MAGNETISM. 



Fig. 14, illustrates the action of electric currents upon a magnetic needle. A A, 

 is a coil of copper wire, B, a magnetic needle freely poised, and pointing north and 

 south. If the wire, c, be connected with the copper pole, and the wire, D, with 

 the zinc pole, of an active voltaic battery, the needle will turn eastward, and upon 

 reversing the wires, the needle will turn westward. Upon this simple principle 

 depends the action of that astonishing apparatus the Electric Telegraph, of which 

 our limits will only allow a brief description. 



Fig. 15, represents a front view of the Telegraph instrument in which two 

 needles are employed. Upon the dial plate are arranged certain letters, figures, 

 and conventional signs. At the top of the instrument, within an ornamental 

 case, is placed a bell or alarum to call attention to the instrument when a com- 

 munication is about to be made. The alarum is put in action by a current of 

 electricity being passed through a coil of wire encircling a piece of soft iron, which 

 is thus converted into an electro-magnet, and attracts a lever, by which clock-work 

 is set in motion, and the hammer caused to strike the alarum. 



Fig. 16, represents the internal mechanism for moving each needle. The handle 

 shown on the front of the instrument is attached to and works the cylinder A. This 

 cylinder has its two ends capped with brass and insulated from each other by a 

 belt of wood B ; from the under part of one end projects a steel pin, c, and from 

 the upper part of the other end, a similar pin, z, these pins representing the copper 

 and zinc poles of the battery with which they are in connection. In giving a sig- 

 nal, the handle is turned either to the right or left according to the direction the 

 needle is required to take. Thus, by turning the cylinder so as to press the pin, z, 

 against the spring D, separating the latter from the point on the brass rod, E, the 

 pin, c, is brought into contact with the boss, F. The electric current now passes 

 from the pin, c, by the boss and metallic conductor through the wire coils, deflects 

 the needle,\ and passing from the terminal, H, to the line wire it similarly deflects 

 the needles of all the instruments in connection, and, being conducted at the extre- 

 mity of the line to a plate of metal buried in the ground, it is transmitted by the 

 earth to the terminal, G, and by the conductor and spring, D, to the pin z. By 

 reversing the movement of the cylinder the direction of the current is also reversed, 

 and the needles are deflected in the opposite direction. 



Magneto-Electricity. As magnetism is derived from electricity, so elcctri 

 city may be obtained from magnetism. If a piece of soft iron, A A, fig. 17, encir- 

 cled by coils of copper wire be brought into, or removed from contact with the 

 poles of a magnet, B B, electrical currents of a considerable magnitude are produced 

 in the Avire, and sparks will pass between the ends of the wire, p and N. To pro- 

 duce the effect, it is essential that the magnet be in motion, or that the conductor 

 be in motion across the magnet. 



