Whilst this apparatus is in use, we can show you with it two other very pretty experiments dependent upon the fact that the discharge of a Leyden jar through a low-resistance circuit is alternating or oscillatory. A moment ago we employed this oscillatory discharge in one circuit to induce a secondary oscillatory discharge in another metallic circuit, and this secondary oscillatory or alternating current was made manifest by its power to illuminate a little incandescent lamp. If, however, we place a large glass bulb, P, which has been partly exhausted of its air, in the interior of the primary discharge coil, you will see that this primary oscillatory discharge of the Leyden jar is able to create in the glass bulb a brilliant luminous ring of light (see Fig. 68). This is called an _electrodeless discharge_, and it is due to the fact that the rapidly oscillatory current existing in the wire wrapped round the bulb creates a similar oscillatory discharge in the rarefied air in the interior of the bulb, this being a conductor, and thus renders it luminous along a certain line. The production of these electrodeless discharges in rarefied gases has been particularly studied by Professor J. J. Thomson. Another experiment illustrating what is called the inductive transformation of electrical oscillations is in the arrangement commonly called a Tesla coil. Such a coil is now before you. It consists of a long coil of insulated wire which is placed in the interior of a tall glass vessel, and on the outside of this glass vessel is wound another insulated and much longer wire. If the alternating or oscillatory discharge of a Leyden jar is allowed to take place through the thicker wire in the interior of the glass cylinder, it generates in the outer or secondary wire a very powerful alternating or oscillatory electromotive force, and we see that this is the case by connecting the ends of this secondary circuit to two insulated brass balls, between which a torrent of sparks now passes. We may vary the experiment by connecting the ends of the secondary circuit of the Tesla coil to two insulated concentric rings of thin, bare, brass wire, and then, when the room is darkened, we see the space between these rings filled with a brilliant purple light, which is due to the discharge taking place through the air under the action of the rapidly oscillatory electromotive force generated in the secondary circuit. [Illustration: FIG. 68.—An electrodeless discharge in an exhausted bulb.] I trust that these experiments will have produced a conviction in your minds that the release of the electric strain in the glass _dielectric_ of a Leyden jar results in the production of electric oscillations or rapidly alternating electric currents in the metallic circuit connecting the two surfaces, just as the sudden release of a compressed spring results in a series of mechanical oscillations.