NEEDLE INSTRUMENTS. 235, 



the electricity will pass beneath the surface of the earth from Liverpool to 

 London, and through the wire from London to Liverpool, thus completing 

 the circuit. The end from which the electricity passes is called the 

 " positive electrode," that to which it returns, the " negative electrode." 

 Fig. 245 will show this arrangement. 



If a bar-magnet be suspended on a pivot so that it may turn freely, it 



Fig. 243. The Needle Telegraph. 



will (as is well known) turn with one end to the north, which is owing to a 

 current of natural electricity passing round the earth in the direction of east 

 and west, the magnet crossing the current at a right angle ; and if a coil of 

 wire coated with silk (to keep one part of the coil from another) be placed 

 round, above, and below the long axis of a bar of steel, as shown at fig. 246, 

 and a current of electricity passed through the wire, the steel becomes a 

 magnet, and will take a direction similar to the natural magnet, more or less,. 



Fig. 244. The Battery. 



at right angles to this coil, as in fig. 247, according to the intensity of the 

 current ; and the instant this electrical current is stopped, it will resume its 

 former direction. This fact has been made use of to form the principal 

 feature of all English telegraphs ; such a needle is mounted in an upright 

 position, and instead of its tendency to turn to the north, a tendency to 

 maintain the upright position is given to it by having one of the arms of 

 the magnet a little heavier than the other ; such a magnet having a coil of 



