Before me are two long brass rods, each of them about 5 feet in length, and the ends of these rods are provided with polished brass balls (see Fig. 71). The rods are placed in one line and supported on pieces of ebonite, and are so fixed that the two balls are separated from one another by a space of about ¹⁄₄ inch. The two rods constitute, therefore, two insulated conductors. These rods are connected by coils of wire with the terminals of an instrument called an induction coil, which I shall not stop to describe, but which you may regard as a kind of electrical machine for producing electromotive force. If we set the induction coil in action, it creates between its terminals an intermittent but very powerful electromotive force, which gradually increases up to a certain value, at which it breaks down the conductivity of the air-gap between the two balls. Let us think carefully what happens as the electromotive force of the induction coil is increasing. One of the rods is in effect being electrified with positive, and the other with negative, electricity, and these charges are increasing in magnitude. The two rods constitute, as it were, the two coated surfaces of a kind of Leyden jar, or condenser, of which the surrounding air is the non-conductor. Accordingly, by all that has been previously explained, you will easily understand that there is an electric strain in the air which exists along certain lines, called lines of electro-static strain, and this state in the air is exactly similar to the condition in which the glass of a Leyden jar finds itself when the jar is charged. If we were to delineate the direction of this electric strain by lines drawn through the space around the rods, we should have to draw them somewhat in the fashion represented by the dotted lines in Fig. 71. As the electrical state of the rods gradually increases in intensity, a point is reached at which the air between the balls can no longer maintain this strain, and it breaks down and passes into a conductive condition. The state of affairs round the rods is then similar to that of a Leyden jar being discharged. An electric current is produced across the air-gap, moving from one rod to the other, and the intensely heated air in between the balls is visible to us as an electric spark. This spark, if photographed, would be found to be an oscillatory spark. The electric current in the rods cannot continue indefinitely: it gradually falls off in strength, but as it flows it creates in the space around the rods an electric strain which is in the opposite direction to that which produced it, although taking place along the same lines.