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DUPLEX TELEGRAPHY. 



of that province at the time of his demise; 

 Eight Rev. John Farrell, Catholic Biship of 

 Hamilton, Ontario ; Hon. Henry Black, Judge 

 of the Admiralty Court of Quebec, and an able 

 jurist ; Hon. J. W. Johnston, Judge of Nova 

 Scotia ; Lady Hincks, wife of Sir Francis 

 Hincks ; and Judge Armstrong, of Ottawa. 



DUPLEX TELEGRAPHY. The progress of 

 the inventions of the method for passing coun- 

 ter-messages simultaneously over a telegraph- 

 wire, which has been brought to perfection 

 and practical availability by Mr. Stearns, an 

 American, is described as follows by the Lon- 

 don Engineer : 



The aim of the duplex system is to send messages 

 in contrary directions at the same moment along the 

 same line without causing any interruption to the 

 transmission in either direction. Of course, it is well 

 known that two currents cannot pass each other on 

 a wire ; but, by the various means which will be de- 

 scribed below,the effects on the sending and receiving 

 instruments at either end of the line are exactly the 

 same as though the currents really passed one an- 

 other. It was in 1853 that the first practical applica- 

 tion of the duplex system was made by Dr. Gentl, a 

 director in an Austrian telegraph company. By 

 using a second battery, which he called his equating 

 battery, he was enabled to send dispatches in con- 

 trary directions at the same time. He employed a 

 relay, the coils of which were wound with two sepa- 

 rate wires in contrary directions, one wire being" in 

 circuit with the line battery and the other with the 

 equating battery, so that when the key was depressed 

 and the whole current of the line battery passed 

 through the relay it remained perfectly unaffected, 

 since the magnetic effect on either coil was equal and 

 opposite. This is of course true for both stations, so 

 that either can send to the other without the relay 

 of the sending station being in any way affected : 

 and thus the first great difficulty m duplex teleg- 

 raphy is overcome. The key that Dr. Gentl used 

 was a double-current key with six contacts. In the 

 circuit of the outer coils of the relay are inserted the 

 equating battery and the front and middle contacts 

 of the right side of the key. The inner coils are 

 connected with the middle contact of the left side 

 of the key and the line wire, the front contact on the 

 same side being connected with line battery, and the 

 back contact being to earth. Thus, the circuit of the 

 line battery is through the two back contacts of the 

 left side of the key, the interior coils, and the line 

 wire to the farther station, where it goes through the 

 interior coils and the two front contacts of the left side 

 of the key to earth, and by earth back to the pole 

 of the line battery. The current from the equating 

 battery at the same time passes through the two back 

 contacts of the right side of the key, the outer coils 

 of the relay, and back to the pole of the same bat- 

 tery. It will be seen that the only use of the equat- 

 ing battery is to neutralize the effect of the line bat- 

 tery on the relay. If now both stations send cur- 

 rents at the same instant, one current will go as above 

 described through the key and inner coils to the 

 farther station's relay, which records the signal, as 

 the equilibrium maintained by the second oattery 

 has been destroyed, and the other current will go by 

 earth to the farfher's station's relay, which will also 

 be affected. In short, if A's line current destroys 

 B's, B's equating current registers the signals, and 

 vice versa. 



This arrangement, which seems so perfect in the- 

 ory, was found sadly deficient in practice ; as it was 

 found quite impossible to keep the currents of .the 

 line and equating batteries equal ; for the local cir- 

 cuit being so much shorter than the line circuit, and 

 the outer coils of the relay being always thicker and 



shorter than the inner ones, the tension of the line 

 batteries remained almost constant, while that of 

 the equating ones fell rapidly. Another fault of this 

 arrangement was that, when contact is broken at the 

 middle contacts of the key and made at the back 

 ones, the circuit is for a moment interrupted at the 

 front contact, and thus some signals are lost. Dr. 

 Gentl afterward applied his principle to a chemical 

 telegraph with somewhat better results. 



The next system, which was invented by Messrs. 

 Frischen and Halse in the following year, certainly 

 did away with the above faults, as no equating bat- 

 tery nor double key was used. Their method was 

 what may be called a '' differential" one. The ar- 

 rangement was very much the same as that of an 

 ordinary Morse circuit. The relay is constructed in 

 the same way as in the former arrangement ; it is 

 connected with the lever of an ordinary Morse key, 

 one of the coils leading to the line and the other 

 through a resistance to earth. This resistance is 

 made equal to the resistance of the other portion 

 of the circuit viz., the other coils, the line, and 

 one branch of the relay at the distant station so 

 that when the key is depressed the current has two 

 paths open to it, each possessing equal resistance. 

 Consequently, by the law that " if two circuits are 

 open to a current it will divide itself between them 

 in the inverse ratio of their resistance," half of itwill 

 flow through one set of coils, and the resistance, to 

 earth ; neutralizing the effect on the home relay of 

 the other half of the current, which will flow through 

 the other coils and line to the farther station. Thus, 

 no signals will be given on the home relay. At the 

 farther station, after producing the signal, the cur- 

 rent will pass to earth, either through the earth con- 

 tact of the key, or else, if the key is depressed, 

 through the other branch of the relay and the resist- 

 ance above mentioned. If, now, both stations be 

 sending to one another at the same time, one current 

 goes by line to the distant station, and the other cur- 

 rent goes by earth. 



But, perfect as this system seemed, it did not an- 

 swer, owing to our defective knowledge of electro- 

 magnetic induction and bad insulation, and nothing 

 was heard of it for many years. Its revival is chiefly 

 due to Mr. Stearns, an American, and it is now much 

 more popular in that country than the old system. 

 To such perfection has it been brought that on one 

 of the most successful lines that between New York 

 and Chicago it is stated that as many as 1,600 mes- 

 sages have been got off in ten hours. On this line 

 there is a repeater half-way. 



Mr. Stearns's system is also based on the differ- 

 ential arrangement, though slightly different from 

 the last-mentioned plan, as it is based on the differ- 

 ence of potentials and not on the difference of cur- 

 rents. The relays are wound with two coils so ar- 

 ranged that, equal currents beingpassed through both, 

 equal magnetization will be produced ; but if those 

 currents be passed through in opposite directions the 

 relays will remain unaffected. By means of the keys 

 ordinary Morse the line is always kept in contact 

 with the earth or battery, except when the key is 

 first depressed, when the battery is for a moment 

 short-circuited. But contact is immediately after 

 broken, which allows the current to flow through 

 two circuits one through the line and the other 

 through the inserted resistance to earth. A con- 

 denser is inserted in the shortest circuit, of such 

 capacity, that the condenser and the resistance coils 

 are equivalent to the whole line. If, now, one circuit 

 be sending, and not the other, a current is sent 

 through both coils of the home relay in opposite 

 directions, so that the instrument remains unaffected. 

 Thence it passes through the line round the inner 

 coils of the distant relay, which causes the instru- 

 ment to record the signals, and to earth by the earth 

 contact of the key. If both stations be sending at 

 the same time, the line currents neutralize each 

 other ; but a current is sent round the outer coils of 



