Nov. 21, 1878] 



NATURE 



57 



the diaphragm was made to press against a little cylinder 

 of crude plumbago. The articulation was poor, though 

 conversation could be understood. Investigation showed 

 that the difference of resistance produced by varying 

 pressure was exceedingly small. As so small a change 

 in a circuit of large resistance was but a small factor, 

 whereas a slight change of resistance in the primary 

 circuit of an induction-coil would be an important factor, 

 it occurred to Edison to associate an induction-coil with 

 his arrangement. But difficulties arose from the high 

 resistance of the plumbago cylinder he first used. Ulti- 

 mately he constructed a transmitter in which a thin slice 

 or button of a semi-conducting substance was placed 

 between two platinum discs. Electrical connection 

 between the button and discs was maintained by the 

 slight pressure of a piece of rubber tubing which was 

 secured to the diaphragm, and also made to rest against 

 the outside disc. The vibrations of the diaphragm were 

 thus able to produce the requisite variations of pressure 

 on the button and thereby create corresponding varia- 

 tions in the resistance in the primary circuit of the induc- 

 tion coil; which in turn gave rise to a corresponding 

 series of induced currents in the secondary. Finally, 

 these induced currents were transmitted through the line 

 and received at the far end by an ordinary magneto- 

 telephone. Fig. I, for which we are indebted to the 

 Telegraphic Journal, shows the arrangement. 



At first a button of solid plumbago, such as is employed 

 by electrotypers, was used, and the results obtained were 

 considered excellent, everjthing transmitted coming out 

 moderately distinct, but the volume of sound was no 

 greater than in that of the magneto-telephone. Nu- 

 merous other semi-conductors were tried until Edison 

 hit upon some lamp-black that had been taken from a 

 smoking petroleum lamp ^d preserved as a curiosity on 

 account of its intense black colour. A small disc was 

 made of this substance, and when placed in the tele- 

 phone it gave splendid results, the articulation being dis- 

 tinct, and the volume of sound several times greater than 

 with a pair of telephones worked on the magneto principle. 

 It was soon found upon investigation that the resistance 

 of a disc formed of this substance could be varied from 

 300 ohms to the fractional part of a single ohm by pressure 

 alone,' and that the bast results were obtained when the 

 resistance of the primary coil, in which the carbon disc 

 was included, was six-tenths of an ohm, and the normal 

 resistance of the disc itself three ohms. 



The rubber tube between the diaphragm and the disc 

 :gave some trouble on account of its tendency to become 

 ^ttened. Experiments imdertaken with a view to remedy 

 this defect led Edison to discover that not only could a 

 rigid substance be interposed with advantage, but that 

 the vibrating diaphragm even was unnecessary ; that, in 

 fact, the sound-waves could be transformed into electrical 

 piilsations without the movement of any intervening 

 mechanism. Edison states that the manner in which he 

 arrived at this result was as follows : — 



"I first substituted a spiral spring of about a quarter- 

 inch in length, containing four turns of wire, for the 

 ■rubber tube which connected the diaphragm Avith the 

 discs. I found, however, that this spring gave out a 

 musical tone which interfered somewhat with the effects 

 produced by the voice ; but, in the hope of overcoming 

 this defect, I kept on substituting spiral springs of thicker 

 wire, and as I did so I found that the articulation became 

 both clearer and louder. At last I substituted a solid 

 substance for the springs that had gradually been made 

 more and more inelastic, and then I obtained very 

 marked improvements in the results. It then occurred 

 to me that the whole question was one of pressure only, 

 and that it was not necessary that the diaphragm should 



the ^cFnl^v^ ^'^^fl '^ ""f,^ *^ statement, but everything dependson 

 bte^Mo^lf ^ °^ ^}'' lampblack, the least trace o£ overheatiSg l</s*ens its 

 »tense blackness and enarmously dirain'^hes its conductivity , 



vibrate at all. I consequently put in a heavy diaphragm, 

 one-and-three-quarter inch in diameter and one-six- 

 teenth inch thick, and fastened the carbon disc and plate 

 tightly together, so that the latter showed no vibration 

 with the loudest tones. Upon testing it I found my sur- 

 mises verified : the articulation was perfect, and the 

 Tolume of sound so great that conversation carried on in 

 a whisper three feet from the telephone was clearly heard 

 and understood at the other end of the line." 



The present and modified form of the instrument is 

 shown in the next diagram, where A A is the thick iron 

 diaphragm, B the rigid connecting-piece pressing together 

 the metal discs D D and the carbon disc c. The pressure 

 can be regulated by the screw s acting upon the sliding 

 stem E, which terminates in an insulating cup that en- 

 closes the carbon and metal discs. Wires lead from 

 D D to binding screws. 



It has been urged that Edison was led to adopt this 

 arrangement by the discovery of the microphone,, it is 

 therefore of historical interest to note that in the American 

 Journal of the Telegraph for April 16, 1878, it is stated 

 " In the latest form of transmitter which Mr. Edison has 

 introduced, the vibrating diaphragm is done away with 

 altogether." A week or two later the discovery of the 

 microphone was announced, and the transmission of 

 speech without a vibrating diaphragm aroused imirersal 

 surprise ; the loose contacts which are essential to the 

 microphone are, however, fatal for telephonic purposes. 



It must be understood that the carbon telephone only 

 acts as a transmitter ; it is incapable of reconverting the 

 electric pulsations into sonorous vibrations. For this pur- 

 pose the ordinary magneto-telephone is employed.^ The 

 accessories and electrical connections requisite for use 

 in a carbon telephone circuit are shown in the ne.xt 

 diagram (Fig. 3). 



P P is the primary wire of an induction coil having a 

 resistance of several ohms and placed outside instead of, 

 as is usual, inside the secondary coU s, which has a re- 

 sistance of some 200 ohms.^ R is the receiving magneto- 

 telephone and T the transmitting carbon telephone; either 

 one or the other can be thrown into the circuit by means 

 of the switch K. When a plug is inserted at the bottom 

 of the switch between 3 and 4, the relay or sounder s, 

 battery B, and key in the centre of the figure, are included 

 in the main line circuit This is the normal arrangement 

 of the apparatus for signalling purposes. To call the 

 distant end the key is pressed do^vn two or three times ; 

 by this means battery- currents are sent through the 

 primary coil p, the currents thus induced in the 

 secondary coil s, pass to line, and actuate the relay 

 or sounder in the distant instrument. When a plug 

 is inserted at the top, between i, 2, and 4, the apparatus 

 is available for telephonic communication. By tracing 

 out the connections it will be seen that in this latter case 

 the battery, B, the primary wire of the coil, p, and the 

 transmitter, T, are in short circuit, and at the same time 

 the line wire is in circuit with the secondary coil, s. A 

 genera] view of the arrangement is shown in Fig. 3, for 

 which we are indebted to the publishers of Coimt du 

 Mon eel's book on the telephone. The lettering is different, 

 but the respective parts can be readily understood. In 

 this case a polarised relay and electric caU-bell are 

 employed instead of the sounder, a necessarj' addition in 

 long circuits. 



Concerning the actual performance of the carbon tele- 



^ The use of the Bell telephone as a. receiver in Edison's instrument is at 

 present the subject of legal proceedings ; Edison, however, claims to have 

 used the magneto-receiver before Bell invented it, and we leam that a letter 

 has recently arrived from Mr. Edison stating that he has now constructed a 

 still better and novel receiver for his telephone. Edison remarks : ' ' Batchelor, 

 one of my assistants, heard a whisper last night fifteen feet away from the 

 receiver, and ordinary conversation comes out as loud as originally spoken." 

 Further information about this receiver is given at the close of this article. 



^ In the last improvements the usual position of the piimary and secondary 

 coils has been reverted to; the resistance of the former for short circuits 

 should be about a third ct an ohm and of the latter somewhat OTer seventy 

 ohmv 



