136 



NA TURE 



[June 7, 1894 



produces no effect,— no stimulating effect, that is, even though 

 the voltage is so high that sparks are ready to jump between 

 the needles in direct contact wiih the nerve. 



AH that such oscillations do, if coniin lel, is to produce a tem- 

 porary paralysis or fatigue of the nerve, so that it is unable to 

 transmit the nerve impulses evoked by other stimuli, from 

 which paralysis it recovers readily enough in course of time. 



Experiment of Gotch and Lodge on the physiol igical effect of rapid pure 

 electric allctnations- Ner\-c and muscle preparation, with four necdle-s 

 or ieUe oon-polari^able electrodes applied to the nerve. C and D arc the 

 terminals of a rapidly alternating electric curient froi) a conductor at zero 

 potential, while A and B are the terminals of an ordinarj' verj' weak galvanic 

 or induction coil stimulus only jutt sufficient to make the muscle twitch. 



This has been expected from experiments on human beings ; 

 such experiments as Tesla's and those of d'Arsonval. But an 

 entire animal is not at all a satisfactory instrument wherewiih 

 to attack the questim ; its nerves are so embedded in conduct- 

 ing tissues that it may easily he doubted whether the alternating 

 type of stimulus ever reaches them at all. By dissecting out a 

 nerve and muscle from a deceased frog, afier the historic manner 

 of physiologists, and applying the stimulus direct to the nerve, at 

 the same time as some other well-known I too h of a volt stimulus 

 is applied to another part of the same netve further from the 

 mu-scle.it can be shown that rapid electric alternations, if entirely 

 unaccompanied by static charge or by resultant algebraic electric 

 transmission, evoke no exciiatory response until they are so 

 violent as logive rise to secondary effects such as heat or mechan 

 ical shock. Vet, notwithstanding this inaction, they gradually 

 and slowly exert a paralysing or obstructive action on the portion 

 of the nerve to which they are applie.1, so that the nerve impulse 

 excited by the feeble just perceptible I'looth volt stimulus above 

 is gradually throttled on its way down to the muscle, and 

 remains so throttled for a time varying from a few minutes to 

 an hour after the cessation of the violence. 



I had intended to exhibit this effect, which is very marked 

 and definite, but it is impossible to show everything in the \\xat 

 at my disposal. 



Air Gap and Elfclroscoft, chargtd by Glass Rod and discharged 

 by moderately distant Sphere excilel by Coil. 



Among trigger methods of detecling electric radiation, I have 

 spoken of the Zchnder vacuum tubes ; another method is one 

 used by Bollzmann.' A pile of several hundred volts is on 

 the verge of charging an electroscope through an air-gap just 

 too wide to break down. Very slight electric surgings precipi- 

 tate the discharge across the gap, and the leaves diverge. I 

 show this in a modified and very simple form. On the cap of 

 an electroscope is placed a highlypolishid knob or rounded 

 end, connected to the sole, and j'tst not touching the cap. Such 

 an electroscope overflows suddenly and completely with any 

 gentle rise of po'ential. Bring excited glass near it, the leaves 

 diverge gradually and then suddenly collapse, because the air 

 space snaps ; remove the glass, and they redivcrge with negative 

 electricity ; the knob above the cap being then charged posi- 

 tively, and to the ver,;e of sparking. In this condition any 

 electrical waves, collected if weak by a foot or so of wire 

 projecting from the cap. will discharge the electroscope by 

 exciting surgings in the wire, and so breaking down the air-gap. 

 The chief interest about this experiment seems to me the 

 extremely definite dielectric strength of so infinitesimal an air 

 •pace. Sloreover, it is a detector for Hertz waves (hat might 

 have been used last century ; it might have been used by 

 Benjamin Franklin. 



For to excite them, no coil or anything complicated is 

 necesaary ; it is sufficient to flick a metal sphere or cylinder 

 with a silk handkerchief, and then discharge it wilh a well- 

 pr>lished knob. H it is not well-polished the discharge is 

 comparatively gradual, and the vibrations are weak ; the more 

 poli.shed are the sides of an air-gap the more sudden is the 

 collapse, and the more vigorous the consequent radiation, 

 especially the radiation of high frequency, the higher harmonics 

 of the disturbance. 



Ftjr delicate experimenti it is sometimes well to repoli'th 

 the knobs every hour or >o. For metrical experiments it is often 

 better to let the knobi get into a less efficient but m ire per- 



' ll'uJ. Ann. 40, p \,'f. 



NO. I 284. VOL. 50] 



manent state. This is true of all senders or radiators. Kor 

 the generation of the. so to speak, "infra-red" Hertz w.ives 

 any knobs will do, but to generate the "ultraviolet" high 

 polish is essential. 



Microphonic Detectors. 



Receivers or detectors which for the present I temporarily 

 call microphonic are liable to respond best to the more rapid 

 vibrations. Their sensitiveness is to me surprising, though of 

 course it does not approach the sensitiveness of the eye ; at the 

 same time, I am by no means sure that the eye differs from them 

 in kind. It is these detectors that I wish specially to bring t< 

 your notice. 



Prof. Minchin, whose long and patient work in connection wilh 

 pho oelectricity is now becoming known, and who h.as devised an 

 instrument more sensitive to radiation than even Boys' radio- 

 micrometer, in that it responds to the radiation of a star while 

 the radio-micrometer does not, found some years ago that some 

 of his light excitable cells lost their sensitiveness capriciously 

 on tapping ; and later he found that they frequently regained it 

 again while Mr. Gregory's Hertz wave experiments were going 

 on in the same room. 



These "impulsion-cells," as he terms them, are troublesome 

 things for ordinary persons to make an 1 work with — at least I 

 have never presumed to try^but in Mr. Minchin's hands they 

 are surprisingly sensitive to electric w.ives. ' 



The sensitiveness of selenium to light is known to everyone, 

 and Mr. Shelford Bidwell has made experiments on the varia- 

 tions of conductivity exhibited by a mix'ure of sulphur and 

 carbon. 



Nearly four years ago, M Edouard Branly found that a 

 burnished coat of porphyrised copper spread on glass diminished 

 its resistance enormously, from some millions to some hundreds 

 of ohmsj when it was exposed to the neighbourhood, even the 

 distant neighbourhood, of Leyden jar or coil sparks. He like- 

 wise found that a tube of metallic filings behaved similarly, but 

 that this recovered its original resistance on shaking. M ■ . Croft 

 exhibited this fact recently at the Physical Society. Branly 

 also made pastes and solid rods of filings in Canada balsam 

 and in sulphur, and found them likewise sensitive.' 



With me the matter arose somewhat differently, as an outcome 

 of the air-gap detector employed with an electroscope by Boltz- 

 mann. For I had observed in 18S9 that two knobs sufficiently 

 close together, far loo close to stand any voltage such as an 

 electroscope can show, could, when a spark p-issed between 

 them, actually cohere ; conducting an ordinary bell-ringing 

 current if a single voltaic cell was in circuit ; and, if there was 

 no such cell, exhibiting an electromotive force of their own 

 sufficient to distuib a low resistance galvanometer vigorously, 

 and sometimes requiring a faintly perceptible amount of force to 

 detach them. Tbc experiment was described to the Institution 

 of Electrical Engineers,'' and Prof. Hughes said he had observed 

 the same thing. 



Coherer in open, responding to Feeble Stimuli ; Small Sphere, 

 Gaslighler, Pis 'ant Sphere, Eleclrophorus. 



Well this arrangement, which I call a coherer, is the most 

 astonishingly sensitive detector of Hertz waves. It difi'ers from 

 the actual air-gap in ih.at the insulating film is not really insu- 

 lating ; the film breaks down not only much more easily, but 

 also in a less discontinuous and more permanent manner than 

 an air gap. A tube of filings, being a .series of bad contacts, 

 clearly woiks on the same plan ; and though a tube of filings i* 

 by no means si sensitive, yet it is in many respecis easier to 

 work wilh, and, except for very feeble stimuli, is more metrical. 

 If the filings used are coarse, say turnings or borings, the tube 

 approximates to a single coherer ; if they are fine, it has a larger 

 range of sensibility. In every case what these receivers feel 

 are sudden jerks of current : smooth sinuous vibralions are 

 ineffective. They seem to me to respond best to waves a few 

 inches long, but doubtless that is determined chiefly by the 

 dimensions of sjme conductor with which they happen to be 

 associated. 



Filings in open, responlim; to Sphere, to Eleclrophorus, to 

 .ipark from Cold leaf Eleitroscopc. 



I picture to myself the action as follows. Suppose two 

 fairly clean pieces of metal in light contact — say two pieces of 



' Phit. Mttj^. vol. 31, p. aaj. 



-' K Kranly. Comfltt Rrmiiii, vol. 111, p. 785 : and vol. 113, p. 90. 

 ■< Jriirnnl Insl. E. E , iSw, vol. 19. pp 35'-4 '. or " Lightnioi Con- 

 ductors and Lightning Giiaids" (\\'l>'«'okerl. pp. 3H2 4. 



