358 NINETEENTH CENTURY. PT. in. 



letter, and so many to the left another letter, and in this 

 way a message can be spelt out, however far off the two men 

 may be. 



This is the whole secret of the electric telegraph ; but to 

 understand how it works you must follow the explanation of 

 the two diagrams (Figs. 60 and 61), very carefully. Suppose 

 that a message is going between London and York, four 

 things are wanted to convey it: i. A battery to produce an 

 electric current. 2. A wire to carry the current. 3. A gal- 

 vanometer, that is a box, A, A', holding a magnetic needle to 

 make the signs. 4. A little box called a commutator, B, B', 

 in which the position of the wires can be changed so as to 

 send the current first one way and then another. 



1. The battery is an ordinary chemical battery such as 

 has already been explained. 



2. The wire is stretched from station to station, resting 

 on little earthenware cups to prevent the electricity running 

 down the poles into the earth, and is arranged in a coil round 

 the magnetic needle at each station in such a way that when 

 the current flows from left to right the needle will turn to the 

 right, when it flows from right to left the needle will turn 

 to the left. You will observe that there is only one wire in 

 the diagram, although we know that no current will pass 

 unless there is a complete circuit from the battery, going 

 out at one pole and coming back to the other. At first 

 telegraphs were made with a second wire to return the 

 current, but Steinheil discovered that this is not needed, 

 for that, if the ends of the wires are sunk in the ground, 

 with plates of copper, f g, fastened to them, the earth 

 itself will act as the second wire, and carry back the re- 

 turn current to the battery. It is not known precisely how 

 the current returns ; it has been suggested that the earth is 



