(820.) 
(821.) 
Arago’s 
rotation- 
magnet- 
ism ex- 
plained. 
(822.) 
(823.) 
Dr Fara- 
day’s dis- 
covery of 
diamag- 
netism, 
Classifica- 
tion of 
magnetic 
and dia- 
magnetic 
bodies. 
980 
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 
and Diamagnetics.—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 awial ; 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 :-— 
Tron. 
Nickel. 
Cobalt. 
Manganese. 
Palladium. 
Crown glass, 
Platinum. 
Osmium. 
MATHEMATICAL AND PHYSICAL SCIENCE. 
(Diss. V1. 
Arsenic. 
Ether. 
Alcohol. 
Gold. 
Water. 
Mercury. 
Flint glass. 
Tin. 
“ Heavy glass.” 
Antimony. 
Phosphorus, 
Bismuth. 
Diamagnetics w.0..sseeersserseseseees 
The equatoreal pointing of diamagnetic bodies evi- _ (824.) 
dently presupposes that they are longer in one di- pp 
mension than in the others. A small bar of sili- Ment snd 
cated borate of lead, or “heavy glass,” about two definition 
inches long, and from a quarter to half an inch of diamag- 
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 phenomenon 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 diamagneties in the presence of |, (825.) 
a magnet may be thus further illustrated. It is jijustea- 
what would occur if a body absolutely inert were tions. 
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 _ (827.) 
precise nature of the Diamagnetic relatively to the Discussion 
Magnetic actions of bodies. Besides Dr Faraday, nature of 
MM, Weber and Edmond Becquerel abroad, and Pro- diamagnet- 
fessors Tyndall and William Thomson in this coun- ism. 
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.) 
