346 



NA TURE 



[February ii, 1892 



time, in a new and forcible way, important questions of 

 electrical theory, and the physiological effects of rapidly 

 alternating currents. That he should have been able 

 unharmed to place himself in the space between two 

 tinfoil plates connected to the terminals of his rapidly 

 alternating machine, was to the ordinary observer in itself 

 sufficiently startling ; but that he should have been able 

 to present a piece of iron to one of the poles of the 

 machine, drawing a spark of several inches in length 

 with impunity, and thereby to interpose his body as a 

 connecting link between the machine and a long vacuum 

 tube which glowed like a flaming sword, must have ap- 

 peared to many of those most conversant with electrical 

 phenomena truly astonishing. 



Hitherto, alternating machines of great frequency and 

 high potential have been deemed peculiarly dangerous, 

 and not without reason. But it did not follow, of course, 

 that with a sufficient increase of the frequency of alterna- 

 tion, the danger might not completely disappear. It will 

 be of great importance to inquire in what way the im- 

 munity of the experimenter from injury is brought about. 

 Are impulses of 20,000 reversals per second and upwards 

 without serious effect on the nervous system of the human 

 body, so that conduction takes place through it without 

 any disagreeable consequences? or is the conduction 

 effected without the nervous system being concerned 

 at all .? 



The delicate network of nerves in the eye is sensitive 

 to a certain range of frequency of electrical vibrations, 

 and perfectly insensitive to vibrations which lie outside 

 that range in frequency. In the same way the insen- 

 sitiveness of the general nerve-system of the human body 

 interposed between a glowing vacuum tube and the ter- 

 minal of a rapidly alternating machine or transformer 

 may begin and end at much lower limits. There is also, 

 of course, the interesting question of the distribution of 

 these rapidly alternating currents in the somewhat com- 

 plicated conductor formed by the human body, which 

 may have a great deal to do with the result. 



The admirable experiments of Mr. Tesla are only 

 another instance of the way in which practical applica- 

 tions of science promote its progress, by enabling appa- 

 ratus to be constructed on an engineering scale, and with 

 all the security for effective action which the constructive 

 art of the engineer furnishes so well. His simple alter- 

 nating machine, running with very little clearance at a 

 speed of about 2000 revolutions per minute, is itself a 

 triumph of skill in design and construction, and well 

 illustrates how desirable and even necessary it is to take 

 advantage of all the aids to exactness, and they are 

 many, which can be obtained from the refined machine 

 tools and truth of design which characterize the engineer- 

 ing workshop of to-day. The ordinary optician of twenty 

 years ago, with his imperfect lathes, and general utter 

 want of power-driven appliances, his continual hand- 

 fitting and shaping, and the absolute non-interchange- 

 ability of the parts of his instruments, has almost passed 

 away ; and even the physical laboratory has become in 

 great measure an engineering workshop, in which are to 

 be found Whitworth lathes and end-measurinj machines 

 adapted for the most exact work. 



It ought to be recalled here that Prof J. J. Thomson 

 has been working in the same field, and has obtained 

 somewhat similar results. These were made the subject 

 of a very interesting demonstration to the members of 

 the Physical Society on the occasion of their visit to 

 Cambridge in May of last year. For a long time Prof. 

 Thomson has investigated this subject both theoretically 

 and experimentally, and his researches have thrown 

 much light on the rationale of the very striking results 

 obtained by Mr. Tesla and himself in their closely allied 

 but independently carried out series of experiments. 



One point in this connection is worthy of notice. 

 Mr. Tesla insists strongly on the essentially electrostatic 



NO. I 163, VOL. 45] 



nature of the phenomena illustrated by his lectures ; 

 while, on the other hand, one object aimed at in 

 Prof. Thomson's experiments was to show that in a 

 tube without electrodes luminosity could be produced by 

 electrodynamic action alone— that is to say, in a field of 

 electric force which is not electrostatic in the sense of 

 admitting of the derivation of its intensity at each point 

 from a potential function. 



The changes produced in the distribution of electricity 

 on neighbouring conductors will cause glow in a vacuum 

 tube when a Holtz machine or Leyden jar is discharged ; 

 and this will in general be more or less operative. But 

 it is not in general possible to separate the electromotive 

 forces due to this cause from those due to electro- 

 magnetic action. Prof. Thomson has succeeded in some 

 cases in screening off these electrostatic effects, and in 

 producing a glow discharge in which electrostatic action 

 could have little or no share. 



The glow or flame discharges from the terminals of Mr. 

 Tesla's induction coil, the glow discharge from the long 

 wires stretched from the induction coil towards the roof 

 of the hall, the glowing vacuous bulbs and phosphorescent 

 tubes in the field between the parallel tin-foil plates 

 attached to the transformer terminals are all phenomena 

 of the highest importance ; though, of course, they are 

 only exceedingly striking and effective illustrations of 

 experimental results already arrived at by the lecturer 

 himself and others, and communicated in a more or less 

 complete manner to the electrical world. The applica- 

 tion of these, which Mr. Tesla suggests as a possible one 

 in the future, would bring about an ideal form of elec- 

 tric lighting, which would transcend in luxury and con- 

 venience our present system of electric lighting by 

 incandescent lamps as far as the latter transcends the 

 oil lamps and tallow dips used by our near ancestors. 

 Every drawing-room would become an electric field in a 

 continual state of rapidly alternating stress, in which the 

 occupants would live, experiencing no unpleasant effects 

 whatever, while vacuous bulbs or phosphorescent globes 

 and tubes, without care or attention, would shed a soft 

 diffused light, of colour and intensity arranged to suit the 

 most luxurious fancy. It would be interesting also to 

 know whether, after all, habitual dwelling in a region of 

 electric stress rapidly changed from one extreme of high 

 intensity to the opposite, produced very slow physiological 

 effects which could be traced in the improved health 

 and longevity of the persons so dwelling, or the reverse. 

 If such applications are made (and there does not seem 

 to be any sufficient reason why they should not come to 

 pass), the magnificent researches of Mr Crookes, as well 

 as those of other investigators to whom the lecturer justly 

 and generously acknowledged his indebtedness, will bear 

 some practical fruit in an almost totally unexpected 

 manner, by becoming at once available in connection 

 with a new and beautiful development of what is at 

 present the most progressive of the physical sciences. 



It does seem that we are on the point of farther great 

 advance into the undiscovered domain of electrical 

 science, and it is significant that it is likely to lie along 

 one of the routes made clearer to us by the discovery and 

 verification of the great theory of electrical radiation. 

 Who knows what further discoveries may be obtained 

 before the present century has come to an end ? We 

 are advancing so rapidly that no one can declare that 

 the record of discovery of the nineteenth century has 

 nearly closed. One important means of further investi- 

 gating electrical radiation will be that which Prof. Fitz- 

 gerald made an attempt to find — a means of maintaining 

 for any required length of time electrical vibrations of 

 sufficiently high frequency. Mr. Tesla's results seem to 

 promise that this problem may perhaps be solved before 

 very long, and many outstanding questions of the electro- 

 magnetic theory of light thereby set at rest. In many 

 other ways hip researches are certain to promote scientific 



