DOVE ON THE ELECTRICITY OF INDUCTION. 143 



ducting cases, or in the form of piles of discs ; moreover, forged 

 iron, soft and hard steel, white and gray pig iron and nickel, 

 when electro-magnetized by the current of a galvanic battery, a 

 thermo-battery, a Saxton's macliine, by the approach oj a closed 

 wire to a magnet, and lastly, by the action of a piece of iron ap- 

 proaching a steel magnet upon a closed ivire which surrounds it, 

 produces electric currents in a wire which surrounds it, when 

 this magnetism becomes evanescent, 



b. The inducing action of the same mass of iron as a continu- 

 ous whole is in general very different from the action of the same 

 mass of iron when it is divided into wires : this difference how- 

 ever varies in kind, according to the mode by which the iron is 

 electro-magn eti zed . 



c. When the iron is magnetized by the connecting wire of a 

 galvanic battery, a thermo-battery, or by a magneto-electrical 

 current, in either of the three modifications distinguished above, 

 the galvanometric action of the current produced by the evanes- 

 cent magnetism on breaking the circuit, remains the same when 

 the iron is broken up into bundles of wires, as does also the pro- 

 perty of this current to magnetize soft iron ; whilst on the other 

 hand, its jjhysiological action, the sparks which it gives rise to on 

 being interrupted, and the magnetism produced by it in steel, 

 are much more powerful. If the bundle of wires is surrounded 

 by a conducting case, as an entire tube or a single coiled spiral 

 with connected ends, its action is that of a solid bar of iron. 

 If, on the contrary, the case is not entire, i. e. consists of a longi- 

 tudinally cut tube or of a simply coiled spiral with unconnected 

 ends, it acts nearly as powerfully as an unenclosed bundle. A 

 spiral composed of a doubled coiled wire surrounding the bun- 

 dle of wires, and having its ends connected, is as inactive as a 

 single coiled wire with unconnected ends. If the mass of iron is 

 divided by sections at right angles to its length into discs, the 

 current induced by this pile of discs is very much weaker in its 

 physiological action. 



d. The differences which have just been noticed between iron 

 rods and bundles of iron wires attain their maximum when they 

 are magnetized by the discharge-shock of a Leyden jar. A spiral 

 wire with a nucleus of iron, for instance, induces a more powerful 

 current in a secondary spiral surrounding it as regards the phy- 

 siological, magnetizing, galvanometric, heating and chemical ac- 



