Nov. 7, 1878] 



NATURE 



13 



tected the slightest vibration but was capable of trans- 

 mitting speech -with remarkable loudness. Although this 

 result has hitherto been unpublished, an application of the 

 same principle was made by Mr. Edison in his " carbon 

 relay," a reprint accoimt of which was published in the 

 Telegraphic Journal ior ]v\y i, 1877, and the simple form 

 of this relay (Fig. 4) resembles in construction and in 

 principle the so-called hammer and anvil microphone. 

 Edison's carbon rheostat, lately illustrated in our pages, 

 also depends upon the same principle, namely, 

 the compression of a semi-conductor, such as 

 carbon, increasing its conductivity, a diminution 

 of pressure increasing the resistance. It is true 

 that it has been suggested that a rheostat of this 

 kind was made by M. Cl^rac in 1865, but so far 

 as we have been able to ascertain, no mention is 

 made of varying pressure in the use of Clerac's 

 instrument, but simply the difference in the resist- 

 ance of columns of plumbago of varying length. 



The curious effect of pressure on the electric 

 resistance of semi-conductors was noticed so long 

 ago as 1856 by M. du Moncel, who wrote as 

 follows in the second edition of his " Expose des 

 Applications de lElectricite " (vol. i. p. 246) : — 

 ** Une chose assez curieuse, &. qui paraif etre au 

 premier abord en contradiction avec la th^orie 

 que Ton s'est faite de 1' electricite, c'est que la plus 

 ou moins grande pression exercee entre les pieces 

 de contact des interrupteurs influe considerable- 

 ment sur I'intensit^ du courant qui les traverse. 

 Cela tient souvent k ce que les metaux de 1' inter- 

 rupteur ne sont pas toujours dans un dtat parfait 

 de decapage au point de contact, mais, peut-etre 

 aussi k une cause physique encore mal apprdcide." 

 Subsequently, in an elaborate memoir on the 

 electric conductivity of bodies, the same dis- 

 tinguished author wTote : — '*' La pression exercee 

 sur les electrodes joue un grand role sur la con- 

 ductibilitd de celui-ci quand il est susceptible d'une 

 certaine compression, comme les bois, les corps 

 mous et pateux et ceux qui sont r^duits a un grand 

 €tat de division.' In 1873 Edison states (Prescott, p. 223) 

 that he independently discovered " the pecuHar property 

 which semi-conductors have of var>'ing their resistance 

 with pressure while constructing some rheostats for arti- 

 ficial cables, in which were employed powdered carbon, 

 plumbago, and other materials in glass tubes." 



In January, 1877, Edison states that he first ap- 

 plied to telephonic purposes the effect of pressure on 

 carbon. It will be remembered that the principle of 

 Gray' s telephone was the variations in the resistance of 

 a liquid, proportional to the motions of a diaphragm 

 vibrated by the voice, but that the decomposition of the 

 liquid by the current introduced practical difficulties in 

 the working of this arrangement. Casting about for a 

 means of overcoming this difficulty, Edison recalled his 

 early observations on the electrical properties of carbon, 

 and, encouraged by his preliminary trials, was led 

 onwards step by step, his unwearied efforts being at last 

 crowned by the construction of his carbon telephone, 

 towards the close of last year. In another article we 

 shall trace the evolution and present achievements of the 

 carbon telephone, an instrument which we believe is 

 destined to have a great future. 



In conclusion, it is interesting to note that just as Gray 

 and Bell were both led to the discovery- of the principle of 

 an articulating telephone through working at the important 

 problem of multiple telegraphy, or the simultaneous 

 transmission of several different signals on one wire, so, 

 too, Edison travelled in the same direction. In Pres- 

 cott's work he writes: "Some time in or about July, 

 1S75, 1 began experimenting with a system of multiple 

 telegraphy which had for its basis the transmission of 

 acoustic vibrations." His transmitters were a chord of 



timing-forks maintained in vibration by the action of a 

 current ; the current not being interrupted at each vibra- 

 tion, but made to A-ary in strength : his, receivers con- 

 sisted of electro-magnets acting upon iron diaphragms, 

 which closed the end of a series of resonant tubes. In 

 practice difficulties arose, but the system contained two 

 important features. 



These were, first, the employment of undulatory electric 

 currents, an electrode attached to one prong of the trans- 



FiG. 2. — Ducretet's stethoscopic microphone. 



mitting fork being made to vary the resistance of the 

 circuit by vibrating in a vessel of water, an idea which 

 Edison claims to have patented as early as 1873 ; and the 

 second is the use of an iron diaphragm and adjacent 

 electro-magnet as a receiver. Substituting a membrane 

 moved by the voice, the transmitter becomes the same 

 as that patented by Gray in 1876, and the receiving 

 instrument is similar to that used in the magnet o-speak- 



FlG. 3. — Hammer and anvil 



microphone 

 speech. 



as arranged for transmitting 



ing or Bell's telephone. Hence, if these facts are unassail- 

 able, and the latter is at present the subject of litigation 

 in America, to Edison is also due the happy inspiration 

 of first using a thin iron diaphragm as a receiver. At 

 the same time Edison, in a recent letter to the present 

 writer, remarks, "Bell had the merit of discovering such 

 a receiver would act as a transmitter," and likewise in 



