384 



NINETEENTH CENTURY. 



PT. III. 



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 



FIG. 69. 



FIG. 70. 



Diagrams showing the general principle of the Electric Telegraph. 

 A, A', Galvanometer, or box containing the magnetic needle. B, B', Commutator, or 

 box in which the telegraph wire an -I earth wire are joined to each other as in B', 

 or to the battery, as in B. c, d, Telegraph wire, e. Earth wire, f, g, Copper 

 plates at the end of the earth wire. The arrows show the direction of the positive 

 current. 



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 



