iron wire, W, and W, (Fig. 41), take the place of the usual resistances between A D and DC (Fig. 40). If

one connects a tele

phone to the points B and D (Fig. 41) and speaks into it, the distribution of the currents along the sides of the little parallelogram W, is disturbed. This disturbance leads to a disturbance between D and B (Fig. 40), which is made evi

dent by the move

ment of the galvanometer needle G. The voice thus alters the circulation of electricity through this network of conductors.

We have now used the fluctuations of electrical resistance to make manifest the waves of heat and light and sound. Can we also use this instrument for the

B

FIG. 42.

detection of electrical waves? This has been accomplished by Prof. Rubens and Prof. Arons, who supplanted the telephone in Fig. 41 by loops of wire, O and P, on glass tubes (Fig. 42) which

could be slipped along the wires traversed by the electrical waves. The fluctuations of electricity at electrical ventral segments and nodes thus caused corresponding

disturbances through the glass tubes on the terminals O and P, which in turn disturbed the distribution of electrical currents in the parallelogram W,. The little glass tubes are really Leyden jars. The portion of the wire in the tubes along which the electrical waves are propagated constitute the inner coating of these jars, and the loops O and P the outer coatings.

In a previous chapter on the rates of change of magnetic induction; we have shown how varied are the transformations of energy which can be effected by the quivering, so to speak, of lines of magnetic flow. In the use of these little Leyden jars we perceive that the electrostatic lines can also, by their rate of change, effect a rate of change of magnetic lines of flow. The fluctuation of the electrical charge on the coatings of the little jars produces a current on the wire connecting them. Indeed, if this fluctuation is accompanied by a sufficiently powerful electro-motive force, we have seen that it can produce the usual evidence of an electric current a spark. On the other hand, if we should speak into the telephone in Prof. Rubens's apparatus, we ought theoretically to obtain an electrostatic disturbance on the little Leyden jars—that is, the fluctuations of the magnetic induction produced in the telephone by the human voice should be transformed into fluctuations of electrostatic lines between the coatings of the Leyden jars.

The effect would be small, but it would be possible. This effect, indeed, has been shown substantially by Dolbear, whose telephone is a Leyden jar to which one listens while a fluctuation on the wires connecting its two coatings is caused by the human voice acting on a carbon transmitter placed in the primary circuit of a Ruhmkorff coil, while the Leyden jar, of which the di

electric is air, is connected with the terminals of the secondary coil of the Ruhmkorff.

Our conceptions of the energy and rapidity of the changes which can be produced by fluctuations of the flow of magnetic induction and of electrostatic induction have been greatly enhanced by the invention of the telephone. And we are now, even in practical applications of electricity, obtaining a realizing sense of the importance of arranging our electrical circuits so that the waves which produce the fluctuations of magnetic and electrostatic induction should have their proper expression in the transformation we desire to accomplish. The scientific imagination even looks forward to transmitting intelligence from America to Japan by suitably modifying the electric charge on the earth. This does not seem at first sight more improbable than the feat of speaking by means of the waves of heat, which is accomplished by Graham Bell's photophone. The following interesting

modification of this instrument has lately come to my attention:

A tube of lampblack, T (Fig. 43), is placed at the focus of a large parabolic mirror, and a speak

FIG. 43.

ing tube, or rather a listening tube, is connected with it. At the distance of half a mile a reflecting mirror is supported so that the rays of the sun shall be reflected to the tube, T. The voice of a speaker at A impinging against the mirror can be thus heard, and conversation carried on through the air. The heat rays are thrown into vibration by means of the speaker's voice upon the lampblack, and the air in the speaking tube is

thus tuned to his voice. Here we have a transformation of sound movements into heat movements and a retransformation into sound waves. Can we not transform the sound waves of the voice into light waves, so that a speaker can cause light to appear at a great distance from him? To accomplish this, we need only to set the electrostatic lines of force to quivering in unison with the voice.

CHAPTER XVIII.

ELECTRIC WAVES.

WHEN We survey the practical development of electricity, which I have outlined, we are struck with the fact that our minds have been led from a consideration of steady currents of electricity and the phenomena produced by them to what may be termed unsteady or periodic currents. The transformations of energy which are possible with periodic or alternating currents are far more varied than those we can accomplish with steady currents. It would seem that even the development of the applications of the alternating current suggests the electro-magnetic theory of light. The swifter the rate of alternation of our alternating dynamo, the nearer we approach to the manifestations of light and the more varied become the electrical phenomena. This seems to me the most remarkable conclusion to be drawn from Tesla's experiments on high frequency discharges. If we could excite electrical currents which would oscillate some billions of times a second we might produce the sensation of light on the retina of the eye without a spark.

The ordinary Leyden jar is the swiftest alternating machine which we can use at present. Joseph Henry showed conclusively, in 1840, that the discharge of a condenser is, in general, oscillatory. His observations