loS 



NA TURE 



[June 7, 1894 



without recharging the lid. Do this several limes ; and at 

 last, when the spark is inaudible, invisible, and otherwise im- 

 perceptible, the coherer some yards away responds more violently 

 than ever, and the spot of light rushes from the scale. 



If a coherer be atiached by a side wire to the gas-pipes, and 

 an elec'.rophorous spark be given to either the gas pipes or the 

 water-pipes, or even to the hotwaler system, in another room of 

 the building, the coherer responds. 



In fact when thus connected to gas-pipes, one day when I 

 tried it, the spot of light could hardly keep five seconds still. 

 Whether there was a distant thunderstorm, or whether it was 

 only picking up telegraphic jeiks, I do not know. The jerk of 

 turning on or ofl an extra Swan lamp can affect it when sensi- 

 tive. I hope to tiy for long-wave radiation from the sun, 

 filtering out the ordinary well-known waves by a black-board or 

 other suflSciently opaque substance. 



We can easily see the detector respond to a distant source of 

 radiation now, viz. to a 6-inch sphere placed in the library 

 between coil knobs. 



Portable Detector. 



Also I exhibit a small complete detector male by my assis- 

 tant Mr. Davies, which is quite portable and easily set up. 

 The essentials are all in a copper cylinder three inches by two. 

 A bit of wire a few inches long, pegged into it, helps it to col- 

 lect waves. It is just conceivable that at some distant date, say 

 by dint of inserting gold wires or powder in the relina, we may 

 be enabled to see waves which at present we are blind lo. 



Observe how simple the production and deleclion of Hertz 

 waves are now. An electrophorous or a frictional machine 

 serves to excite them ; a voltaic cell, a rough galvanometer, and 

 a bad contact, serve to detect them. Indeed they might have 

 been observed at the beginning of the century, before galvano- 

 meters were known. A frog's leg or an iodide of starch paper 

 would do almost as well. 



A bad contact was at one time regarded as a simple nuisance, 

 because of the singularly uncerlain and capricious character of 

 the current transmuted by it. Hughes observed its sensitive- 

 ness to sound-waves, and it became the microphone. Now it 

 turns out to be sensitive to electric waves, if it be made of any 

 oxidisable metar(not of carbon), and we have an instrument 

 which might be called a micro-something, but which, as it 

 appears to act by cohesion, I call at present a coherer. Per- 

 haps some of the capriciousness of an anathematised bad con- 

 tact was sometimes due lo the fact that it was responding to stray 

 electric radiation. 



The breaking down of cohesion by mechanical tremor is an 

 ancient process, observed on a large scale by engineers in rail- 

 way axles and girders ; indeed, the cutting of small girders by 

 persi>tent blows of hammer and chisel reminded me the other 

 day of the lapping back of our cohering surfaces after they have 

 l>een exposed lo ihe welding efTecl of the electric jerk. 



Put Copper Hat oier Tube. Shut up everything in Box 

 completely. 



If a coherer is shut up in a complete metal enclosure, waves 

 cannot get at it, but if wires arc led from it lo an outside ordi- 

 nary galvanometer, it remains nearly as sensitive as it was 

 before (nearly, not quite), for the circuit picks up the waves, 

 and they run along the insulated wires into the closed box. To 

 screen it effectively it is necessary to enclose battery and gal- 

 vanometer and every bit of wire connection ; the only thing that 

 mayl)e left outside is the needle of the galvanometer. Accord- 

 ingly here we have a compact arrangement of battery and coil 

 and coherer, all shut up in a copper box. The coil is fixed 

 against the side of the box at .'<uch height that it can act con- 

 veniently on an oui<.i(Ic suspended compass needle. The slow 

 action of Ihe coil has no difTicully in getting through copper, as 

 everyone knows ; only a perfect conductor could screen oif that, 

 but the Ileriz waves are eflectively kept out by sheet copper. 



Chink; Kound Uole ; FrolruJing Wire. 



It miul be saif), however, that the box must be exceedingly 

 well closed for ihe screening to be pcrlect. The very narrowest 

 chink permits Iheir entrance, and at one time I thought I should 

 have to solder a lid on before they could be kept entirely out. 

 Clamping a copper lid on lo a flange in six pl.ices w,i8 not 

 enough. But by Ihe use of pads of tinfoil, chinks can he 

 avoided, and the inside of Ihe box becomes then electrically 

 dark. 



If even an inch of Ihe circuit protrudes, it at once becomes 

 slightly sensitive .igain ; and if a single branch wire protrudes 



NO. 1284, VOL. 50] 



through the box, provided.it is insulated where it passes through , 

 the waves will utilise it as a speaking tube, and run blithely in. 

 And this whether the wire be connected lo anything inside or 

 not, though it acts more strongly when connected. 



Receiver Hat and Metal Tube for Connecting Wires. 



If wires are to be laken out of the box to a coherer in ^onu 

 olher enclosure, they must be enclosed in a metal tube, andii, 

 tube must be well connected with the melalof both enclosure-, 

 if nothing is lo get in but wh.it is w.inted. 



Similarly, when definite radiation is desired, it is well to put 

 the radiator in a copper hat, open in only one direction. And 

 in order to guard against reflected and collateral surgings run- 

 ning along Ihe wires which pass oulsideto Ihe coil and battery, as 

 they are liable lo do, I am accustomed lo put all these things in 

 a packing case lined wiih tinfoil, to the outside of which the 

 sending hat is fixed, andlo pull the key of the primary excitint; 

 circuit by a string Irom outside. 



Sender in Hat and Box, with Lid (adjustable) clamped on. 



Even then, with the lid of the hat well clamped on, some- 

 thing gets out, but it is not enough to cause serious disturbance 

 of qualitative results. The sender must evidently be thought 

 ol as emitting a momentary blaze of light which escapes through 

 every chink. Or, indeed, since the waves are some inches long, 

 Ihe difficulty of keeping them out of an enclosure may be 

 likened to the difiicully of excluding sound ; ihough the diffi- 

 culty is not quite so great as Ihal, since a reasonable thick- 

 ness of n elal is really opaque. I fancitd once or twice I 

 delected a trace of transparency in such metal sheets as ordinary 

 linplale, but unnoticed chinks elsewhere may have deceived 

 me. It is a thing easy to make sure of as soon as I have more 

 time. 



One thing in this connection is noticeable, and that is how 

 little radiation gels either in or out of a small round hole. A 

 narrow long chink in ihe receiver box lets in a lot ; a round 

 hole the size of a shilling lets in hardly any, unless indeed a 

 bit of insulated wire protrudes through it like a collecting ear- 

 trumpet. 



Gas-lighter with Tinfoil. 



It may be asked how the waves get out of the metal tube of 

 an electric gas-lighter. Hut they do not ; they get out through 

 the handle, which being of ebonite is transparent. Wrap up 

 the handle tightly in tinfoil, and a gas-lighter is powerless. 



Optical Experiments. 



And now in conclusion I will show some of Ihe ordinary 

 optical experimenis with Heriz waves, using as source either one 

 of two devices : either a 6-inch sphere wilh sparks lo ends of a 

 diameter, an arrangement which emits ginch waves, but of so 

 dead-beat a character I hat it is wise I o enclose it in acopperhatto 

 prolong them, and send them out in the desired direction ; or 

 else a 2-inch hollow cylinder with spark knobs at ends of an 

 internal diatueler. This last emits 3-inch waves of a very fairly 

 persistent character, but with nothing like the intensity of one 

 of ihe outside radiators. 



As receiver there is no need to use anything sensitive, so I 

 employ a glass lube full of coarse iron filings, put at the back of a 

 copper hat wilh its mouth turned well askew to ihe source, which 

 is put outside the door at a distance of some yards, so that 

 only a little direct radiation can reach the tube. Sometimes 

 the lube is put lengthways in ihe hat instf.id of crossw.iys, which 

 makes it less sensitive, and has also Iheadvaul.age of doing away 

 wilh the polaiising or rather analysing power of a crossway 

 tube. 



Various apertures in Lid. 



The radiation from the sphere is slill too strong, but it can be 

 stopped down by a diaphiagin plate wilh holes in it of var)ing 

 size clamped on the sending hat. 



Heflecting Plate, Wet Cloth, Glass Plate. 

 Having thus reduced the excursion of the spot of light lo a 

 foot or JO, a mclnl plate is held as reflector, and al once ihe 

 spot Iravcis a couple of yards. A wet clolh reflects something, 

 bul a ihin glass plate, i( dry, reflects next lo nothing, being, as 

 is well known, loo thin to give anything but "the black spot." 

 I have fancied that it teflccts something of the 3-inch waves, 



Refractius; Prism and Lens, 

 A block of parafl^n about a cubic foot in volume is cast Into 

 the shape o( a priim with argles 75°, 60 , and 45°. Using the 



