. 73.—Electric wave radiator (A) and receiver (B).]