198 Mr. W. H. Preece [May 23, 



strength may be, and the other relay, which is connected to the light 

 sounder, responds to the increased strength of the current whatever its 

 direction may be ; so that on depressing the key which reverses the 

 current the deep-toned sounder responds ; and if I touch the other 

 key I simply make the light sounder speak by increasing the strength 

 of that current. (Mr. Preece here very minutely traced the different 

 currents from the battery through key and relay to sounder, so as to 

 produce sounds by currents, varying in either direction or strength, 

 and acting upon one or both sounders at the same time, or separately 

 at will.) The increase of current is brought into action by means of 

 the one key having attached to it a greater proportion of cells of the 

 battery. 



That is diplex telegraphy. 



How is this diplex used for quadruplex working ? I showed you 

 in duplex working that we simply split the current in two paths made 

 exactly equal to each other. It matters not whether we have one or 

 any number of relays in. Theoretically, it is possible to insert a great 

 number of relays, but practically only two are used ; and in quadruplex 

 working we simply duj^lex what I have called the diplex arrangement. 

 On one end of the table before you we have two sides of the quadruplex, 

 or one diplex ; and on the other end of the table we have the other 

 side of the quadruplex which also works diplex, and by this apparatus 

 we are able to send four messages at the same time on the single wire, 

 which you see goes from the table to the wall, and so proceeds to the 

 Central Telegraph Station. (The various courses of the currents and 

 adjustments of balance necessary, chiefly owing to variations of 

 temperature, &c., were here described, but without the aid of a 

 diagram it would be next to impossible to follow them.) The system, 

 although difficult in description, is really wonderfully and beautifully 

 simple, and if it were only requisite to follow out what I have just 

 said, quadruplex working would be a very easy matter indeed. But 

 there is such a thing in England as a climate, and there also is 

 unfortunately such a thing as rain, and rain interferes very consider- 

 ably with the action of our duplex and quadruplex working. In a 

 word, the effect of rain is precisely the same as reducing the length 

 of our line. Supposing we have a wire between Liverpool and 

 London, about 200 miles long, then when we have rain it covers the 

 insulators with moisture, and the moistened insulators allow the 

 current to escape to earth, and the result is just the same as though 

 the line itself were reduced in length ; and to compensate for this it 

 is necessary to take out resistance equal to the loss. But by means 

 of the rheostat it is not a very difficult matter to adjust a balance, and 

 by carefully noting and watching storms of rain, snow, sleet, or fogs 

 and mist, which are so troublesome to the working, a clerk has simply 

 to vary his rheostat, and can so maintain working on wires of ordinary 

 length. 



When we come to very long lengths of wire the weather disturb- 

 ance interferes very much. 



