December i i, 1902] 



NA TURE 



133 



able, and has obviously proved a stumbling-block to the 

 Marconi Company, as it forms the subject-matter of two 

 or three patents taken out by Prof. Fleming and the 

 Company. Some of the methods described therein are 

 exceedingly ingenious, but, unfortunately, space does not 

 allow us to describe them here, especially as their bearing 

 on wireless telegraphy is only indirect. 



With the exception of the magnetic detector devised 

 by Mr. Marconi and tested during the cruise of the 

 Carlo Alberto, practically all the different systems make 

 useof the coherer principle for receiving. Theactua! type 

 of coherer used differs considerably in the several cases. 

 For long-distance work, it has generally been found most 

 suitable to use a coherer which requires no tapping back, 

 but spontaneously returns to its norma! condition, this 

 being connected in parallel with a telephone. One of the 

 chief advantages of this arrangement lies in the fact that 

 the energy required to give audible signals in the tele- 

 phone is much less than that needed to work a relay. 

 There are several different coherers working on this 

 principle — the principle really of the microphone ; in the 

 system devised by M. Popoff, carbon granules form the 

 loose contacts, the resistance, which is normally high, 

 being broken down by the received waves and the coherer 

 then restoring itself to its original condition ; the change 



Serial 



Fig. 2.— Castelli Coherer and Connections. 



in the current through the coherer causes a click in the 

 telephone. In the de Forest system, an electrolytic 

 "anticoherer" is used ; this has a paste, composed of a 

 viscous material, loose conducting particles and an 

 electrolyte, between suitable electrodes. In the normal 

 condition, the conducting particles bridge the gap and 

 give the receiver a low resistance ; electrolysis is set 

 up by the received oscillations and the consequent 

 polarisation greatly increases the resistance. Of the 

 coherers of this type, the greatest interest attaches to 

 the Castelli coherer. This, invented by a semaphorist in 

 the Italian navy, was used by Mr. Marconi in his first 

 Transatlantic experiments. Its construction is shown in 

 Fig. 2. Two iron or carbon electrodes, c C, fit into the 

 tube T and are connected by a single drop of mercury 

 Bg. The connections shown are, of course, the same in 

 the case of the two other coherers just described. When 

 electrical oscillations reach the tube, the mercury coheres 

 to the electrodes, but returns at once to its normal con- 

 dition when the stimulus ceases. The magnetic detector 

 to which we have made reference above was described by 

 Mr. Marconi in a paper read before the Royal Society last 

 June. Fig. 3 shows the principle of its construction. It 

 consists of a core of thin iron wires, I, over which are 

 wound two coils of fine copper wire, C, and C. The outer 

 core, c„ is connected to a telephone receiver and the inner, 

 c 2 to the aerial and earth or to the secondiry of a trans- 

 former the primary of which is connected to the aerial 



NO. 1728, VOL. 67] 



and earth. The iron core is magnetised by a permanent 

 magnet, M, at one end, which is rotated by clockwork so 

 as to produce a continual slow change in the magnetis- 

 ation, which, however, owing to the hysteresis, lags 

 behind the magnetising force. When oscillatory currents 

 pass through the inner coil, there is a sudden decrease 

 in the hysteresis, due apparently to the molecules being 

 released from restraint : a corresponding sudden vari- 

 ation in the magnetisation of the iron results, and this 

 induces a current in the outer winding connected to the 

 telephone. 



Such, in brief, are the more important advances that 

 have been made in the practice of wireless telegraphy 

 during the past year. In addition, much work has been 

 done on the purely scientific side of the subject, the 

 action of the coherer in particular having been submitted 

 to somewhat rigorous examination, work which has 

 already produced results which may prove both of great 

 physical and great practical value. It may fairly be said 

 that we know now, with a considerable degree of 

 certainty, some of the more useful services which wire- 

 less telegraphy may be relied upon to perform. Already 

 its commercial application is considerable ; many ships, 

 in the navies of this and other countries and in the 

 merchant services, are equipped with wireless telegraphic 

 apparatus which has, we believe, fully justified its instal- 



Jkrial 



J 



- mMm tt 



\Earth 



Fig. 3. — Diagram of Marconi's Detector. 



lation. It is in this direction that we look with the most 

 confidence for a steady increase in its application, and 

 we would rather hear of a few more ships being thus 

 equipped than of another " S " being transmitted across 

 the Atlantic. MAURICE SOLOMON. 



NOTES. 

 The Paris correspondent of the Times announces ihe death of 

 M. Deherain, professor of vegetable physiology in the Museum 

 of Natural History, and of M. Hautefeuille, mineralogist at the 

 Faculty of Sciences. Both were members of the Paris Academy 

 of Sciences. The death is also announced of M. Alexandre 

 Bertrand, one of the original founders of the fine museum of St. 

 Germain, of which he had been curator since 1862. He was 

 also professor at the Ecole de Louvre of national archaeology, 

 and his fame as an archaeologist was world-wide. 



The great dam on the Nile at Assuan is to be inaugurated by 

 the Duke and Duchess of Connaught as we go to press with this 

 number. Sir Benjamin Baker, K. CM. G., has been appointed to 

 be a Knight Commander of the Order of the Bath, in recognition 

 of his services in connection with the construction of the Nile 

 reservoir. Other honours conferred in connection with the 

 work are : — To be G.C.M.G., Sir William Edmund Garstin, 

 K.C.M.G., Under-Secretaiy of State for Public Works in 

 Egypt. Tobe K.C M.G., Major R. H. Brown, R.E., C.M.G., 



