May 14, 1 891] 



NATURE 



ZZ 



gated along the strings, waves of alternate motion down 

 and up. 



In order to reflect these waves, we require a metallic 

 sheet of considerable area some twoor three wave-lengths 

 away from the generator ; so far away in order that we 

 may have room for our detector to find the loops and 

 nodes formed every half wave-length where the outgoing 

 waves meet those reflected from the screen. Not too 

 far away, or our waves will be too feeble even at the 

 loops to affect our detector. The waves are thrown off 

 all round, but are most intense in the horizontal plane 

 through the spark, so that our detector had better be 

 placed as near to this plane as possible. The detector 

 may be either a very nearly closed circle of wire, or two 

 conductors, each somewhat longer and thinner than the 

 combined lengths of the generating conductors, and 

 placed vertically over one another, and separated by a 

 minute air gap. As the theory of this latter form of 

 detector is simpler than that of the circle, it will simplify 

 matters to consider it alone. The two conductors should 

 each have a period of electrical oscillation up and down 

 it, the same as that of the charges on the generator. 

 The generator consists of two conductors certainly, but 

 then during the time the spark lasts they are virtually 

 one conductor, being connected by the spark across 

 which the electric charges are rushing alternately up and 

 down. Hence the period of oscillation of the charges on 

 the generator corresponds to that on a single conductor 

 of the same size as its two parts combined. Various ex- 

 periments have been made as to the best form for these 

 conductors that form the detector. They might be made 

 identical with the generator, only that the spark gap in 

 the generator should be represented by a connecting wire. 

 They may be longer and thinner. If longer, they should 

 be thinner, or they will not have the same period of 

 vibration. On the whole, the best results have been got 

 with conductors somewhat longer and thinner than the 

 generator. It is not generally convenient that the spark 

 between the two conductors that form the detector should 

 take place directly from one to the other. It is not easy 

 to make arrangements by which distance apart of these 

 conductors can be regulated sufficiently accurately. The 

 most convenient way is to connect the lower end of the 

 upper conductor and the upper end of the lower one 

 each with a short thin wire leading, one to a fixed small 

 knob, and the other to a very fine screw impinging on 

 the knob. The screw may then be used to adjust the 

 spark gap between it and the small knob with great ac- 

 curacy. This spark gap must be very small indeed, if 

 delicate work be desired. A thousandth of a centimetre 

 would be a fair-sized spark gap. The minute sparks that 

 are formed in these gaps when doing delicate work 

 are too faint to be seen, except in a darkened room. 

 Having placed the detector in position between the 

 generator and the screen, the difificult part of the obser- 

 vation begins. It is heartrending work at first. A bright 

 spark now and then arouses hope, and long periods of 

 darkness crush it again. The knobs of the generator 

 require repolishing ; the spark gap of the detector gets 

 closed up ; dust destroys all working ; and not without 

 much patience can the art be attained of making sure 

 of getting sparks whenever the conditions are favourable, 

 though it is easy enough not to get sparks when the con- 

 ditions are unfavourable. Before making any measure- 

 ments, all this practice must be gone through. It is hard 

 enough with the success of others before us to encourage 

 us, with their advice to lead us, with a clear knowledge 

 of what is to be expected to guide us. How much credit, 

 then, is due to Hertz, who groped his way to these won- 

 derful experiments from step to step, without the success 

 of others to encourage him, without the advice of others 

 to lead him, without any certainty as to what was to be 

 expected to guide him. Patiently, carefully, through many 

 by-paths, with constant watchfulness, and checking every 



NO. I I 24, VOL. 44] 



advance by repeated and varied experiments, Hertz worked 

 up to the grand simplicity of the fundamental experiment 

 in electricity that is engaging our attention. 



Having gained command over the apparatus, we may 

 look about for places where sparks occur easily, and for 

 others where they cannot be produced. Two or three 

 places may be found where no sparks can be observed. 

 These places will be found to be nearly equidistant. 

 They are the nodes we are in search of The distance 

 between any pair is half the distance an electric wave is 

 propagated during the period of an oscillation. Their 

 presence proves that the electric force is not propagated 

 instantaneously, but takes time to get from place to place. 

 If the electric force were propagated instantaneously, 

 there might be one place where the action of the currents 

 induced in our reflecting sheet neutralized the direct 

 action of our generator ; but there could not be a series 

 of two or more such places between the generator and 

 the reflecting sheet. That there are more than one proves 

 that electric force is propagated from place to place, and 

 does not occur simultaneously everywhere. It sets the 

 crowning stone on Maxwell's theory that electric force is 

 due to a medium. Without a medium there can be no 

 propagation from place to place in time. It only remains 

 to confirm by calculation that the rate of propagation is 

 the same as that of light. This is a complicated matter. 

 It involves the question of how fast should, on any 

 theory, the charge oscillate up and down a conductor. 

 The problem has only been accurately solved in a few 

 special cases, such as that of a sphere by itself. The 

 conductors that have been employed are not this shape, 

 are not by themselves, and so only rough approximations 

 are possible as to the rate at which these oscillations 

 occur. Knowing the wave-length will not determine the 

 velocity of propagation unless we know the period of 

 vibration ; and consequently this direct measure of the 

 velocity has only been roughly made ; but it agrees as 

 accurately as could be expected with Maxwell's theory 

 that it must be the same as the velocity of light if 

 electrical phenomena are due to the same medium as 

 light. The conviction that more accurate determinations 

 will confirm this agreemfent is founded upon safe ground. 

 It was pointed out that the ether that transmits light 

 and is set in vibration by the molecules of matter can 

 hardly avoid moving them itself. This ether can hardly 

 help having other properties than merely transmitting 

 a comparatively small range of vibrations. It can hardly 

 help producing other phenomena. When it has been 

 shown that, if there is a medium concerned in con- 

 veying electric and magnetic actions, it must possess 

 properties which would enable it to transmit waves like 

 light ; and when it has been shown that there is a medium 

 concerned in conveying electric and magnetic actions, 

 and that the rate at which they are conveyed is approxi- 

 mately the same as the rate at which light is propagated ; 

 the conclusion is almost unavoidable that we are dealing 

 with the same medium in both cases, and that future ex- 

 periments, capable of accurate calculation and observa- 

 tion, will confirm the conclusion that electric force is 

 propagated through, and by means of, the luminiferous 

 ether with the velocity of light. We really know very 

 little about the nature of a wave of light. We know a 

 great deal more about electric and magnetic forces, and 

 much may be learnt as to the nature of a wave of light 

 by studying it under the form of a wave of electric force. 

 The waves produced by the Hertzian generator may be a 

 metre long or more. The difficulty is to get them short 

 enough. We know a good deal about how they are pro- 

 duced, and from this, and also by means of suitable de- 

 tectors, we can study a great deal about their structure. 

 They are truly very long waves of light. Atoms are 

 Hertzian generators whose period of vibration is hundreds 

 of millions of millions per second. A Hertzian generator 

 may vibrate rapidly, but it is miserably slow compared 



