148 



ELECTRICITY. 



Lesson VIII. 



P. If we present one pole of a bar- 

 mugni t held in the hand to 

 the pole a (Fig 26,) it will 

 be repelled, while h will be 

 attracted. If we reverse 

 the magnet, and present 

 the other pole to a, it will 

 Fig. L'6. be attracted and b repelled. 



96. T. What is the reason of the 

 phenomenon you have described ? 



P. Because it is a law in magnetism 

 that similar poles repel each other, and 

 contrary poles attract each other. The 

 two poles of the magnet in the hand are 

 opposite poles, because they are of a 

 different nature. 



97. T. Is it possible to impart tern- 

 porary magnetic influence to iron? 



P. Yes. [Exp. 14]. Here is a bar 

 magnet (a b). and 

 a small bar of iron 

 Cc). You observe 

 when I dip the bar 

 \d of iron into some 

 iron filings that no 

 magnetic effect takes 

 place; but when the 

 bar of iron is pre- 



. 27. 



sented to the magnet, then it is attracted 

 and supported by it- I will now dip the 

 bar of iron into the iron filings, and ob- 

 serve what takes place. [Does so, and 

 the iron filings are attracted in a tuft, as 

 in Fig. 27.] This clearly demonstrates 

 that under the influence of a magnet, iron 

 itself becomes magnetic. I will remove the 

 iron bar, and you will then see the iron 

 tilings fall off, because no further attrac- 

 tive force remains. [Does so, and the 

 iron filings drop off,] [Experiment 15.] 

 Instead of using iron filings I will em- 

 ploy small cylinders, attaching four of 

 them to the first (as d Fig. 27), and you 

 will then find that a chain is formed, of 

 which the magnet is the first link, but 

 that as soon as the magnet is removed, the 

 chain will fall apart, because there is no 

 magnetic power to hold the links toge- 

 ther. [Performs the Experiment]. 



98. T If the pole of a bar magnet 

 supports a piece of iron which is in danger 

 of lalling from its weight, what will he 

 the effect of bringing the contrary pole 

 of another magnet immediately over it? 



P. The iron would f ,11 off, because, 

 the second magnet disturbs the actions of 



the first, and decomposes the magnetic 

 influence in the iron. [Experiment 16.] 

 Here is a b.tr-magnet (a b, 

 Fiy. 28), with a piece of 

 iron (e) attached; if I 

 bring another bar - magnet 

 near to it, so that the 

 opposite pole b' is placed 

 over the pole a, the piece 

 of iron (e} will fall off. 

 [Does so, and it falls as 

 Fig. 28. represented in Fig. 28.] 



99. T 7 . Can }ou render any other 

 metal magnetic besides iron ? 



P. Yes, nickel and cobalt may also be- 

 come magnetic. 



100. T. What do you mean by the 

 I armature of a magnet? 



P. It is generally a piece of soft iron 

 brought into contact with the magnet, in 

 order to preserve its power by means of 

 the magnetic decomposition going on in 

 the soft iron. 



101. T. What is meant by the direc- 

 tion of magnets? 



P. It has been observed, that when 

 magnets are suspended horizontally, or 

 magnetic needles revolve upon a fixed 

 point in an horizontal plane, that they 

 alwavs point one particular end to a parti, 

 cular spot or point in the horizon. If 

 you turn this point away from this point, 

 back it comes again ; consequently there 

 must be a magnetic force which acts at 

 all points of the earth's surface ; and we 

 find that this force distinguishes between 

 the two poles, attracting one and repelling 

 the other like a magnet. From observing 

 this, we have been enabled to name the two 

 poles of the magnet ; thus one is called 

 the north pole, and the other the south 

 pole. 



102. T. Then the north pole of the 

 magnet points to the north, and the south 

 pole to the south ? 



P. Certainly not. The north pole of 

 the magnet points to the terrestial mag- 

 netic south pole, and the south pole to the 

 magnetic north pole. 



103. T. What do you mean by the 

 declination of a magnetic needle? 



P. It is the deviation of the needle from 



the astronomical meridian. Thus it may 



decliiirtte easr or west, which is the same 



! as saying that it turns towards one or the 



other side of the astronomical meridian. 



