1877.] 
Recent Advances in Telegraphy . 
369 
proved fatal to the early duplex systems, and although there 
was up till 1865 considerable ingenuity shown, nothing 
practical was done in double transmission. 
The recent revival of duplex, however, again revived the 
hope of double transmission and of quadruplex. In 1872 
Mr. Stearns made duplex a practical success by applying 
the condenser to the artificial line, and thereby overcoming 
the “ kicks;” and in 1874 Mr. T. Alva Edison, of Newark, 
New Jersey, and Mr. G. B. Prescott, while testing Stearns’s 
system, invented the first practical system of quadruplex. 
It was worked, in the same year, on the New York to Boston 
line, a distance of 240 miles. Since then it has been intro- 
duced on several long lines in the United States, including 
the line from New York to Chicago, which is nearly 
1000 miles in length. On this line there is a “ repeater ” 
at Buffalo. The speed of working is about 120 words a 
minute. 
The basis of Messrs. Prescott and Edison’s quadruplex 
system is the “ Wheatstone bridge ” duplex system. With 
this is united their system of double transmission, which 
resembles the early method of Bosscha, but differs from it 
in many important points. To understand their system it 
is only necessary to consider their plan of double trans- 
mission, since we know that the mere duplexing of this 
arrangement ensures quadruplex. The combination of cur- 
rents produced by the two keys are for— 
1. A positive current of strength 1 sent. 
2. A negative current of strength 3 or 4 sent. 
3. A positive current of strength 3 or 4 sent. 
4. A negative current of strength 1 sent. 
There is a practical advantage in this system over the 
earlier ones, in the greater disparity between the different 
currents. To interpret these currents a Siemens polarised 
relay and a neutral relay are used. The neutral relay will 
work by currents, either positive or negative, provided they 
are sufficiently strong. The currents sent by 2 and 3 work 
this relay. The polarised relay depends for its working only 
on the kind of the current, not on its strength. Thus a 
negative current keeps the armature or tongue of the relay 
against its back stop, thereby keeping the local circuit open, 
1 whereas a positive current causes the tongue to close the 
local circuit. This relay therefore works with 1 and 3 
currents. 
Fig. g represents the arrangement of the keys and bat- 
teries for sending, ab c is the “ Wheatstone bridge,” with 
VOL, vii. (n.s.) 2 c 
