CH. XXXV. 



THE ELECTRIC TELEGRAPH. 



359 



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 fastening two 

 needles side by side, with the north pole of the one lying 



Fig. 60. 



Fig. 61. 



IJIIWI / ^ > > 



Diagrams showing the general principle of the Electric Telegraph. 



a'. Galvanometer, or box containing the magnetic needle. • B, b', Commutator, or 

 box in which the telegraph wire and 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 



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

 attracts each needle in a different way, the pull is neutralized. 

 This double needle is called an astatic needle^ and it is so 

 placed in the box a in the form of telegraph v/e are de- 

 scribing, that one needle is inside surrounded by the wire, 

 while the other is outside on the face of the box. 



