The outer ends of the choking coils are connected to an induction coil or electrical machine, say a small Wimshurst machine, suitable for producing electric sparks about 2 or 3 inches in length. If then sparks are taken between the balls, we have an arrangement which is, in fact, a small Hertz oscillator or radiator. It has been fully explained in the last chapter that the action of the induction coil or electrical machine is first to create a difference in the electric condition of the balls, such that one is positively electrified and the other negatively. The balls and rods and the surrounding air, as already explained, then form a sort of Leyden jar or condenser, and in virtue of the electromotive force the air is electrically strained around the balls. When this strain reaches a particular value, the air between the balls passes at once into a conductive condition, and we have a discharge which is oscillatory in nature produced between the conductors. We may consider that the electrical charges on the two rods rush backwards and forwards, setting up on the rods an oscillatory surface electric current, and that this is accompanied by a very rapid reversal of the strain in the surrounding non-conductor or dielectric. This state of affairs results in sending out into space an effect called an electric wave. [Illustration: FIG. 74.—Electric radiation detector (Miller).] Turning, then, to the receiver B (Fig. 73), we notice that this consists of a similarly shaped metal box, having in it a board to which are fixed two short nickel wires. These are crossed without touching in the interior of a small ebonite box (see Fig. 74). The wires are just covered inside the box with a very small quantity of fine nickel filings. To the end of the zinc receiver-box is fixed a long lead pipe, in the interior of which are two insulated wires, _c_, _d_.