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



the same time I influence Brighton's needle. This gives us the first 

 principle of duplex telegraphy. Of course, with a similar arrangement 

 at Brighton, on the key at that end being depressed the needle there 

 remains unaffected, while at the same time the needle at this end is 

 deflected. When, however, under these conditions, currents are sent 

 from both instruments, signals or deflections are noticed at each station. 

 By using resistance we are able to make an artificial line attached 

 to the second wire of the electro-magnet exactly equal to the line wire 

 attached to the other wire of the electro-magnet. I have in a box here 

 resistance equal to three or four hundred miles of line. Resistance is 

 measured in ohms, an ohm being a unit used by electricians, and which 

 rei^resents about a yard of fine platinum wii'e, or about one-tenth of a 

 mile of ordinary iron wire. The resistance contained in this box is 

 divided into quantities ranging from one to two thousand ohms, any 

 portion of which can be brought into use by simply taking out pegs 

 representing the amount required. With this ready means at hand 

 you will easily comprehend the facility with which a balance can be 

 adjusted, and the opposition to the two halves of the current passing 

 through the electro-magnet be made equal. Whatever the distance of 

 the line, whether between here and the Central Telegraph Station, or 

 between here and Calcutta, or any distant place to which you choose 

 to exert your imagination, the effect is just the same. To show you 

 this system in actual working I have here instruments joined to a wire 

 which passes through the streets to the Central Telegraph Station in 

 the city. It is just as easy for us to connect a wire up in actual opera- 

 tion as to have assistants secreted in the adjoining room, which is some- 

 times supposed to be the case. I must first of all explain to you how 

 telegraphic signals are interpreted into ordinary language. It is by 

 means of what is known as the Morse alphabet, which consists of 

 combinations of dots and dashes used to represent the ordinary letters. 

 A dot and a dash, for instance, represents a, a dash and three dots 

 represents h, dash dot dash dot c, and so on throughout the alphabet. 

 A dot itself is rei^resented on the instrument before us by a short 

 sound or beat, and a dash by a longer one. The sounds caused by the 

 signals appeal to the consciousness through the ear, and are translated 

 into the proper language. (The central station was called up, and a 

 message was sent to him at the same time that one was being received 

 from him, in illustration of duplex working.) 



The operation which you have just seen going on exactly corre- 

 sponds with that which I just described to you. The division of the 

 current is arranged at each end by the use of a resistance box or 

 " rheostat," and the signals are sent without any interruption between 

 the one and the other. 



The duplex system of telegraphy is employed to a very large 

 extent in this country, no less than 320 circuits being so fitted. The 

 system is aj^plicable to long and short lines, and even long cables in 

 the Red Sea, Indian Ocean, and across the Atlantic (in one case 2400 

 miles long), have boon successfully so fitted. 



