LESSONS IN ELECTRICITY. 



341 



pole in the opposite direction, the needle must 

 necessarily set itself as a tanffenttv the curve. 

 I \vill not seek to simplify this subject fur- 

 ther. If there be anything obscure or con- 

 fused or incomplete in my statement, you 

 ought now, by patient thought, to be able 

 to clear away the obscurity, to reduce the 

 confusion to order, and to supply what is 

 needed to render the explanation complete. 

 Do not quit the subject until you thoroughly 

 understand it ; and if you are able to look 

 with your mind's eye at the play of forces 

 around a magnet," and see distinctly the 

 operation of those forces in the production of 

 the magnetic curves, the time which we have 

 spent together has not been spent in vain. 



In this thorough manner we must master 

 our materials, reason upon them, and, by de- 

 termined study, attain to clearness of concep- 

 tion. Facts thus dealt with exercise an ex- 

 pansive force upon the boundaries of thought; 

 they widen the mind to generalization. We 

 soon recognize a brotherhood between the 

 larger phenomena cf Nature and the minute 

 effects which we have observed in our private 

 chambers. Why, we inquire, does the mag- 

 netic needle set north and south ? Evidently 

 it is compelled to do sj by tho earth ; the 

 great globe which we inherit is itself a mag- 

 net. Let us learn a lit tie more about it. By 

 means of a bit of wax or otherwise, attach 

 your silk fibre to your magnetic needle by a 

 single point at its middle, the needle will thus 

 be uuinterfered with by the paper loop, and 

 will enjoy to some extent a power of dipping 

 its point or its eye below the horizon. Lay 

 your magnet on a table, and hold the needle 

 over the equator of the magnet. The needle 

 sets horizontal. Move it toward the north 

 end of the magnet ; the south end of the 

 needle diijs, the dip augmenting as you ap- 

 proach the nortli pole, over which the needle 

 tf free to move, will set itself exactly vertical. 

 Move it back to the centre, it resumes its 

 konzontality ; pass it on toward the south 

 pole, its north end now dips, and directly 

 over the south pole the needle becomes ver- 

 tical, its north end being now turned down- 

 ward. Thus we learn that on the one side 

 of the magnetic equator the north end of the 

 needle dips ; on the other side the south end 

 dips, the dip varying from nothing to ninety 

 degrees. If we go to the equatorial regions 

 of the earth with a suitably suspended needle, 

 AVC shall find there the position of the needle 

 horizontal. If we sail north, one end of the 

 needle dips; if we sail south, the opposite 

 end dips ; and over the north or south terres- 

 trial magnetic pole the needle sets vertical. 

 The south magnetic pole has not yet been 

 found,, but Sir James Ross discovered the 

 north magnetic pole on the 1st of June, 1881. 

 In this manner we establish a complete par- 

 allelism between the action of the earth and, 

 that of an ordinary magnet. 



The terrestrial magnetic poles do not coin- 

 cide with the geographical ones ; nor does tho 

 earth's magnetic equator quite coincide with 

 the geographical emiator. The direction of 

 the magnetic neceiltt in London, which is 



called the magnetic meridian, incloses an an- 

 gle of 24 degrees with the 1n:e astronomical 

 meridian, this angle being called the declina- 

 tion of the needle f<T London. The north 

 pole of the needle now lirs to the west of the 

 true meridian ; the declination is westerly. 

 In the year 1GGO, however, the declination 

 was nothing, while before that lirnu it was 

 casteily. All this proves that the earth's 

 magnetic constituents are gradually changing 

 iheir distribution. This change is very 

 slow: it is technically called \l\esecularchange, 

 and the observation of it has notj'et extended 

 over a sufficient period of time to enable us 

 to trucks, even approximately, at its laws. 



Having thus discovered, to some extent, 

 the secret of the earth's power, we can turn 

 it to account. 1 hold in my hand a poker 

 formed of good soft iron ; it is now in the line 

 of dip, a tangent, in fact, to the earth's line of 

 magnetic force. The earth, acting as a mag- 

 net, is at this moment constraining the two 

 fluids of the poker to separate, making th 

 lower end of the poker a north pole, and th 

 upper end a south pole. Matk the experi- 

 ment : I hold the knob uppermost, and it at- 

 tracts the north end of a magnetic needle. 1 

 now reverse the poker, bringing its knob un- 

 dermost ; the knob is now a north pole and 

 attracts the south end of a magnetic needle. 

 Get such a poker and carefully repeat this 

 experiment ; satisfy yourselves that the fluids 

 Biiift their position according to the manner 

 in which the poker is presented to the caith. 

 It has already been stated that the softest 

 iron possesses a certain amount of coerciyo 

 force. The earth, at this moment, finds in 

 this force an antagonist which opposes the 

 full decomposition of the neutral fluid. The 

 component fluids may be figured as meeting 

 an amount of friction, or possessing an 

 amount of adhesion, which prevents them 

 from gliding over the atoms of the poker. 

 Can we assist the earth in this case ? If w 

 wish to remove the residue of a powder from 

 the interior surface of a glass to which the 

 powder clings, we invert the glass, tap it, 

 loosen the hold of the powder, and thus en- 

 able the force of gravity to pull it down. 80 

 also by tapping the end of the poker we loosen 

 the adhesion of the fluid to the atoms and en- 

 able the earth to pull them apart. But what 

 is the consequence ? The portion of fluid 

 which has been thus forcibly dragged over 

 the atoms refuses to return when the poker 

 has been removed from the line of dip ; tho 

 iron, as you see, has become a permanent 

 magnet. By reversing its position and tap- 

 ping it again we reverse its magnetism. A 

 thoughtful and competent teacher will well 

 know how to place these rcmarkabl' 1 facts 

 before his pupils in a manner which will ex- 

 cite their interest ; he will know, and if not, 

 will try to learn, how, by the use of sensiblo 

 images, more or less gross, to give those he 

 teaches definite conceptions, purifying these 

 conceptions more and more as the minds of 

 his pupils become more capable of abstraction. 

 He will cause his logic to run like a line of 

 light through these images, and by thus act- 



