You have already seen that an alternating or two-way electric current existing in one circuit can produce another alternating or two-way electric current in a neighbouring circuit. Before me, on the table, is an arrangement by which a battery of six Leyden jars, L, is continually being charged and discharged through a thick wire which is wound a dozen times round a square wooden frame, P (see Fig. 67). In proximity to this wooden frame there is another wooden frame, S, also having on it a dozen or two turns of insulated wire; the circuit of this last conductor is completed by a small incandescent lamp, G. You will notice that when the Leyden jars are charged and discharged rapidly through the primary conductor, the little glow-lamp of the secondary circuit lights up brilliantly, and, in virtue of what has already been explained, you will see that this experiment is a proof that the discharge of the Leyden jars through the primary circuit must consist in an alternating or two-way current; in other words, it must be oscillatory. [Illustration: FIG. 67.] A still further proof may be given that the discharge of a Leyden jar or condenser, when taking place through a low-resistance circuit, is oscillatory in the following manner:— We employ the vacuum tube that we brought to your notice a few moments ago. When an electric current is sent always in the same direction through such a tube, it is well known that the two ends of the tube are not alike in appearance. The tube, as you have seen, is filled with a luminous glow; but this glow is interrupted, forming what is called a dark space near one terminal of the tube, this terminal being that which is termed the negative pole. Accordingly, this unsymmetrical appearance in the light in the tube is a proof that the electric current is passing through it always in one direction. We can, however, vary the experiment, and instead of illuminating the tube by means of a direct-discharge or induction coil, which is always in one direction, we are able to illuminate it by means of a rapid series of discharges from a Leyden jar. You will then see that the glow-light in the tube is symmetrical—the tube, in other words, is alike at both ends; and this shows us that the discharge from the tube under these circumstances must be alternating—that is, first in one direction and then in the other.