NATURE 



[June 8, 1S93 



the first so-called electrical phenomena noted. But though they 

 have been known to U5 for many centuries, the precise nature of 

 the mechanism concerned in these actions is still unknown to us, 

 and has not been even quite satisfactorily explained. What kind 

 of mechanism must that be? We cannot help wondering when 

 we observe two magnets attracting and repelling each other with 

 a force of hundreds of pounds with apparently nothing between 

 them. We have in our commercial dynamos magnets capable of 

 sustaining in mid-air tons of weight. But what are even these 

 forces acting between magnets when compared with the tremen- 

 dous attractions and repulsions produced by electrostatic force, to 

 which there is apparently no limit as to intensity. In lightning 

 discharges bodies are often charged to so high a potential that 

 they are thrown away with inconceivable force and torn asunder 

 or shattered into fragments. Still even such effects cannot com- 

 pare with the attractions and repulsions which exist between 

 charged molecules or atoms, and which are sufficient to project 

 them with speeds of many kilometres a second so that under their 

 violent impact bodies are rendered highly incandescent and are 

 volatilized. It is of special interest for the thinker who inquires 

 into the nature of these forces to note, that whereas the actions 

 between individual molecules or atoms occur seemingly under any 

 condition, the attractions and repulsions of bodies of measurable 

 dimensions imply a medium possessing insulating properties. So, 

 if air, either by being rarefied or heated, is rendered more or less 

 ■conducting, these actions between two electrified bodies practi- 

 cally cease, while the actions between the individual atoms 

 continue to manifest themselves. 



Single-wire Transmission, 



It has been for a long time customary, owing to the limited 

 experience with vibratory currents, to consider an electric 

 current as something circulating in a closed conducting path. It 

 was astonishing at first to realise that a current may flow through 

 the conducting path even if the latter be interrupted, and it 

 was still more surprising to learn, that sometimes it may be 

 even easier to make a current tlow under such conditions than 

 through a closed path. But that old idea is gradually dis- 

 appearing, even among practical men, and will soon be entirely 

 forgotten. 



It is thought useful to devote here a few remarks to the sub- 

 ject of operating devices of all kinds by means of only one 

 leading wire. It is quite obvious, that when high-frequency 

 currents are made -use of, ground connections are — 

 at least, when the E. M.F. of the currents is great — better 

 than a return wire. Such ground connections are objectionable 

 with steady or low frequency currents on account of destructive 

 chemical actions of the former and disturbing influences exerted 

 by both on the neighbouring circuits ; but with high frequencies 

 these actions practically do not exist. Still, even ground con- 

 nections become superfluous when the E. M.F. is very high, for 

 soon a condition is reached when the current may be passed 

 more economically through open, than through closed con- 

 ductors. Remote as might seem an industrial application of such 

 single wire transmission of energy to one not experienced in 

 such lines of experiment, it will not seem so to any one who for 

 some time has carried on investigations of such nature. Indeed 

 I cannot see why such a plan should not be practicable. Nor 

 should it be thought that for carrying at such a plan currents of 

 very high frequency are implicitly required, for just as soon as 

 potentials of say 30,000 volts are used, the single wire trans- 

 mission may be effected with low frequencies, and experiments 

 have been made by me from which these inferences are made. 



Electrical Resonance. 



Some remarks and experiments were then made with regard to 

 electrical resonance. Continuing, Mr. Tesla said : — • 



In connection with resonance effects and the problem 

 of transmission of energy over a single conductor which 

 was previously considered, I would say a few words on 

 a subject which constantly fills my thoughts and which 

 concerns the welfare of all. I mean the transmission of 

 intelligible signals or perhaps even power to any distance 

 without the use of wires. I am becoming daily moie convinced 

 of the practicability of the scheme, and though I know full well 

 that the great majority of scientific men will not believe that 

 such results can be practically and immediately realised, yet I 

 think that all consider the developments in recent years by a 

 number of workers to have been such as to encourage thought 

 and experiment in this direction. My conviction has grown 



NO. 1232, VOL. 48] 



so strong that I no longer look upon this plan of energy or 

 intelligence transmission as a mere theoretical possibility, 

 but as a serious problem in electrical engineering, which must 

 be carried out some day. The idea of transmitting intelligence 

 without wires is the natural outcome of the most recent 

 results of electrical investigations. Some enthusiasts 

 have expressed their belief that telephony to any distance 

 by induction through the air is possible. I cannot stretch my 

 imagination so far, but I do firmly believe that it is practicable 

 to disturb by means of powerful machines the electrostatic con- 

 dition of the earth, and thus transmit intelligible signals and 

 perhaps power. In fact, what is there against the carrjing out 

 of such a scheme ? We now know that electric vibiaiiou may 

 be transmitted through a single conductor. Why then not try 

 to avail ourselves of the earth for this purpose? We nrcd not 

 be frightened by the idea of distance. To the weary wanderer 

 counting the mile-posts the earth may appear very large, but 

 to that happiest of all men, the astronomer, who gazes at the 

 heavens and by their standard judges the magnitude of our 

 globe, it apppears very small. And so, I think, it must seem to 

 the electrician, for when he considers the speed with which an 

 electric disturbance is propagated through the earth all his 

 ideas of distance must completely vanish. 



A point of great importance would be first to know what is the 

 capacity of the earth? and what charge does it contain if electri- 

 fied ? Though we have no positive evidence of a charged body 

 existing in space without other oppositely electrified bodies being 

 near, there is a lair probability that the earth is such a body, for 

 by whatever process it was separated from other bodies — and this 

 is the accepted view of its origin — it must have retained a 

 charge, as occurs in all processes of mechanical separation. If 

 it be a charged body insulated in space its capacity should be 

 extremely small, less than one-thousandth of a farad. But the 

 upper strata of the air are conducting, and so, perhaps, is the 

 medium in free space beyond the atmosphere, and these may 

 contain an opposite charge. Then the capacity might be in- 

 comparably greater. In any case it is of the greatest importance 

 to get an idea of what quantity of electricity the earth contains. 

 It is difficult to say whether we shall ever acquire this necessary 

 knowledge, but there is hope that we may, and that is by means 

 of electrical resonance. If ever we can ascertain at what period 

 the eailh's charge, when disturbed, oscillates with respect to an 

 oppositely electrified system or known circuit, we shall know a 

 fact possibly of the greatest importance to the welfare of the 

 human race. I propose to seek for the period by means of an 

 electrical oscillator, or a source of alternating electric currents. 

 One of the terminals of the source would be connected to earth, 

 as, for instance, to the city water mains, the other to 

 an insulated body of large surface. It is possible that 

 the outer conducting air strata or free space contains 

 an opposite charge and that, together with the earth, they form 

 a condenser of very large capacity. In such case the period of 

 vibration may be very low and an alternatmg dynamo machine 

 might serve for the purpose of the experiment. I would then 

 transform the current to a potential as high as it would be found 

 possible and connect the ends of the high tension secondary to 

 the ground and to the insulated body. By varying the frequency 

 of the currents and carefully observing ihe potential of the in- 

 sulated body and watching for the disturbance at various neigh- 

 bouring points of the earth's surface resonance might be 

 detected. Should, as the majority of scientific men in all prob- 

 ability believe, the period be extremely small, then a dynamo 

 machine would not do and a proper electrical oscillator would 

 have to be produced and perhaps it might not he possible to 

 obtain such rapid vibrations. But whether this be possible 

 or not, and whether the earth contains a charge or not, 

 and whatever may be its period of vibration, it certainly 

 is possible — for of this we have daily evidence— to produce 

 some electrical disturbance sufficiently powerful to be perceptible 

 by suitable instruments at any point of the earth's surface. 



Production of Light. 



The light effects which it has been the chief object to investi- 

 gate can be divided into four classes: (i) Incandescence 

 of a solid. (2) Phosphorescence. (3) Incandescence or 

 phosphorescence of a rarefied gas, and (4) Luminosity pro- 

 duced in a gas at ordinary pressure. The first question 

 is, ■ How are these luminous effects produced ? In order to 

 answer this question as satisfactorily as I am able lo do 

 in the light of accepted views and with the experience 



