KUSCTRO-MAQNETS.] UNDULATORY FORCES. ELECTRO-MAGNETISM. 



240 



with spokes, instead of the circular form we have de- 

 8. scribed. Such will combine the 



principles of construction illus 

 trated in our two last engravings 

 for. whilst it has a rotary motion, 

 each spoke, as it leaves the mer- 

 cury, is, as it were, similarly jerkei 

 out in the way we mentioned ir 

 the case of the wire. The an- 

 nexed cut will assist our readers 

 in making a wheel of this kind 

 all other matters are the same as 

 in that form of instrument we have just noticed. 



If a small voltaic battery be suspended round each 01 

 the poles of a horse-shoe magnet, the metals of which il 

 is composed will revolve rapidly round the magnet, anc 

 in reverse directions at each pole. These arrangements 

 are exceedingly ingenious, but very troublesome in use. 

 In the following engraving one of them is illustrated. 

 They are sold at the instrument-makers; and give an 

 additional proof, when in action, of the mutual influence 

 of electric currents and magnets. 



In Fig. 9, o represents a pole of a powerful circular 



Fig. 9. 



horse-shoe magnet ; 6 is a copper 

 bucket, suspended at d on the 

 top of the magnet, round which 

 it may freely rotate; c is a 

 zinc cylinder which hangs inside 

 the copper one. The latter is to 

 be filled with some acid and 

 water, when a voltaic battery is 

 at once formed. As soon as this 

 is charged the cylinders turn 

 round rapidly on their axes at d, 

 which action is due to the same 

 laws as those we mentioned in 

 connection with our three last 

 experiments. 



Whilst referring to the magnetism of the earth, we 

 spoke of electric currents as affecting the magnetic 

 needle ;* and also, when describing the dipping-needle, 

 we mentioned that there existed magnetic poles. .Mr. 

 Barlow, many years ago, invented a most ingenious 

 arrangement, which consisted of a number of wires 

 (corresponding with the circles of latitude on a globe), 

 which were wound round a wooden ball, and were in- 

 tended to represent the direction of electric currents as 

 they pass round the earth. If a current of electricity be 

 sent from a voltaic battery round a ball so arranged, and 

 a vertical magnetic needle be presented to it in different 

 parts, the various inclinations or dips are at once pro- 

 duced. Such an arrangement forms a curious and 

 instructive illustration of the production of magnetic 

 phenomena by means of a voltaic current, and also 

 affords to the mind an idea of the effects which occur on 

 the large scale in nature. 



ELECTRO-MAGNETS. 



WE have hitherto confined our attention to simple 

 instances of the mutual action of electric and magnetic 

 currents ; and shall now proceed to point out how, by 

 applying the laws already discussed, we may produce an 

 immense development of magnetic attraction in soft iron, 

 by means of properly directed voltaic currents. If a 

 piece of steel be nibbed by means of an ordinary magnet, 

 of any form, it retains a portion of the magnetic force ; 

 but soft iron does not ; and it ceases to exhibit any sign 

 of nt traction so Boon as the magnet which had induced it 

 is withdrawn. In fact, the result is simply one of 

 induction, and, therefore, ceases on the inducing power 

 l-in^' ruraovcd. The same holds good in reference to the 

 magnetic force induced by electricity; as we shall dis- 

 cover in trying any of the following experiments : 



Experiment 7. Twist some covered copper wire round 

 a roller, BO as to form a spiral of a few inches in length, 

 and, removing the roller, insert in its place a rod of soft 

 iron. Connect the two ends of the spiral with the wires 



Sec ante, f. 243. 

 VOl. I. 



proceeding from a voltaic battery. Immediately that 

 the current of electricity passes round the iron, it 

 induces, at right angles to its direction, t magnetism 

 in the rod ; and if some nails or iron filings are presented 

 to the extreme ends, they will be powerfully attracted. 

 The annexed cut illus- Fig. 10. 



trates this simple ar- 

 rangement. At each 

 end of the coil the iron 

 rod is seen projecting; 

 and the wires termi- 

 nating the coil are also 

 shown. 



Experiment 8. So 

 long only as the current of electricity passes, does the 

 iron exhibit magnetic attraction ; forj on removing either 

 of the battery wires, the iron nails will at once fall down, 

 and the iron rod returns to its normal condition. 



We must here note the fact, that the magnetic effects 

 are induced at right angles to the direction of the elec- 

 trical current; and to assist the student, we introduce 

 the following cut. 



Fig. 11 represents a skeleton view of the spiral coil 

 Fig. ll. 





I 



w- 



and iron rod: n and s show the poles of magnetic force 

 induced in the bar of iron t. Now, the direction of the 

 current of electricity from one of the wires, w, to the 

 other on its return to the battery, is indicated by the 

 arrows ; and if the spiral bo wound quite straight round 

 the rod, then it and the current will necessarily be at 

 right angles to the iron which they encircle. Wo have 

 already seen that the sides of a spiral are magnetic when 

 it conveys a voltaic current ;| and it follows that the 

 iron enclosed therein partakes of that change or effect. 

 Indeed, the simple arrangement illustrated in Fig. 11 is 

 quite of sufficient power, when in action, to induce 

 magnetism in a small soft iron wire, if such be placed in 

 its centre ; and thus the application of the laws of which 

 wo have so repeatedly spoken, is evident. 



This law, however, may be made more special in the 

 instances to which we are now referring, by stating, " that 

 a current of electricity passing round soft iron, converts 

 the metal into what is termed an electro-magnet, when 

 its direction is at right angles to the iron. 



We have seen that the magnetic effect produced on 

 soft iron is but temporary ; that is, provided that the 

 metal be perfectly pure. But this is rarely the case, 

 because iron is mixed with other substances which affect 

 its nature. Carbon is invariably present to a small 

 extent, and hence the metal does not lose its mag- 

 netism instantly on the current being withdrawn. Tho 

 magnetic effect remains for some time ; and this is called 

 residuary magnetism, which frequently becomes a source 

 of great difficulty and annoyance in many of the appli- 

 cations of electro-magnetism. A perceptible interval of 

 time is required to produce the highest effect of magnetic 

 induction ; and as in the case of ordinary magnets, the 

 poles do not exist exactly at the extremity of the 

 metal rod. We shall now examine the effect produced 

 on steel by the voltaic current as it passes round a rod of 

 that metal. 



Experiment 9. Place inside a coil of covered copper 

 wire (which, for this purpose, may be wound round a 

 lapcr or gutta-percha tube), a bar of good steel, which 

 las been previously tested, so that the absence of all 

 magnetism has been proved. As soon as a current of 

 electricity passes over it, the steel will become magnetic, 

 as may be shown by holding any piece of iron near it. 

 3ut it will be noticed, on the current ceasing to pass by 

 t See ante, p. 247. J See aalr, p. 213. 



2K 



