160 THE ELECTRIC TELEGRAPH. 



and control very necessary on some lines by an arrange- 

 ment of translation, by which, a message transmitted by the 

 employe from station A, for example, is not only received on 

 the relay and Morse at station B, but also retransmitted 

 by the Morse apparatus at B to station A$ where it can be 

 examined at once to be sure of its correctness. 



95. Methods of Transmitting Two Messages along a Single 

 Line in the Same Direction at the Same Time. The first 

 success attained in this direction was by Stark of Vienna in 

 1855. His method consists of sending from the transmitting 

 station, by two keys, two currents of different intensities, 

 which, on arriving at the receiving station, each set a relay 

 in motion. 



The relays are arranged in such a way that when the 

 weaker currents traverse the line, only one of the relays is 

 put in motion ; when the stronger current traverses the line 

 the other relay is affected ; and lastly, when both currents go 

 together, both the relays respond to them. 



At the sending station Stark arranged two keys, as in the 

 plan Fig. 88 ; K being a simple Morse key, and K' a similar 

 lever, supplied at the back with an insulated earth contact, 

 which it moves against the two anvils 5 and 6. The usual 

 front and back contacts of the keys are marked in the figure 

 1 and 3 respectively, and the levers 2. The battery, which 

 is connected up in series, or one element after the other, is 

 used in two unequal parts, a and I, the number of elements 

 represented by b being double that of a. The battery a is 

 put into circuit with the line by pressing down the key K ; 

 b y by the key K' ; and both together by depressing both the 

 keys at the same time. 



The copper-pole of a is, therefore, connected to the contact 

 1 of K, the zinc-pole of same to 5 of K'. Copper-pole of b 

 is connected with 1, and zinc-pole with 6 of K. Lever of 

 K' is in connection with the back contact 3 of K ; line is 

 brought to the lever 2 of K, and the back contact of K' goes 

 to relay, &c. 



When K alone is depressed, the currents of a pass from z 

 (5 and 4 of K') to earth, and from c (1 and 2 of K) to line. 



