CH. xxxvi. ELECTRIC TELEGRAPH. 385 



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, /-, fastened to them, the earth itself 

 will act as the second wire and carry back the return 

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

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

 great reservoir, as it were, of electricity, so that when the 

 current runs into it at one place an equal amount must run 

 out at another; but all that is really known is that the 

 whole globe acts practically as a return wire. 



3. The magnetic needle is made of two or more parts, for 

 since it would be very inconvenient if the pointer were 

 always trying to turn to the north, this is avoided by fasten- 

 ing two needles side by side, with the north pole of the one 

 lying against the south pole of the other, and thus, as the 

 earth attracts each needle in a different way, the pull is 

 neutralised. This double needle is called an astatic needle, 

 and in the form of telegraph we are describing, it is placed 

 in the box A, and surrounded by the wire, while a light 

 strip of whalebone outside the box is so fastened as to show 

 on the face of the dial how the needle is pointing inside. 



4. The commutator, B, is a box with an apparatus inside 

 which is so arranged that by turning a handle (not shown 



'in the diagram) different ways, the earth wire and telegraph 

 wire can be joined together, or either of them can be joined 

 to one of the poles of the battery. 



The commutator and galvanometer are really made in 

 one instrument, but I have drawn them separate to make 

 it more clear. 



Now, when the man in London wants to send his mes- 

 sage to York, he first sends off a current which rings a little 

 bell at all the stations along the line to call attention, and 



