April 5, 19 17] 



NATURE 



105 



the introduction of con^paratively small masses on 

 the earth's surface would have no perceptible effect 

 on the gravitational field, 5 may be taken as constant 

 in any exp)eriments on the earth's surface. 



Dr. P. E. Shaw (Nature, March 29) argues that 

 my hypothesis involves a violation of the conservation 

 of energy. To avoid the difficulty of perpetual motion 

 I would suggest making dQ/dr positive instead of 

 negative. This will not alter the expression for F, 

 but the turning moment on Dr. Shaw's disc will 

 then bring it to rest. George W. Todd. 



Newcastle-upon-Tyne. 



THERMIONIC DETECTORS IN WIRELESS 

 TELEGRAPHY AND TELEPHONY. 



'^PHE arts of wireless telegraphy and telephony 

 -»- involve the use in the receiving circuit of 

 some device named a detector, which is sensitive 

 to electric oscillations of very high frequency. 

 In the earliest years of radiotelegraphy the appli- 

 ance used was the so-called coherer, in which a 

 small mass of metallic filings or an imperfect 

 contact between two pieces of metal was con- 

 verted into a better conductor by the passage 

 through it of the high-frequency oscillations. All 

 the various forms of coherer have now been aban- 

 doned and are no longer used as detectors. In 

 modern radiotelegraphy, so far as regards the 

 spark or damped-wave system, only three types 

 of detector are at present in practical use. The 

 first of these is the magnetic detector, chiefly the 

 rotating band form, invented by Marconi; the 

 second type is some form of rectifying contact 

 or crystal, such as the carborundum detector due 

 to Dunwoody, or the zincite-chalcopyrite rectifier 

 of Pickard; and the third is some modification 

 of the thermionic detector, or Fleming oscillation 

 valve. 



In the magnetic detector the electric oscillations 

 to be detected are caused to circulate round a 

 magnetised iron wire and alter its magnetic per- 

 meability or hysteresis in such a fashion as to 

 create a sudden change in the magnetisation of 

 the iron. This in turn is made to create an 

 induced electric current in a second coil and 

 reveal itself by a sound made in a telephone in 

 series with that coil. The rectifying contacts 

 or crystals depend upon the fact that a contact of 

 small surface between certain substances, gener- 

 ally crystalline, has a greater electric conductivity 

 in one direction than in the other. Hence, if such 

 a contact as that, for example, between a frag- 

 ment of zincite or native oxide of zinc and a piece 

 of chalcopyrite or copper pyrites is traversed by 

 a train of electric oscillations, these will be con- 

 verted into a movement of electricity chiefly in 

 one direction. Accordingly, if a rapid sequence 

 of such oscillations passes through such recti- 

 fying contact placed in series with a telephone 

 receiver, the latter will be traversed by a series 

 of intermittent gushes of electricity in the same 

 direction, and will emit a sound the pitch of which 

 is determined by the group frequency of the oscil- 

 lations. A very commonly used rectifying con- 

 [tact is a crystal of carborundum, or carbide of 

 |silicon, held between metal clips. Although this 



NO. 2475, VOL. 99] 



rectifying property of certain contacts and crystals 

 has been much studied, the reasons for it are not 

 yet fully elucidated, but it is probably connected 

 with the thermoelectric properties of the materials. 



The third type of detector is the thermionic 

 detector first suggested and used by Dr. J. A. 

 Fleming, of University College, London. The 

 construction and mode of operation of this form 

 of detector may be briefly described as follows : — 

 It had been known for many years prior to the 

 advent of radiotelegraphy that the electric conduc- 

 tivity of a highly rarehed gas was greatly deter- 

 mined by the temperature of the negative elec- 

 trode by which the current left the exhausted 

 vessel containing it. It had been found by Hittorf 

 and also by Elster and Geitel that if the negative 

 electrode was a platinum wire which could be 

 rendered incandescent, the conductivity of the 

 highly rarefied gas was greatly increased. The 

 emission of positive and of negative ions from 

 incandescent solids in vacuo had been studied 

 particularly by Elster and Geitel, beginning in 

 1880. 



In 1884 Edison made known an interesting fact 

 connected with carbon-filament glow-lamps. He 

 sealed into the bulb of one of his bamboo-filament 

 lamps a metal plate placed between the legs of 

 the horseshoe-shaped filament, the said plate being 

 carried on a platinum wire sealed through the 

 glass bulb. He found that when the filament w^as 

 rendered incandescent by a continuous current, 

 a galvanometer connected between the terminal 

 of the plate and the external negative ter- 

 minal of the filament indicated no current, but 

 that if connected between the plate and the posi- 

 tive filament terminal, it showed a current of a 

 few milliamperes. Edison gave no explanation 

 of this, nor did he make any application of the 

 discovery. He supplied a certain number of lamps 

 made with middle plates to the late Sir William 

 Preece, and the latter communicated to the Royal 

 Society in 1885 a paper describing various ex- 

 periments with these lamps. This " Edison effect " 

 was more completely examined by Dr. J. A. 

 Fleming in researches described by him in pap>ers 

 to the Royal Society in 1890 and to the Physical 

 Society in 1896. Dr. Fleming showed that the eff^ect 

 was in some way due to the scattering of particles 

 charged with negative electricity from the hot 

 filament, and that it could be prevented by en- 

 closing the negative leg of the carbon in a glass 

 tube, or placing a sheet of mica between the 

 carbon and the plate. He also proved, as Elster 

 and Geitel had done in another way, that a vacuum 

 tube having two carbon filaments as electrodes 

 had a very large conductivity for small voltages 

 when the negative electrode was made incan- 

 descent. 



It was not until 1899, when Sir J. J. Thomson 

 announced his epoch-making discovery of elec- 

 trons, or corpuscles smaller than atoms, carrying 

 a negative charge, that it was clearly recognised 

 that incandescent solids in high vacua emit elec- 

 tric ions, some positive and some negative. 



This electronic emission from hot bodies has 



