June 7, 1894] 



NA TURE 



oD 



wachs made the important observation, which Righi, Stoletow, 

 Branly, and others have extended, that a freshly-polished zinc 

 or other oxidisable surface, if charged negatively, is gradually 

 discharged by ultraviolet light. 



It is easy to fail in reprolucing this experimental result if the 

 right conditions are not satisfied ; but if they are, it is absurdly 

 easy, and the thing might have been observed nearly a century 

 ago. 



Zinc discharging Negative Electricity in Light; Gold Leaf 

 Electroscope ; Glass ami Quartz Panes ; Quartz Prism. 



Take a piece of linc, clean it with emery paper, connect it to 

 a gold leaf electroscope, and expose it to an arc lamp. If 

 charged positively nothing appears to happen, the action is very 

 slow, but a negative charge leaks away in a few seconds if the 

 light is bright. Any source of light rich in ultra-violet ravs 

 will do ; the light from a spark is perhaps most powerful of all. 

 A pane of glass cuts off all the action ; so does atmospheric 

 air in sufficient thickness (at any rate, town air), hence sunlight 

 is not powerful. -A. pine of quartz transnits the action almost 

 undiminished, but fluor spar may be more transparent still. 

 Condensing the arc rays with a quartz lens and analysing them 

 with a quartz prism or reflexion grating, we find that the most 

 effective part of the light is high up in the ultra-violet, sur- 

 prisingly far beyond the limits of the visible spectrum.' 



This is rather a digression, but I have taken some pains to 

 show it properly because of the interest betrayed by Lord 

 Kelvin in this matter, and the caution which he felt about 

 accepting the results of the Continental experimenters too 

 hastily. 



It is clearly a chemical phenomenon, and I am disposed to 

 express it as a modification of the Volta contact effect - with 

 illumination. 



Return now to the Hertz vibrator, or Leyden jar with its 

 coatings well separated so that we can get into its electric as 

 well as its magnetic field. Here is a great one, giving waves 

 30 metres long, radiating while it lasts with an activity of a 

 hundred horse-power, and making ten million complete electric 

 vibrations per second. 



Large Hertz Vibrator in action ; Abets Fuse ; Vacuum Tube ; 

 y.trike an Arc. 



Its great radiating power damps it down very rapidly, so that 

 1; does not make above two or three swings ; but, nevertheless, 

 each time it is excited, sparks can be drawn from most of the 

 reasonably elongated conductors in this theatre. 



A suitably situated gas-leak can he ignited by these induced 

 sparks. An Abel's fuse connecting the water-pipes with the 

 ;;as. pipes will blow off; vacuum tubes connected to nothing 

 will glow (this fact has been familiar to all who have worked 

 with Hertz waves since 1S89) ; electric leads, if anywhere near 

 each other, as they are in some incandescent lamp-holders, 

 may spark across to each other, thus striking an arc and blow- 

 ing their fuses. 



This blowing of fuses by electric radiation frequently hap- 



1 While preparing for Ihe lecture it occurred to me to try, if possible. 

 'luring the lecture itself, some new experiments -^n the elTect of light on 

 negatively charged bits of rock and ice, because if the effect is not limited 

 to metals it must be important in connection with atmospheric electricity. 

 When Mr. Branly coated an aluminium plate with an insulating varnish. 

 Vie found that its charge was able to soak in and out of Ihe varnish during 

 kluminaticn (C(7;////w Rcndits. vol. 110, p. SgS, iSgo). Now. the mountain 

 I >[,^ of a negatively charged earth are exposed to very ultra-violet rays, and 

 ''le airis a dielectric in which quiet up-carrying and sudden downpour of 

 ■l';ctricity could go on in a manner not very unlike the wellknown behaviour 

 f water vapour : and this perhaps may be the reason, or one of the reasons, 

 why it is not unusual to experience a thunderstorm after a few fine days. 1 

 have now tried these experiments on such geological fragments as were 

 handy, and find that many of them discharge negative electricity under the 

 action of a naked arc, especially from the side of the specimens which w.as 

 somewhat dusty, but that when wet they discharge much less rapidly, and 

 when positively charged hardly at all. Ice and garden soil discharge nega- 

 tive electrilication toj, under ultra-violet illumination, but not so quickly as 

 Milestone, mica schist, ferruginous quartz, clay, and some other specimens. 

 I ^tanite barely acts ; it seems to insulate loo well. The ice and soil were 

 iried in their usual moist condition, but, even when thoroughly dry, soil dis- 

 charges quite rapidly. 



No rock tested was found to discharge as quickly as does a surface of 

 perfectly bright metal such as iron, but many discharged much more quickly 

 than ordinary dull iron, and rather more quickly than whtn the bright iron 

 surface was thinly oiled or wetted with water. 



To-day (June 5) I find that the leaves of a gcanium disch.argc positive 

 electritic.ition five times as quickly as negative, under the action of an arc- 

 light, and that glass cuts the effect off while quart/ transmits it. 



- See Brit, .\ssoc. Report, 18S4, pp. 502, 519 ; or /'////. .lAi^. vol. 19, pp. 

 267, 352. 



NO. 128^, VOL. 50] 



pened at Liverpool till the suspensioas of the theatre lamps were 

 altered. 



The striking of an arc by the little reverberating sparks 

 between two carbon points connected with the loo volt mains 

 I incidentally now demonstrate. 



There are some who think that lightning flashes can do 

 none of these secondary things. They are mistaken. 



Specimens and Diagrams. 



On the table are specimens of various emitters and receivers 

 such as have been used by different people. The orthodox 

 Hertz radiator of the dumb-bell type, and the orthodox Hertz 

 receivers — a circular ring for interference experiments, because 

 it is but little damped ; and a straight wire for receiving at a 

 distance, because it is a much better absorber. Beside these 

 are the spheres and ellipsoids (or elliptical plates) which 

 I have mainly used, because they are powerful radiators 

 and absorbers, and because their theory has been worked out 

 by Horace Lamb and J. J. Thomson. Also dumb-bells with- 

 out air-gap, and many other shapes, the most recent of mine 

 being the inside of a hollow cylinder with sparks at ends of a 

 diameter ; this last being a feeble radiator but a very persistent 

 vibrator,' and therefore well adapted for interference and diffrac- 

 tion experiments. But indeed spheres can be made to vibrate 

 longer than usual by putting them into copper hats or enclosures, 

 in which an aperture of varying size can be made to let the 

 waves out. 



Many of these senders will do for receivers too, givin.j oft' 

 sparks to other insulated bodies or to earth ; but besides the 

 Hertz type of receiver, many other detectors of radiation have 

 been employed. Vacuum tubes can be used, either directly, or 

 on the trigger principle, as by Zehnder,'- the resonator spark pre- 

 cipitating a discharge from some other auxiliary battery or 

 source of energy, and so making a feeble disturbance very visible. 

 Explosives may be used for the same purpose, either in the form 

 of mixed water-gases or in the form of an Abel's fuse. Fitz- 

 gerald found that a tremendously sensitive galvanometer could 

 indicate that a feeble spark had passed, by reason of the con- 

 sequent disturbance of electrical equilibrium which settled down 

 again through the galvanometer.^ This was the method he used 

 in this theatre two years ago. BIyth used a one-sided electto- 

 meter, and young Bjerkness has greatly developed this method, 

 abolishing the need for a spark, and making the electrometer 

 metrical, integrating, and satisfactory.^ With this detector miny 

 measurements have been made at Bonn, by Bjerkness, Vule, 

 Barton, and others, on waves concentrated and kept from spice- 

 dissipation by guiding wires. 



Mr. Boys has experimented on the mechanical force exerted 

 by electrical surgings, and Hertz also made observations of the 

 same '/cind. 



Going back to older methods of detecting electrical radiation, 

 we have, most important of all, a discovery made long before 

 man existed, by a creature that developed a sensitive cavity on 

 its skin ; a creature which never so much as had a name to be 

 remembered by (though perhaps we now call it trilobite). Then, 

 in recent times, we recall the photographic plate and the thermo- 

 pile, with its modification the radio-inicromcter ; also the so- 

 called bolometer, or otherwise known Siemens' pyrometer, 

 applied to astronomy by Langley ; ap|)lied to the detection of 

 electric waves in wires by Rubens and Ritter and Paalzow and 

 .\rons. The thermal junction was applied to the same purpose 

 by D. E. Jones and others. 



And, before all these, the late Mr. Gregory, of Cooper's Hill, 

 made his singularly sensitive expansion meter, whereby waves 

 in free space could be detected by the minute rise of temperature 

 ihey caused in a platinum wire : a kind of early and sensitive 

 form of Cardew voltmeter. 



Going back to the physiological method of detecting surgings, 

 Hertz tried the frog's-leg nerve and muscle preparation, which 

 to the steadier types of electrical stimulus is so surpassingly 

 sensitive, and to which we owe the discovery ol current elec- 

 tricity. But he failed to get any result. Ritier h.as succeeded ; 

 but, in my experience, failure is the normal and proper result. 

 Working with iny colleague Prof. Gotch, at Liverpool, I too 

 have tried the nerve muscle preparation of the frog, and we 

 find that an excessively violent stimulus of a rapi lly alternating 

 character, if pure and unaccompanied by secondary actions, 



' J T. Thomson, " Recent Researches," p. 344. 



- tidied. .-/««. 4-. p. 77. 



^ Fitzgerald, Nature, v.j1. 41, p. 295. and vol. 42, p. 172. 



■* l^'tcit. .-i/tn. 44, p. 74. 



