CH. xxxvi. ELECTRIC TELEGRAPH. 383 



in June 1837 ; and during the same year Dr. Steinheil, of 

 Munich, and Professor Morse, of America, both invented 

 telegraphs of rather different kinds. I shall not attempt 

 ico describe all of these, but only to explain the simplest 

 principle of an electric telegraph as it is used in England, 

 and to show how it depends upon electricity and magnetism. 



You will see, if you turn back to Figs. 64 and 65, p. 359, 

 that when the electric current flowed round one way, a b c 

 d, Fig. 64, the north pole of the needle turned to the west ; 

 when it flowed round the other way, a b c d, Fig. 65, the 

 north pole turned to the east. Now the signals of the 

 electric telegraph depend upon this fact, that the direction 

 of the current alters the direction of the magnet. When 

 one man wants to send a message to another, he does it by 

 sending an electric current from a battery along a telegraph 

 wire, so that it passes a magnetic needle either from right 

 to left or from left to right. When it flows round one way, 

 the needle, even if it is a hundred miles off, turns to the 

 right, when it flows round the other way the needle turns 

 to the left ; and it is agreed that so many strokes to the 

 right mean one 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. 69 and 70) 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 

 galvanometer, 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. 



