July lij, 1884.] 



KNOWLEDGE 



69 



between every two poles there is a region of neutrality — 

 neutral, not in consequence of the absence of magnetism, 

 but because the two poles are opposite and equal. We are 

 furthermore accustomed to say, and that truly, that the 

 two polarities in any magnet are equal, and for this, if for 

 no other reason, neiiher of the north poliuitics (Fig. 3) can 

 equal the south polarity. Were we able to eliminate every 

 modifying influence, we should tind that the sitigle south 

 pole equalled in strength the sum of the two norths. 



These effects may be easily and efl'ectively obtained by 

 coiling a little wire around a knitting-needle and sending a 

 current through it. The needle being steel, it will retain 

 the magnetism induced, and by placing its various parts in 

 in the neighbourhood of one or other of the poles of a sus- 

 pended or floating magnet, experimental confirmation of its 

 assumed magnetic condition may be afforded. 



Supposing, in the next place, that our battery-power is 

 limited, and that the effect jiroduced by sending the current 

 through a single helix (such as Figs. 1 or 2) is incapable of 

 performing a certain task, the question arises : How are we 

 to increase the magnetic effect exerted upon the iron with- 

 out submitting to a corresponding increase in the battery- 

 strength ? This may be attained by increasing the coil of 

 ■wire, or, in other words, by increasing the number of turns 

 or loops. Presuming the coil to be closely wound, it is 

 only possible to increase the number of turns in a single 

 helix by using a thinner wire. It must, however, be borne 

 in mind that by so doing we shall materially raise the re- 

 sistance in circuit, and consei[uently cause a proportional 

 diminution in the strength of the current traversing it 

 Thus, supposing that we substitute a wire half the 

 diameter of that previously used, we should be able to 

 envelope the core with, approximately, twice the number 

 of rings or loops. This means twice the length of wire ; 

 and the resistance offered by a piece of wire varies directly 

 as its length — or, more simply, twenty yards of wire offer 

 twice the resistance that would be presented by only 

 ten yards. But this is not all, for resistance also 

 varies inversely as the cross section, which is the 

 same as saying that it varies inversely as the 

 square of the diameter. Now our thinner wire has 

 only half the diameter of the other, or their diameters are 

 as 1:2, the squ.ires of whicli are 1 and 4. Thus, for equal 

 lengths of the thin and thick wires the respective resist- 

 ances will be 4 and 1. Bvit it was said that the iron core 

 ■would carry twice the length of thin wire, so that the 

 resistance of such a helix would be eight times as great as 

 the one composed of the thicker wire. The effect upon the 

 current-strength is easily seen. Suppose we have two cells 

 giving an electro-motive force of two volts, and having an 

 internal resistance of 1 ohm per cell. Then, with the 

 stout wire helix having a resistance of, say, 1 ohm, we get, 

 according to Ohm's law (which declares that the current 

 produced varies directly as the electro-motive force (E), 

 and inversely as the total resistance, including that of the 

 battery (r) and of the external circuit (E)), 



^=-l-:=l-3. 

 R-t-r l-f2 



And with the thin wire helix (having eight times the 



resistance of the other, and therefore offering 8 Ohms) 



■we get : — 



_^ = ^ = -4. 



R-f-r 8-f2 



The bearing of these figures in demonstrating that in the 

 second case we get less magnetic force developed than in 

 the first case may be readily seen, and will be dealt with 

 next, 



(To be continued.) 



OTHER WORLDS THAN OURS. 



A WEEK'S CONTEESATIOX OX THE PLURALITY OF 

 WORLDS. 



By Mons. de Fontenelle. 



with xotf.s by richakd a. proctor. 



{Continued from p. 44.) 



" A LL philosophy," said T, " madam, is founded upon 

 JrX. two things, either that we are too shortsighted, or 

 that we are too cuiious ; for if our eyes were better than 

 they are we should soon see whether the stars were worlds 

 or not ; and if, on the other hand, we were less curious, we 

 should not care whether the stars are worlds or not, which, 

 I think, is much to the same purpose. But the business is, 

 we have a mind to know more than we see. And, again, 

 if we could discern well what we do see, it would be too 

 much known to us ; but we see things quite otherwise than 

 they are. So that your true philoso]iher will not believe 

 what he does see, and is always conjectuiing at what he 

 does not, which is a life, I think, not much to be envied. 

 Upon this I fancy to myself that nature very much re- 

 sembles an opera ; where you stand, you do not see the 

 stage as it really is, but as it is placed with advantage, and 

 all the wheels and movements are hid, to make the repre- 

 sentation the more agreeable. Nor do you trouble your- 

 self how or by what means the machines are moved, though 

 certainly an engineer in the pit is aflected with what does 

 not touch you ; he is pleased with the motion, and is 

 demonstrating to himself on ■what it depends, and how it 

 comes to pass. 



" This engineer, then, is like a philosopher, though the 

 difficulty is greater on the philosopher's part, the machines 

 of the theatre being nothing so curious as those of Nature, 

 who disposes her wheels and springs so out of sight that 

 we have been long a guessing at the movement of the 

 universe. Suppose, then, the sages to be an opera — i.e., 

 Pythagoras, Plato, Aristotle, and all the wise men who 

 have made such a noise in the world for these many ages ; 

 we will suppose them at the representation of Phneton, where 

 they see the aspiring youth lifted up by the winds, but do 

 not discover the wires by which he mounts, nor know 

 they anything of what is done behind the scenes. Would 

 you have all these philosophers own themselves to be stark 

 fools, and confess ingeniously they know not liow it comes 

 to pass ? No, no, they are not called wise men for no- 

 thing ; the', let me tell you, most of their wisdom 

 depends upon the ignorance of their neighbours. Every 

 man presently gives his opinion, and how improbable so- 

 ever, there are fools enough of all sorts to believe 'em : 

 One tells you Phaeton is drawn up by a hidden magnetick 

 virtue, no matter where it lies ; and perhaps the grave 

 gentleman will take pet if you ask him the question. 

 Another says Phaeton is compos'd of certain numbers that 

 make him mount; and, after all, the Philosopher knows 

 no moie of those numbers than a sucking child does of 

 Algebra. A third tells you. Phaeton hath a secret love 

 for the top of the theatre ; and, like a true lover, cannot 

 be at rest out of his mistress's company ; with an hundred 

 such extravagant fancies, that a man must conclude the 

 old sages were very good banterers. 



" But now comes Monsieur Descartes with some of the 

 moderns, and they tell you Phaeton ascends, because a 

 greater weight than he descends ; so that now we do not 

 believe a body can move unless it is push'd and forc'd by 

 another body, and as it were drawn by cords, so that no- 

 thing can rise or fall but by means of counter-poise ; to 



