182 



MATHEMATICAL AND PHYSICAL SCIENCE. 



piss. VI. 



(820.) 



(821.) 

 Arago's 

 rotation- 

 magnet- 

 ism ex- 

 plained. 



(822.) 



(823.) 

 DP Fara- 

 day's dis- 

 covery of 

 diamag- 

 netism. 



Classifica- 

 tion of 

 magnetic 

 and dia- 

 magnetic 

 bodies. 



production of a spark. This, then, was the disco- 

 very of magneto-electricity. 



The mere motion of a permanent magnet was now 

 substituted for the induction of magnetism in soft 

 iron. By pushing one end of a bar-magnet into the 

 coil, electricity was developed so long as the motion 

 continued; on withdrawing it an opposite current 

 took place. Even the feeble magnetism of the 

 earth induced a sensible electric current in a wire 

 moved transversely to the direction of the dipping- 

 needle. 



By making a copper plate revolve in the neigh- 

 bourhood of a powerful magnet, a continuous cur- 

 rent of electricity may be detected passing from the 

 centre to the circumference of the plate, and may be 

 collected by proper means. Here, then, is a mag- 

 neto-electric machine. This current perpetually pre- 

 sent in a conducting plate revolving beneath a mag- 

 net, cannot fail (by the common laws of electro- 

 magnetism) to react on that magnet. Dr Faraday 

 showed in the most satisfactory manner that its 

 action is exactly what is required to explain M. 

 Arago's experiment of " transient magnetism by 

 rotation" namely, to cause the magnet (if free) 

 to follow the direction of motion of the plate 

 (515). 



On the whole, this research of Dr Faraday may be 

 cited as one of the most original and admirably 

 conducted which the annals of science present, 

 and as such may be usefully recommended to the 

 student. 



III. The influence of the magnet on all bodies, and 

 their consequent division into two classes ; Magnetics 

 andDiamagnetics. By many, perhaps most persons, 

 this will be regarded as the greatest of Dr Faraday's 

 discoveries. It dates from 1846. By using electro- 

 magnets of very great power, and suspending bodies 

 of a somewhat elongated form between the poles, he 

 has proved that every substance, solid, liquid, or 

 gaseous which he has put to the test, is either drawn 

 into a line joining the poles of the magnet, as soft 

 iron would be, returning to that line if displaced, or 

 else it settles in a position at right angles to this, or 

 across the line of poles. The former he calls para- 

 magnetic or simply magnetic bodies, and their posi- 

 tion axial ; the latter diamagnetic bodies, and their 

 position equatoreal. Bodies may be arranged in a 

 list commencing with those most paramagnetic, dimi- 

 nishing to neutrality, then feebly diamagnetic, and 

 finally the strongest diamagnetics. The following 

 is such a list of a few solid and liquid bodies thus 

 classified : 



(iron. 

 ' Nickel. 

 Cobalt. 

 Manganese. 

 Palladium. 

 Crown glass. 

 Platinum. 

 Osmium. 

 Zero Vacuum. 



Magnetics or Paramagnetics. . 



Diamagnetics .. 



Arsenic. 



Ether. 



Alcohol. 



Gold. 



Water. 



Mercury. 



Flint glass. 



Tin. 



" Heavy glass." 



Antimony. 



Phosphorus. 



Bismuth. 



(824.) 



n 



The equatoreal pointing of diamagnetic bodies evi- 

 dently presupposes that they are longer in one di- 

 mension than in the others. A small bar of sili- 

 cated borate of lead, or " heavy glass," about two definition 

 inches long, and from a quarter to half an inc 

 broad and deep, suspended in a stirrup of paper by 

 six or eight lengths of cocoon silk, was the appa- 

 ratus first employed by Dr Faraday. When a 

 sphere or a cube is used, of course it cannot point. 

 The diamagnetic action is shown in that case by the 

 little body being repelled indifferently from either 

 pole of the magnet, in the same manner as soft iron 

 is indifferently attracted by either. This repulsive 

 tendency includes the phenomenoa of equatoreal 

 pointing, and its law is thus comprehensively ex- 

 pressed : " The diamagnetic tendency is to move the 

 body from stronger to weaker places of magnetic 

 force." 



The behaviour of diamagnetics in the presence of ( 825 



i -i r ji MI i j TJ. Farther 



a magnet may be thus further illustrated. It is iji ugtra . 

 what would occur if a body absolutely inert weretions. 

 suspended in a fluid pressing upon it, that fluid 

 being at the same time more or less magnetic, that 

 is, more or less attracted by either pole of the mag- 

 net. The result would evidently be, that the body 

 would seem to be repelled, and would set equato- 

 really for the same reason that a piece of wood 

 plunged in water rises to the surface as if repelled 

 by gravity. Thus Dr Faraday suspended feebly 

 paramagnetic bodies in ferruginous solutions more 

 magnetic than themselves, when they acted as dia- 

 magnetic bodies would do. 



It is impossible for the most part to guess before- 

 hand to which class a substance will belong. China- 

 ink, porcelain, silkworm gut, shell-lac, and charcoal, 

 rank amongst paramagnetic substances ; whilst sul- 

 phur, resin, wood, leather, and most animal sub- 

 stances, are diamagnetic. Thus, if a living man 

 could be delicately enough suspended between the 

 poles of a huge magnet, he would settle equato- 

 really. 



Philosophers are not yet entirely agreed as to the 

 precise nature of the Diamagnetic relatively to the 

 Magnetic actions of bodies. Besides Dr Faraday, na t ur e of 

 MM. Weber andEdmond Becquerel abroad, and Pro- diamagnet- 

 fessors Tyndall and William Thomson in this coun- lsm - 

 try, have examined the subject both practically and 

 theoretically in great detail. The more probable 

 opinion seems to be, that bismuth and its analogues 



(826.) 



(827.) 



