face, and are reflected by the surface. The phenomenon is similar to that we should obtain if, having excited a powerful tuning fork some feet from a smooth

M

FIG. 47.

wall, we should obtain evidences of nodal points between the prongs of the fork and the wall. A simple way to do this is to walk toward the wall with the fork while it is sounding and note that there is a point where the sound of the fork becomes louder. This is where the reflected wave of sound re-enforces the movement of the prong of the fork. The column of air between the fork and the wall then vibrates in time with the prong of the fork, just as in the experiment when the fork is held over a cylinder which is moved up and down in water until a resonating column of air is obtained. Sarasin and De la Rive, working in a large room at Geneva, obtained reflected electric waves by the use of the circular resonator which we have described. The wave motion of electricity has thus been traced

in the air, or rather in the ether, free from all conductors.

A still more striking way of showing that electrical waves can be reflected is due to Hertz, who used parabolic mirrors. Before speaking of the use of parabolic mirrors to transmit and receive electrical vibrations, let us examine their use in the subjects sound, heat, and light. If a watch is held at the focus of a parabolic mirror, A (Fig. 48), and a listener should station himself at the focus, B, of a similar mirror at a considerable distance, he can hear the tick of the watch. If a more powerful source of sound were placed at A, a sound inaudible at the distance of forty feet can be heard distinctly by the aid of the second mirror. The sound waves emanating from the focus A converge to the focus B of the receiving mirror. The light of a candle placed at A will also be reflected to the focus B; and the heat of a metallic ball placed at A can be

B

FIG. 48.

detected when the focus B is at least one hundred feet from A. It only remains to see if electrical vibrations emanating from the focus A can be detected also at B. Hertz has shown that this is possible. A simple method of constructing his mirrors is the following: A framework of wood is made, and parabolic curves, cut

out of wood, are nailed to this framework. Over these parabolic guides stiff cardboard is nailed and is then covered with tin foil. In this manner one can make large mirrors of which every horizontal section is a parabola. With this species of parabolic mirror we have a linear focus instead of a focus at a point. If now an oscillatory spark is formed at the focus, A, of one mirror, electrical waves are sent out which are reflected from the surface of the mirror to the surface of the second parabolic mirror, and these converge to the linear focus B. If, then, a suitable conductor, including a spark gap, is placed along this linear focus, the waves decay along this conductor, and a spark is obtained as an evidence of this decay at the spark gap B. When B was placed forty feet from A, sparks could still be detected at B. It is probable that this distance can be greatly exceeded, and the imagination immediately pictures the possibility of sending and receiving electrical waves through a fog between one steamship and another. When the mirrors are so far apart that no evidence of a spark can be obtained at B, the focus of the receiving mirror, the waves can still be detected by an interesting process of transformation of energy. One terminal of a galvanometer of great resistance is connected to one side of the spark gap at B, and the other to its opposite side. When the electrical waves are received on the conductors at B a sufficient disturbance of the electrical state is caused to produce a slight electrical current through the galvanometer. Prof. Lodge employed another device, somewhat similar, to detect electrical waves. A glass tube is filled with metallic filings and is connected through a galvanometer with a battery. The inclosed air prevents electrical contacts. When electrical waves fall on the

tube they cause minute sparks among the filings, and the battery is connected by these sparks with the galvanometer. He calls such tubes coherent tubes. The sparks serve the purpose of a relay to throw a stronger electrical impulse through the galvanometer.

We

Greater refinements can doubtless be made in apparatus to detect electrical waves. Indeed, our present form of apparatus will doubtless appear to the worker fifty years from now much as the rude mirrors of the ancients now appear to us. We have now given evidence that electrical waves can be reflected like light and heat waves; it remains to see if they can be refracted also. This refraction has been accomplished by Hertz, who placed a large prism of pitch between the parabolic mirrors, and found that, in order to obtain evidences of electrical waves at the focus of the receiving mirror, it had to be moved in order to receive the waves. owe to Prof. Rhigi, of Bologna, a great simplification of Hertz's apparatus, and it is interesting to reflect that by means of this simplification we are brought back to the use of the electrical machine. We are again in the position of Benjamin Franklin in respect to apparatus. With great experience gained by means of the researches of Galvani, Volta, and Faraday, we have been led back by a distinguished Italian to the study of electricity in its most untrammelled manifestation. Instead of a Ruhmkorff coil or a transformer excited by a battery, Rhigi uses a small electrical machine such as we have shown (Fig. 2, page 27) in comparison with a Franklin machine. Two little spheres in oil (B and C, Fig. 49) receive their charges from the prime conductors, P and P', of the electrical machine. Three sparks are formed, but the middle spark, B C, is the oscillatory one of very rapid period; for the capacity of B

and C is very small, and so also is the self-induction of the little portion of the circuit, A B C D. The dimensions of the spheres and the circuit can be made so small that electric waves of six millimetres (about

one quarter of an inch) can be sent out from this oscillator, whereas the shortest waves obtained by Hertz were several feet long. In order to detect these waves, it was necessary also to have a resonator or electrical eye of very small capacity and self-induction. Rhigi accomplished this by coating a plate of glass with tin foil and making with a diamond a thin cut in the tin foil. Minute sparks passed across this cut when the detector was in electrical tune with the little spheres in the oil.

With this apparatus, experiments on electrical waves are brought within the range of almost any experimenter, and with it Rhigi has performed by means of electrical waves almost all the ordinary optical experiments, such as refraction of waves by prisms of pitch or other light opaque insulating substances; interference of waves; reflection of waves, from the focus of one cylindrical or spherical mirror to the focus of another; and polarization of electric waves.

The experiment of polarization has been shown by a block of wood in the following manner: In Fig. 50

* Lebedew, Ann. der Physik. und Chemie, vol. lvi, 1895.