July 11, 188t.] 



* KNOWLEDGE 



29 



" Cheylesmore " for all ia all, I consider it a cheap, safe, 

 pretty, coiufortalile, and, in the hands of a good rider, a 

 fairlj swift machine. F. 



THE ELECTRO-MAGNET. 



By W. Slisgo. 



PRESUMABLY most readers are acquainted with the 

 general features of an electromagnet. It is, how- 

 ever, an instrument or tool capable of performing such 

 svonderful dexterities, and of affording such a valuable in- 

 sight into the principles of electricity, that a few brief 

 remarks upon its structure and its uses may, it is thought, 

 be welcome. Electricity is not such a young science but 

 that most people know the electro- magnet to consist 

 primarily of a coil or helix of insulated wire and a piece of 

 soft iron, and that a current of electricity, in traversing the 

 wire, renders the iron a magnet. For the benefit of the 

 uninitiated, and in order to present a more continuous 

 series of exjieriiiients and illustrations, a few lines may be 

 advantageously devoted to the simpler fundamental features 

 pertaining to the subject prior to venturing upon that 

 portion of it which will be kept more prominently 

 in view — viz, the best form to be adopted for a parti- 

 cular purpose, supplemented by directions for easily 

 making and using the instrument. The two extremi- 

 ties of au ordinary steel magnet exhibit a difference 

 in their behaviour, and are therefore in themselves dif- 

 ferent. Either end of a magnet attracts eqaally the 

 same piece of unmagnetised iron, the difference being 

 observable in the action of the ends or poles upon 

 -each other. Two magnetised sewing needles suspended a 

 few inches apart on water will, after a few second.% assume 

 iparallel positions, the extremities pointing more or less 

 north and south. If two of the similarly pointing ends 

 be made to approach, repulsion ensues, while attraction 

 results if the north-pointing eud of one needle is placed 

 sufficiently near to the south-pointing end of the other. 

 To magnetise the sewing-needles, they may be drawn a few 

 times over one of the poles of a magnet, always drawing 

 in one direction, either from eye to point, or point to eye. 

 The end of the needle which last touches the magnet 

 becomes of opposite polarity to that of the end of the 

 magnet used. Thus, if the north end of a magnet be used, 

 and a needle passed over it a few times, beginning at the 

 ■ftye, then the |ioint viill be south (and the eye, of necessity, 

 north). This follows as a matter of course, from the law 

 that like polarities repel, and unlike attract, for we may 

 ■easily conceive that the north makes itself evident at the 

 ^e (the part most reniote from the magnet) in obedience 

 to the repulsion by the north, and similarly that the point 

 becomes south because of the closer proximity of the 

 magnet-pole to the point than to the eye. 



Suppose now that above or below a copper wire capable 

 of carrying a current of electricity from two or three cells 

 we suspend an unmagnetised needle at right angles with 

 the copper wire. On passing the current, the needle will 

 become a more or less powerful magnet, particularly if the 

 wire is placed east to west, so that the needle is north to 

 south, when the inductive effect exerted by the current is 

 aided by the magnetism of the earth, that magnetism which 

 caused the suspended magnetised needles to poiiit north 

 and south. If the needle be afterwards turned so as to 

 point in some other direction, it will, when free, turn 

 again to the north and south position, and will, in fact, 

 .assume all the [iroperties of an ordinary magnet. Had 

 the needle been an iron in'Stead of a steel one, 



a different result would have followed. The iron 

 would have been magnetised on the passage of 

 the current, just as the steel was, only more 

 powerfully. On the cessation of the current, however, its 

 effect upon the iron needle would have instantly ceased, 

 although the needle would continue in obedience to inertia 

 and to the feeble influence of terrestrial magnetism to point 

 north and south. The difference between iron and steel is 

 further seen on moving the former intu any other position 

 than that of north and south. When this is done all trace 

 of magnetisation is gone, and the iron exhibits no tendency 

 to reassume the longitudinal direction. Had the iron been 

 placed originally in any other than a north and south posi- 

 tion it would not have felt, and, therefore, could not have 

 retained the small effect produced by the magnetism of the 

 earth. 



So far as the magnetisation due to the current is concerned, 

 it is quite immaterial whether the previously unmagnetised 

 needle be placed in a longitudinal or any other direction 

 whatever, so long as the current is made to cross it at right 

 angles. It follows, obviously, that if we encircle a needle 

 by a ring or loop of copper-wire, and send a current 

 through the wire, the electricity in any one part of the 

 circle conspires with or aids the current in every other 

 portion. A proportionate increase in the intensity of 

 magnetism nattirally ensues. Carrying this jirinciple a 

 little farther, if, instead of one circular loop we emjiloy 

 several similar loops (wound round the needle in one con- 

 stant direction, so that the current in traversing the wire 

 is compelled to keep travelling from end to end without 

 doubling back on itself) then the magnetising effect due to 

 a single litie or loop is proportionably multiplied. Again, 

 if we employ two or more concentric loops, such as we 

 should get, for e.xample, by winding tie wire in a manner 

 similar to that of a watch-spring, we shall produce a corre- 

 sponding increase. It is apparent, then, how proportion- 

 ately great must be the effect produced by a current 

 passing through a long coil composed of several layers, as 

 compared with that resulting from the passage of the same 

 current through a single wire at right angles to the needle. 



The polarity which the needle assumes is dependent on 

 the relative direction of the current. With a current pass- 

 ing over the needle from, say, south to north, the eye or 

 the point, whichever it may be that is in the west, assumes 

 a north polarity, while that extremity of the needle which 

 lies to the east of the wire becomes a south pole. Gene- 

 rallv, to adopt an illustration of Ampere's, assuming a little 

 man to be swimming in the wire in the same direction a.s 

 the current, and to have his face turned constantly towards 

 the needle, that end will become north which is on his left 

 hand. The same principle is involved here as in the case of 

 galvanometers, and demonstrates the multiplying power of 

 coils of wire. 



When a soft iron wire is used instead of the steel needle, 

 the effect produced by a current traversing the enveloping 

 coil of wire ceases with the current, just as the effect pro- 

 duced by a current in a single length of wire was no longer 

 felt when the current disappeared. Of the relation sub- 

 sisting between the effective power of a sii gle wire and a 

 coil, more will be said presently. 



B.fore we can predetermine the polarity an iron wire 

 or rod will assume in response to the inductive effect ex- 

 erted by a coil of wire, we mast bear in mind that there 

 are two ways of winding a coil, and that the electro- 

 magnetic effects exerted by these two coils are opposite. 

 This may be experimentally demonstrated with the aid of an 

 iron rod such as, for want of something else, a clean poker, 

 a yard or two of cotton-covered or other insulated wire 

 (rather stout, say No. 16, B.W.G.), a magnetised sewing- 



