April lo, 1884] 



NATURE 



555 



Why this is so will be more readily comprehended if it 

 be remembered that a telephone is sensitive to the changes 

 in the strength of the current if those changes occur with 

 a frequency of some hundreds or in some cases thousands 

 of times per second. On the other hand, currents vibrating 

 with such rapidity as this are utterly incompetent to affect 

 the moving parts of telegraphic instruments, which cannot 

 at the most be worked so as to give more than 200 to Soo 

 separate signals per viinute. 



The simplest arrangement for carrying out this method 

 is shown in Fig. i, which illustrates the arrangements at 

 one end of a line. M is the Morse key for sending 



Fig, 3 



messages, and is shown as in its position of rest for 

 receiving. The currents arriving from the line pass first 

 through a "graduating" electromagnet, Eo, of abDUt 500 

 ohms resistance, then through the key, thence through 

 the electromagnet R of the receiving Morse instrument, 

 and so to the earth. A condenser, c, of 2 microfarads 

 capacity is aho introduced between the key and earth. 

 There is a second "graduating" electromagnet, e,, of 

 500 ohms resistance introduced between the sending 

 battery b and the key. When the key M is depressed in 

 order to send a signal, the current from the battery must 

 charge the condenser C, and must magnetise the cores of 



the two electromagnets Ej and Ej, and is thereby retarded 

 in rising to its full strength. Consequently no sound is 

 heard in a telephone, T, inserted in the line-circuit. 

 Neither the currents which start from one end nor those 

 which start from the other will affect the telephones 

 inserted in the line. And, if these currents do not affect 

 telephones in the actual line, it is clear that they will not 

 affect telephones in neighbouring lines. Also the tele- 

 phones so inserted in the main line might be used for 

 speaking to one another, though the arrangement of the 

 telephones in the same actual line would be inconvenient. 

 Accordingly M. Van Rysselberghe his devised a further 

 modification in which a separate branch taken from the 

 telegraph line is made available for the telephone service. 

 To understand this matter one other fact must be ex- 

 plained. Telephonic conversation can be carried on 

 even though the actual metallic communication be severed 

 by the insertion of a condenser. Indeed, in quite the 

 early days of the Bell telephone, an operator in the 

 States used a condenser in the telegraph line to enable 

 him to talk through the wire. If a telephonic set at Tj 

 (Fig. 2) communicate through the line to a distant station, 

 T,, through a condenser, C, of a capacity of half a micro- 

 farad, conversation is still perfectly audible provided the 

 telephonic system is one that acts by induction cur- 

 rents. And since in this case the interposition of the 

 condenser prevents any continuous flow of current 

 through the line, no perceptible weakening will be felt 

 if a shunt, .S, of as high a resistance as 500 ohms and of 

 great electro-magnetic rigidity,that is to say, having a high 

 coefficient of self-induction, be placed across the circuit 

 from line to earth. In this, as well as in the other figures, 

 the telephones indicated are of the Bell pattern, and if 

 set up as shown in Fig. 2, without any battery, would be 

 used both as transmitter and receiver on Bell's original 

 plan. But as a matter of fact any ordinary telephone 

 might be used. In practice the Bell telephone is not 

 advantageous as a transmitter, and has been abandoned 

 except for receiving ; the Bl.ike, Ader, or some other modi- 

 fication of the microphone bsing used in conjunction with 

 a separate battery. To avoid complication in the draw- 

 ings, however, the simplest case is taken. And it must 

 be understood that instead of the single instrument shown 

 at Ti or T., a complete set of telephonic instruments in- 



cluding transmitter, battery, induction-coil, and receiver 

 or receivers, may be substituted. And if a shunt, .s, of 

 500 ohms placed across the circuit makes no difference to 

 the talking in the telephones because of the interposition 

 of the separating condenser C, it will readily be under- 

 stood that a telegraphic system properly "graduated," 

 and having also a resistance of 500 ohms, will not affect 

 the telephones if interposed in the place of s. This 

 arrangement is shown in Fig. 3, where the " graduated " 

 telegraph-set from Fig. i is intercalated into the tele- 

 phonic system of Fig. 2, so that both work simultaneously, 

 but independently, through a single line. The combined 



Fig. 4 



system at each end of the line will then consist of the 

 telephone-set T,, the telegraph instruments (comprising 

 battery B,, key Mj, and Morse receiver R,), the "graduat- 

 ing" electromagnets El and E,, the "graduating" con- 

 denser Cj, and the "separating" condenser C,. It was 

 found by actual experiments that the same arrangement 

 was good for lines varying from 28 to 200 miles in length. 

 A single wire between Brussels, Ghent, and Ostend is 

 now regularly employed for transmission by telegraph of 

 the ordinary messages and of the telemcteorographic 

 signals between the two observatories at those places, 

 and by telephone of verbal sinmltaneous correspondence 



