1879.] on Multiple Telegraphy. 195 



telegraphy, in a way which I will now show to you. It is nsnally 

 imagined by those who think of dni)lex telegraphy that sonietliing 

 crosses or passes in the same way that railway trains pass or cross 

 each other. It is not at all necessary to conceive that anything 

 whatever passes. For instance, here I have some glass balls placed 

 in a row between two parallel wooden rods. If I take the ball at one 

 end, say No. 1, a sliort distance from its normal position, and drive it 

 along the rods back again, you will notice that a something, a form of 

 force, passes through the other balls, and the one at the far end is by 

 this developed motion forced a distance away from its neighbours. 

 The same thing occurs if 1 take No. 12, as No. 1 is then disj^laced by 

 the concussion. This resultant motion represents, and may be called a 

 telegraphic signal. If I am skilful enough to take No. 1 and No. 12, 

 and let them return together, we shall see that they are both driven 

 back simultaneously, the intervening balls remain undisturbed, and 

 two such signals will be made. It is this latter phenomenon that 

 I wish you to bear in mind as being analogous to the principle used 

 in duplex telegraj^hy. 



The next fact that I wish to impress upon you is that if a current 

 of electricity have many 2)aths open to it, it will always separate or 

 spread itself among those paths in inverse proportion to the resistance 

 which each opposes to its progress. The greater the resistance, the 

 smaller the current ; the less the resistance, the greater the current. 



If we have two paths or lines open to a current, and these two lines 

 be of exactly equal resistance, the current will divide itself between 

 them in exactly equal proportions. 



The next point, almost as self-evident as the previous one, but 

 which time will not allow me to illustrate by experiment, is that the 

 magnitude of the magnetism produced in the electro-magnet is simply 

 proportional to the strength of current passed through that magnet. 



The last elementary fact I have to bring before you is that the 

 polarity or direction of the magnetism simply depends upon the direc- 

 tion of the current. 



On the wall I have two electro-magnets connected with each 

 other by means of a wire. Around each ai-e coiled two wires of 

 exactly equal length and equal resistance. If I send a current 

 through one of the wires I produce a deflection in one direction; 

 if I send a current through the other wire I get a deflection in the 

 opposite direction. You can imagine that one of these magnets is in 

 Brighton and the other in London, with the connecting line wire 

 between them. I want to arrange matters so that when I make a 

 signal at Brighton I shall not in any way affect my own instrument, 

 and to do so I divide my current in halves ; one half goes through 

 one wire of the magnet, and tends to deflect the needle in one direc- 

 tion, and the other half goes through the other wire of the magnet, 

 with a tendency to throw the needle in the opposite direction. If by 

 an arrangement, such as an artificial line, I make these two halves 

 exactly equal, then no deflection of my needle takes place ; though at 



