with the turns all in the same plane (Prof. Henry's spirals). While performing some experiments with them at a lecture he was giving, he noticed that the discharge of a very small Leyden jar, or of a small induction coil, passed through the one was able to excite induced currents in the other, provided that a small spark-gap was made in the circuit of the first spiral. Thus was made the all-important discovery of the "effective spark-gap" which started Hertz on the road of his marvellous investigations.

A very little consideration of this phenomenon enabled him, even at this early stage, to lay down the following propositions :

1. If we allow a condenser, such as a Leyden jar, of small capacity, to discharge through a short and simple circuit with a spark-gap of suitable length, we obtain a sharply defined discharge of very short duration, which is the long-sought-for sudden disturbance of electrical equilibrium-the exciter of electrical vibrations.

2. Such vibrations are capable of exciting in another circuit of like form resonance effects of such intensity as to be evident even when the two circuits are separated by considerable distances. In this second circuit Hertz had found the long-sought-for detector of electric waves.

With the exciter to originate electric waves and the detector to make them evident at a distance, all the phenomena of light were, one after another, reproduced in corresponding electro-magnetic effects, and the identity of light and electricity was completely demonstrated.1

In his paper "On very Rapid Electric Oscillations," Hertz occupied himself with some of these phenomena. As an exciter he used wire rectangles, or simple rods (fig. 30) to the ends of which metallic cylinders or spheres were con

1 See Appendix A for a clear exposition of the views regarding the relation of the two before and after Hertz.

nected, the continuity being broken in the middle where the ends were provided with small spherical knobs between

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Fig. 30.

which the sparks passed. The exciter was charged by an ordinary Ruhmkorff induction coil of small size.

Fig. 31.

When

The detector was mostly a simple rectangle or circle of wire (fig. 31), also provided with a spark-gap. vibrations are set up in the detector and sparks pass across the gap, the greater length of these sparks indicates the greater intensity of the received wave impacts. When, therefore, the dimensions of the detector are so adjusted as to give the maximum sparks with a given exciter the two circuits are said to be in resonance, or to be electrically tuned. Fortunately this condition of resonance or syntony is not essential to the excitement of sparks, else wireless telegraphy by Hertzian waves would not be so advanced as it is to-day. Thus, when a good exciter is in action it will cause little sparks between any conducting body in its vicinity and a wire held in the hand and brought near to the body, showing that the influence of the exciter extends to all conducting bodies, and not merely to those which are tuned to it. Of course it still holds good that, cæteris paribus, the maximum effect is obtained with

resonance.

In the course of his experiments on electric resonance, Hertz observed a phenomenon which for a time was inexplicable. It was seen that the length and brightness of the sparks at the detector were greatly modified by the sparks given off at the exciter. If the latter were visible from the detector spark-gap the sparks given off there were small and hardly perceptible, but became larger and brighter as soon as a screen was placed between the two instruments. By carefully thought-out experiments he showed that this singular action was due solely to the presence of ultra-violet light, breaking down the insulation of the gap and making it, so to say, more conductive. This effect can be shown in another way, by widening the spark-gap of an induction coil beyond the ordinary sparking distance, when, by simply directing a beam of ultra-violet light into the gap, sparking will be resumed.1

Having made himself familiar with the phenomena of electrical resonance, Hertz went on to study the propagation of electric vibrations through space-the most difficult, as it is probably the most important, of all his researches.

1 Prof. K. Zickler has proposed to use this property for telegraphy. At the sending station an arc lamp, which is rich in ultra-violet rays, is provided with a shutter and a lens for directing flashes towards the receiving station. There they are made to impinge on the sparkgap, unduly widened, of an induction coil in action, and allow sparks to pass. These give rise to electric waves which act on the coherer, which in its turn operates a bell, a telephone, or a Morse instrument in the way we shall see later on when we come to speak of the action of the Marconi apparatus. The reflecting lens is made of quartz and not of glass, which does not transmit the ultra-violet rays; but for signalling or interrupting the rays in long and short periods a glass plate is used as the shutter. The interruption of the ultra-violet rays is thus effected without altering the light, which assures secrecy of transmission. Prof. Zickler has in this way signalled over a space of 200 metres, and thinks that with suitable lamps and reflectors the effect would be possible over distances of many kilometres.— 'Elektrische Zeitung,' July 1898.

The results he gave to the world in 1888, in his paper "On the Action of a Rectilinear Electric Oscillation on a Neighbouring Circuit." When sparks pass rapidly at the exciter electric surgings occur, and we have a rectilinear oscillation which radiates out into surrounding space. The detectors, whose spark-gaps were adjustable by means of a micrometer screw, were brought into all kinds of positions with respect to the exciter, and the effects were studied and measured. These effects were very different at different points and in the different positions of the detector. In short, they were found to obey a law of radiation which was none other than the corresponding law in optics.

In his paper, "On the Velocity of Propagation of Electrodynamic Actions," he gave experimental proof of the hitherto theoretical fact that the velocity of electric waves in air was the same as that of light, whereas he found the velocity in wires to be much smaller-in the ratio of 4 to 7. For the moment he was puzzled by this result: he suspected an error in the calculations, or in the conditions of the experiment, but—and here he showed himself the true philosopher -he did not hesitate to publish the actual results, trusting to the future to correct or explain the discrepancy. The explanation was soon forthcoming. Messrs E. Sarasin and L. de la Rive of Geneva took up the puzzle, and ended by showing that the deviations from theory were caused simply by the walls of Hertz's laboratory, which reflected the electric waves impinging on them, so causing interferences in the observations. When these investigators repeated the Hertzian experiment with larger apparatus, and on a larger scale, as they were able to do in the large turbine hall of the Geneva Waterworks, they found the rate of propagation to be the same along wires as in air.1

1 'Comptes Rendus,' March 31, 1891, and December 26, 1892. See also the 'Electrician,' vol. xxvi. p. 701, and vol. xxx. p. 270.

In his paper, "On Electro-dynamic Waves and their Reflection," Hertz further developed this point, and showed the existence of these waves in free space. Opposite the exciter a large screen of zinc plate, 8 feet square, was suspended on the wall; the electric waves emitted from the exciter were reflected from the plate, and on meeting the direct waves interference phenomena were produced, consisting of stationary waves with nodes and loops. When, therefore, Hertz moved the circle of wire which served as a detector to and fro between the screen and the exciter, the sparks in the detector circuit disappeared at certain points, reappeared at other points, disappeared again, and so on. Thus there was found a periodically alternating effect corresponding to nodes and loops of electric radiation, showing clearly that in this case also the radiation was of an undulatory character, and the velocity of its propagation finite.

In a paper, "On the Propagation of Electric Waves along Wires," March 1889, Hertz shows that alternating currents or oscillations of very high frequencies, as one hundred million per second, are confined to the surface of the conductor along which they are propagated, and do not penetrate the mass. This is a very important experimental proof of Poynting's theory concerning electric currents, which he had deduced from the work of Faraday and Maxwell. According to this theory, the electric force which we call the current is in nowise produced in the wire, but under all circumstances enters from without, and spreads itself in the metal comparatively slowly, and according to similar laws as

1 It should be stated here that long ago Prof. Henry, the Faraday of America, held the same views, and proved them, too, by an experiment which is strangely like one of Hertz's, though, of course, he did not explain them as Hertz does. Henry's views are given clearly in two letters addressed to Prof. Kedzie of Lansing, Michigan, in 1876. Being of historical interest, as well as of practical value, I give them entire in Appendix B.