GALVANISM, OR VOLTAIC ELECTRICITY. 



27 



Fig. 14. 



fact had been long known that electricity was capable of inducing 

 and destroying magnetism, as witnessed in the effects of lightning on 

 the compass-needle, it was not until the year 1819 that the laws of 

 these phenomena were established by CErsted, and the science of 

 electro-magnetism truly developed. It is found that if a galvanic 

 current be set in motion near a magnetic needle, the latter will 

 arrange itself across the current, so that its axis may be perpen- 

 dicular to the wire. 



When an electric current is passed at right angles to a piece of 

 iron or steel, the latter acquires magnetic polarity, 

 either temporary or permanent ; the direction of 

 the current determining the position of the poles. 

 This effect is very much increased by causing the 

 current to circulate a number of times around the 

 bar, which soon acquires extraordinary magnetic 

 power. A piece of soft iron in the form of a 

 horseshoe, surrounded thus by a coil of copper 

 wire, insulated by being covered with silk, may 

 be made to become so highly magnetic simply by 

 connecting the two ends of the iron with a small 

 battery of a single pair of plates, as to be capable 

 of sustaining a very heavy weight. 



As electricity can produce a magnetic influence, 

 in the same manner it is found that magnetism can 

 call into activity electric currents. \{ the two ex- 

 tremities of the coil of the electro-magnet just de- 

 scribed, be connected with a galvanoscope, and the iron magnetized by 

 the application of a steel horseshoe magnet to the ends of the bar, a 

 momentary current will be developed in the wire, and pointed out 

 by the movement of the needle. On removing the magnet, whereby 

 the polarity of the iron is at once destroyed, 

 a second current will become apparent, but 

 in the opposite direction to that of the first. 

 By using a very powerful steel magnet, 

 surrounding its iron keeper or armature 

 with a very long coil of wire, and then 

 making the armature itself rotate in front 

 of the faces of the magnet, so that its in- 

 duced polarity shall be rapidly reversed, 

 magneto-electric currents may be produced 

 of such intensity as to give bright sparks, 



and powerful shocks, and exhibit all the phenomena of voltaic elec- 

 tricity. The accompanying figure represents such an arrangement. 

 There is a great variety of form in electro-magnetic machinery ; but 

 in all, even the most complicated, the essential jninciple is the same, 

 viz. : the development of an electrical current by magnetic action. 



Fig. 15. 



