§ 5. _Terrestrial Magneto-electric Induction._ § 6. _Force and Direction of
Magneto-electric Induction generally._
Read January 12, 1832.
§ 5. _Terrestrial Magneto-electric Induction._
140. When the general facts described in the former paper were discovered,
and the _law_ of magneto-electric induction relative to direction was
ascertained (114.), it was not difficult to perceive that the earth would
produce the same effect as a magnet, and to an extent that would, perhaps,
render it available in the construction of new electrical machines. The
following are some of the results obtained in pursuance of this view.
141. The hollow helix already described (6.) was connected with a
galvanometer by wires eight feet long; and the soft iron cylinder (34.)
after being heated red-hot and slowly cooled, to remove all traces of
magnetism, was put into the helix so as to project equally at both ends,
and fixed there. The combined helix and bar were held in the magnetic
direction or line of dip, and (the galvanometer needle being motionless)
were then inverted, so that the lower end should become the upper, but the
whole still correspond to the magnetic direction; the needle was
immediately deflected. As the latter returned to its first position, the
helix and bar were again inverted; and by doing this two or three times,
making the inversions and vibrations to coincide, the needle swung through
an arc of 150° or 160°.
142. When one end of the helix, which may be called A, was uppermost at
first (B end consequently being below), then it mattered not in which
direction it proceeded during the inversion, whether to the right hand or
left hand, or through any other course; still the galvanometer needle
passed in the same direction. Again, when B end was uppermost, the
inversion of the helix and bar in any direction always caused the needle to
be deflected one way; that way being the opposite to the course of the
deflection in the former case.
143. When the helix with its iron core in any given position was inverted,
the effect was as if a magnet with its marked pole downwards had been
introduced from above into the inverted helix. Thus, if the end B were
upwards, such a magnet introduced from above would make the marked end of
the galvanometer needle pass west. Or the end B being downwards, and the
soft iron in its place, inversion of the whole produced the same effect.
144. When the soft iron bar was taken out of the helix and inverted in
various directions within four feet of the galvanometer, not the slightest
effect upon it was produced.
145. These phenomena are the necessary consequence of the inductive
magnetic power of the earth, rendering the soft iron cylinder a magnet with
its marked pole downwards. The experiment is analogous to that in which two
bar magnets were used to magnetize the same cylinder in the same helix
(36.), and the inversion of position in the present experiment is
equivalent to a change of the poles in that arrangement. But the result is
not less an instance of the evolution of electricity by means of the
magnetism of the globe.
146. The helix alone was then held permanently in the magnetic direction,
and the soft iron cylinder afterwards introduced; the galvanometer needle
was instantly deflected; by withdrawing the cylinder as the needle
returned, and continuing the two actions simultaneously, the vibrations
soon extended through an arc of 180°. The effect was precisely the same as
that obtained by using a cylinder magnet with its marked pole downwards;
and the direction of motion, &c. was perfectly in accordance with the
results of former experiments obtained with such a magnet (39.). A magnet
in that position being used, gave the same deflections, but stronger. When
the helix was put at right angles to the magnetic direction or dip, then
the introduction or removal of the soft iron cylinder produced no effect at
the needle. Any inclination to the dip gave results of the same kind as
those already described, but increasing in strength as the helix
approximated to the direction of the dip.
147. A cylinder magnet, although it has great power of affecting the
galvanometer when moving into or out of the helix, has no power of
continuing the deflection (39.); and therefore, though left in, still the
magnetic needle comes to its usual place of rest. But upon repeating (with
the magnet) the experiment of inversion in the direction of the dip (141),
the needle was affected as powerfully as before; the disturbance of the
magnetism in the steel magnet, by the earth's inductive force upon it,
being thus shown to be nearly, if not quite, equal in amount and rapidity
to that occurring in soft iron. It is probable that in this way
magneto-electrical arrangements may become very useful in indicating the
disturbance of magnetic forces, where other means will not apply; for it is
not the whole magnetic power which produces the visible effect, but only
the difference due to the disturbing causes.
148. These favourable results led me to hope that the direct
magneto-electric induction of the earth might be rendered sensible; and I
ultimately succeeded in obtaining the effect in several ways. When the
helix just referred to (141. 6.) was placed in the magnetic dip, but
without any cylinder of iron or steel, and was then inverted, a feeble
action at the needle was observed. Inverting the helix ten or twelve times,
and at such periods that the deflecting forces exerted by the currents of
electricity produced in it should be added to the momentum of the needle
(39.), the latter was soon made to vibrate through an arc of 80° or 90°.
Here, therefore, currents of electricity were produced by the direct
inductive power of the earth's magnetism, without the use of any
ferruginous matter, and upon a metal not capable of exhibiting any of the
ordinary magnetic phenomena. The experiment in everything represents the
effects produced by bringing the same helix to one or both poles of any
powerful magnet (50.).
149. Guided by the law already expressed (114.), I expected that all the
electric phenomena of the revolving metal plate could now be produced
without any other magnet than the earth. The plate so often referred to
(85.) was therefore fixed so as to rotate in a horizontal plane. The
magnetic curves of the earth (114. _note_), i.e. the dip, passes through
this plane at angles of about 70°, which it was expected would be an
approximation to perpendicularity, quite enough to allow of
magneto-electric induction sufficiently powerful to produce a current of
electricity.
150. Upon rotation of the plate, the currents ought, according to the law
(114. 121.), to tend to pass in the direction of the radii, through _all_
parts of the plate, either from the centre to the circumference, or from
the circumference to the centre, as the direction of the rotation of the
plate was one way or the other. One of the wires of the galvanometer was
therefore brought in contact with the axis of the plate, and the other
attached to a leaden collector or conductor (86.), which itself was placed
against the amalgamated edge of the disc. On rotating the plate there was a
distinct effect at the galvanometer needle; on reversing the rotation, the
needle went in the opposite direction; and by making the action of the
plate coincide with the vibrations of the needle, the arc through which the
latter passed soon extended to half a circle.
151. Whatever part of the edge of the plate was touched by the conductor,
the electricity was the same, provided the direction of rotation continued
unaltered.
152. When the plate revolved _screw-fashion_, or as the hands of a watch,
the current of electricity (150.) was from the centre to the circumference;
when the direction of rotation was _unscrew_, the current was from the
circumference to the centre. These directions are the same with those
obtained when the unmarked pole of a magnet was placed beneath the
revolving plate (99.).
153. When the plate was in the magnetic meridian, or in any other plane
_coinciding_ with the magnetic dip, then its rotation produced no effect
upon the galvanometer. When inclined to the dip but a few degrees,
electricity began to appear upon rotation. Thus when standing upright in a
plane perpendicular to the magnetic meridian, and when consequently its own
plane was inclined only about 20° to the dip, revolution of the plate
evolved electricity. As the inclination was increased, the electricity
became more powerful until the angle formed by the plane of the plate with
the dip was 90°, when the electricity for a given velocity of the plate was
a maximum.
154. It is a striking thing to observe the revolving copper plate become
thus a _new electrical machine_; and curious results arise on comparing it
with the common machine. In the one, the plate is of the best
non-conducting substance that can be applied; in the other, it is the most
perfect conductor: in the one, insulation is essential; in the other, it is
fatal. In comparison of the quantities of electricity produced, the metal
machine does not at all fall below the glass one; for it can produce a
constant current capable of deflecting the galvanometer needle, whereas the
latter cannot. It is quite true that the force of the current thus evolved
has not as yet been increased so as to render it available in any of our
ordinary applications of this power; but there appears every reasonable
expectation that this may hereafter be effected; and probably by several
arrangements. Weak as the current may seem to be, it is as strong as, if
not stronger than, any thermo-electric current; for it can pass fluids
(23.), agitate the animal system, and in the case of an electro-magnet has
produced sparks (32.).
155. A disc of copper, one fifth of an inch thick and only one inch and a
half in diameter, was amalgamated at the edge; a square piece of sheet lead
(copper would have been better) of equal thickness had a circular hole cut
in it, into which the disc loosely fitted; a little mercury completed the
metallic communication of the disc and its surrounding ring; the latter was
attached to one of the galvanometer wires, and the other wire dipped into a
little metallic cup containing mercury, fixed upon the top of the copper
axis of the small disc. Upon rotating the disc in a horizontal plane, the
galvanometer needle could be affected, although the earth was the only
magnet employed, and the radius of the disc but three quarters of an inch;
in which space only the current was excited.
156. On putting the pole of a magnet under the revolving disc, the
galvanometer needle could be permanently deflected.
157. On using copper wires one sixth of an inch in thickness instead of the
smaller wires (86.) hitherto constantly employed, far more powerful effects
were obtained. Perhaps if the galvanometer had consisted of fewer turns of
thick wire instead of many convolutions of thinner, more striking effects
would have been produced.
158. One form of apparatus which I purpose having arranged, is to have
several discs superposed; the discs are to be metallically connected,
alternately at the edges and at the centres, by means of mercury; and are
then to be revolved alternately in opposite directions, i.e. the first,
third, fifth, &c. to the right hand, and the second, fourth, sixth, &c. to
the left hand; the whole being placed so that the discs are perpendicular
to the dip, or intersect most directly the magnetic curves of powerful
magnets. The electricity will be from the centre to the circumference in
one set of discs, and from the circumference to the centre in those on each
side of them; thus the action of the whole will conjoin to produce one
combined and more powerful current.
159. I have rather, however, been desirous of discovering new facts and new
relations dependent on magneto-electric induction, than of exalting the
force of those already obtained; being assured that the latter would find
their full development hereafter.
* * * * *
160. I referred in my former paper to the probable influence of terrestrial
magneto-electric induction (137.) in producing, either altogether or in
part, the phenomena observed by Messrs. Christie and Barlow[A], whilst
revolving ferruginous bodies; and especially those observed by the latter
when rapidly rotating an iron shell, which were by that philosopher
referred to a change in the ordinary disposition of the magnetism of the
ball. I suggested also that the rotation of a copper globe would probably
insulate the effects due to electric currents from those due to mere
derangement of magnetism, and throw light upon the true nature of the
phenomena.
[A] Christie, Phil. Trans. 1825, pp. 58, 347, &c. Barlow, Phil. Trans.
1825, p. 317.
161. Upon considering the law already referred to (114.), it appeared
impossible that a metallic globe could revolve under natural circumstances,
without having electric currents produced within it, circulating round the
revolving globe in a plane at right angles to the plane of revolution,
provided its axis of rotation did not coincide with the dip; and it
appeared that the current would be most powerful when the axis of
revolution was perpendicular to the dip of the needle: for then all those
parts of the ball below a plane passing through its centre and
perpendicular to the dip, would in moving cut the magnetic curves in one
direction, whilst all those parts above that plane would intersect them in
the other direction: currents therefore would exist in these moving parts,
proceeding from one pole of rotation to the other; but the currents above
would be in the reverse direction to those below, and in conjunction with
them would produce a continued circulation of electricity.
162. As the electric currents are nowhere interrupted in the ball, powerful
effects were expected, and I endeavoured to obtain them with simple
apparatus. The ball I used was of brass; it had belonged to an old
electrical machine, was hollow, thin (too thin), and four inches in
diameter; a brass wire was screwed into it, and the ball either turned in
the hand by the wire, or sometimes, to render it more steady, supported by
its wire in a notched piece of wood, and motion again given by the hand.
The ball gave no signs of magnetism when at rest.
163. A compound magnetic needle was used to detect the currents. It was
arranged thus: a sewing-needle had the head and point broken off, and was
then magnetised; being broken in halves, the two magnets thus produced were
fixed on a stem of dried grass, so as to be perpendicular to it, and about
four inches asunder; they were both in one plane, but their similar poles
in contrary directions. The grass was attached to a piece of unspun silk
about six inches long, the latter to a stick passing through a cork in the
mouth of a cylindrical jar; and thus a compound arrangement was obtained,
perfectly sheltered from the motion of the air, but little influenced by
the magnetism of the earth, and yet highly sensible to magnetic and
electric forces, when the latter were brought into the vicinity of the one
or the other needle.
164. Upon adjusting the needles to the plane of the magnetic meridian;
arranging the ball on the outside of the glass jar to the west of the
needles, and at such a height that its centre should correspond
horizontally with the upper needle, whilst its axis was in the plane of the
magnetic meridian, but perpendicular to the dip; and then rotating the
ball, the needle was immediately affected. Upon inverting the direction of
rotation, the needle was again affected, but in the opposite direction.
When the ball revolved from east over to west, the marked pole went
eastward; when the ball revolved in the opposite direction, the marked pole
went westward or towards the ball. Upon placing the ball to the east of the
needles, still the needle was deflected in the same way; i.e. when the ball
revolved from east over to west, the marked pole wont eastward (or towards
the ball); when the rotation was in the opposite direction, the marked pole
went westward.
165. By twisting the silk of the needles, the latter were brought into a
position perpendicular to the plane of the magnetic meridian; the ball was
again revolved, with its axis parallel to the needles; the upper was
affected as before, and the deflection was such as to show that both here
and in the former case the needle was influenced solely by currents of
electricity existing in the brass globe.
166. If the upper part of the revolving ball be considered as a wire moving
from east to west, over the unmarked pole of the earth, the current of
electricity in it should be from north to south (99. 114. 150.); if the
under part be considered as a similar wire, moving from west to east over
the same pole, the electric current should be from south to north; and the
circulation of electricity should therefore be from north above to south,
and below back to north, in a metal ball revolving from east above to west
in these latitudes. Now these currents are exactly those required to give
the directions of the needle in the experiments just described; so that the
coincidence of the theory from which the experiments were deduced with the
experiments themselves, is perfect.
167. Upon inclining the axis of rotation considerably, the revolving ball
was still found to affect the magnetic needle; and it was not until the
angle which it formed with the magnetic dip was rendered small, that its
effects, even upon this apparatus, were lost (153.). When revolving with
its axis parallel to the dip, it is evident that the globe becomes
analogous to the copper plate; electricity of one kind might be collected
at its equator, and of the other kind at its poles.
168. A current in the ball, such as that described above (161.), although
it ought to deflect a needle the same way whether it be to the right or the
left of the ball and of the axis of rotation, ought to deflect it the
contrary way when above or below the ball; for then the needle is, or ought
to be, acted upon in a contrary direction by the current. This expectation
was fulfilled by revolving the ball beneath the magnetic needle, the latter
being still inclosed in its jar. When the ball was revolved from east over
to west, the marked pole of the needle, instead of passing eastward, went
westward; and when revolved from west over to east, the marked pole went
eastward.
169. The deflections of the magnetic needle thus obtained with a brass ball
are exactly in the same direction as those observed by Mr. Barlow in the
revolution of the iron shell; and from the manner in which iron exhibits
the phenomena of magneto-electric induction like any other metal, and
distinct from its peculiar magnetic phenomena (132.), it is impossible but
that electric currents must have been excited, and become active in those
experiments. What proportion of the whole effect obtained is due to this
cause, must be decided by a more elaborate investigation of all the
phenomena.
170. These results, in conjunction with the general law before stated
(114.), suggested an experiment of extreme simplicity, which yet, on trial,
was found to answer perfectly. The exclusion of all extraneous
circumstances and complexity of arrangement, and the distinct character of
the indications afforded, render this single experiment an epitome of
nearly all the facts of magneto-electric induction.
171. A piece of common copper wire, about eight feet long and one twentieth
of an inch in thickness, had one of its ends fastened to one of the
terminations of the galvanometer wire, and the other end to the other
termination; thus it formed an endless continuation of the galvanometer
wire: it was then roughly adjusted into the shape of a rectangle, or rather
of a loop, the upper part of which could be carried to and fro over the
galvanometer, whilst the lower part, and the galvanometer attached to it,
remained steady (Plate II. fig. 30.). Upon moving this loop over the
galvanometer from right to left, the magnetic needle was immediately
deflected; upon passing the loop back again, the needle passed in the
contrary direction to what it did before; upon repeating these motions of
the loop in accordance with the vibrations of the needle (39.), the latter
soon swung through 90° or more.
172. The relation of the current of electricity produced in the wire, to
its motion, may be understood by supposing the convolutions at the
galvanometer away, and the wire arranged as a rectangle, with its lower
edge horizontal and in the plane of the magnetic meridian, and a magnetic
needle suspended above and over the middle part of this edge, and directed
by the earth (fig. 30.). On passing the upper part of the rectangle from
west to east into the position represented by the dotted line, the marked
pole of the magnetic needle went west; the electric current was therefore
from north to south in the part of the wire passing under the needle, and
from south to north in the moving or upper part of the parallelogram. On
passing the upper part of the rectangle from east to west over the
galvanometer, the marked pole of the needle went east, and the current of
electricity was therefore the reverse of the former.
173. When the rectangle was arranged in a plane east and west, and the
magnetic needle made parallel to it, either by the torsion of its
suspension thread or the action of a magnet, still the general effects were
the same. On moving the upper part of the rectangle from north to south,
the marked pole of the needle went north; when the wire was moved in the
opposite direction, the marked pole went south. The same effect took place
when the motion of the wire was in any other azimuth of the line of dip;
the direction of the current always being conformable to the law formerly
expressed (114.), and also to the directions obtained with the rotating
ball (101.).
174. In these experiments it is not necessary to move the galvanometer or
needle from its first position. It is quite sufficient if the wire of the
rectangle is distorted where it leaves the instrument, and bent so as to
allow the moving upper part to travel in the desired direction.
175. The moveable part of the wire was then arranged _below_ the
galvanometer, but so as to be carried across the dip. It affected the
instrument as before, and in the same direction; i.e. when carried from
west to east under the instrument, the marked end of the needle went west,
as before. This should, of course, be the case; for when the wire is
cutting the magnetic dip in a certain direction, an electric current also
in a certain direction should be induced in it.
176. If in fig. 31 _dp_ be parallel to the dip, and BA be considered as the
upper part of the rectangle (171.), with an arrow _c_ attached to it, both
these being retained in a plane perpendicular to the dip,--then, however BA
with its attached arrow is moved upon _dp_ as an axis, if it afterwards
proceed in the direction of the arrow, a current of electricity will move
along it from B towards A.
177. When the moving part of the wire was carried up or down parallel to
the dip, no effect was produced on the galvanometer. When the direction of
motion was a little inclined to the dip, electricity manifested itself; and
was at a maximum when the motion was perpendicular to the magnetic
direction.
178. When the wire was bent into other forms and moved, equally strong
effects were obtained, especially when instead of a rectangle a double
catenarian curve was formed of it on one side of the galvanometer, and the
two single curves or halves were swung in opposite directions at the same
time; their action then combined to affect the galvanometer: but all the
results were reducible to those above described.
179. The longer the extent of the moving wire, and the greater the space
through which it moves, the greater is the effect upon the galvanometer.
180. The facility with which electric currents are produced in metals when
moving under the influence of magnets, suggests that henceforth precautions
should always be taken, in experiments upon metals and magnets, to guard
against such effects. Considering the universality of the magnetic
influence of the earth, it is a consequence which appears very
extraordinary to the mind, that scarcely any piece of metal can be moved in
contact with others, either at rest, or in motion with different velocities
or in varying directions, without an electric current existing within them.
It is probable that amongst arrangements of steam-engines and metal
machinery, some curious accidental magneto-electric combinations may be
found, producing effects which have never been observed, or, if noticed,
have never as yet been understood.
* * * * *
181. Upon considering the effects of terrestrial magneto-electric induction
which have now been described, it is almost impossible to resist the
impression that similar effects, but infinitely greater in force, may be
produced by the action of the globe, as a magnet, upon its own mass, in
consequence of its diurnal rotation. It would seem that if a bar of metal
be laid in these latitudes on the surface of the earth parallel to the
magnetic meridian, a current of electricity tends to pass through it from
south to north, in consequence of the travelling of the bar from west to
east (172.), by the rotation of the earth; that if another bar in the same
direction be connected with the first by wires, it cannot discharge the
current of the first, because it has an equal tendency to have a current in
the same direction induced within itself: but that if the latter be carried
from east to west, which is equivalent to a diminution of the motion
communicated to it from the earth (172.), then the electric current from
south to north is rendered evident in the first bar, in consequence of its
discharge, at the same time, by means of the second.
182. Upon the supposition that the rotation of the earth tended, by
magneto-electric induction, to cause currents in its own mass, these would,
according to the law (114.) and the experiments, be, upon the surface at
least, from the parts in the neighbourhood of or towards the plane of the
equator, in opposite directions to the poles; and if collectors could be
applied at the equator and at the poles of the globe, as has been done with
the revolving copper plate (150.), and also with magnets (220.), then
negative electricity would be collected at the equator, and positive
electricity at both poles (222.). But without the conductors, or something
equivalent to them, it is evident these currents could not exist, as they
could not be discharged.
183. I did not think it impossible that some natural difference might occur
between bodies, relative to the intensity of the current produced or
tending to be produced in them by magneto-electric induction, which might
be shown by opposing them to each other; especially as Messrs. Arago,
Babbage, Herschel, and Harris, have all found great differences, not only
between the metals and other substances, but between the metals themselves,
in their power of receiving motion from or giving it to a magnet in trials
by revolution (130.). I therefore took two wires, each one hundred and
twenty feet long, one of iron and the other of copper. These were connected
with each other at their ends, and then extended in the direction of the
magnetic meridian, so as to form two nearly parallel lines, nowhere in
contact except at the extremities. The copper wire was then divided in the
middle, and examined by a delicate galvanometer, but no evidence of an
electrical current was obtained.
184. By favour of His Royal Highness the President of the Society, I
obtained the permission of His Majesty to make experiments at the lake in
the gardens of Kensington-palace, for the purpose of comparing, in a
similar manner, water and metal. The basin of this lake is artificial; the
water is supplied by the Chelsea Company; no springs run into it, and it
presented what I required, namely, a uniform mass of still pure water, with
banks ranging nearly from east to west, and from north to south.
185. Two perfectly clean bright copper plates, each exposing four square
feet of surface, were soldered to the extremities of a copper wire; the
plates were immersed in the water, north and south of each other, the wire
which connected them being arranged upon the grass of the bank. The plates
were about four hundred and eighty feet from each other, in a right line;
the wire was probably six hundred feet long. This wire was then divided in
the middle, and connected by two cups of mercury with a delicate
galvanometer.
186. At first, indications of electric currents were obtained; but when
these were tested by inverting the direction of contact, and in other ways,
they were found to be due to other causes than the one sought for. A little
difference in temperature; a minute portion of the nitrate of mercury used
to amalgamate the wires, entering into the water employed to reduce the two
cups of mercury to the same temperature; was sufficient to produce currents
of electricity, which affected the galvanometer, notwithstanding they had
to pass through nearly five hundred feet of water. When these and other
interfering causes were guarded against, no effect was obtained; and it
appeared that even such dissimilar substances as water and copper, when
cutting the magnetic curves of the earth with equal velocity, perfectly
neutralized each other's action.
187. Mr. Fox of Falmouth has obtained some highly important results
respecting the electricity of metalliferous veins in the mines of Cornwall,
which have been published in the Philosophical Transactions[A]. I have
examined the paper with a view to ascertain whether any of the effects were
probably referable to magneto-electric induction; but, though unable to
form a very strong opinion, believe they are not. When parallel veins
running east and west were compared, the general tendency of the
electricity _in the wires_ was from north to south; when the comparison was
made between parts towards the surface and at some depth, the current of
electricity in the wires was from above downwards. If there should be any
natural difference in the force of the electric currents produced by
magneto-electric induction in different substances, or substances in
different positions moving with the earth, and which might be rendered
evident by increasing the masses acted upon, then the wires and veins
experimented with by Mr. Fox might perhaps have acted as dischargers to the
electricity of the mass of strata included between them, and the directions
of the currents would agree with those observed as above.
[A] 1830. p. 399.
188. Although the electricity obtained by magneto-electric induction in a
few feet of wire is of but small intensity, and has not yet been observed
except in metals, and carbon in a particular state, still it has power to
pass through brine (23.); and, as increased length in the substance acted
upon produces increase of intensity, I hoped to obtain effects from
extensive moving masses of water, though quiescent water gave none. I made
experiments therefore (by favour) at Waterloo Bridge, extending a copper
wire nine hundred and sixty feet in length upon the parapet of the bridge,
and dropping from its extremities other wires with extensive plates of
metal attached to them to complete contact with the water. Thus the wire
and the water made one conducting circuit; and as the water ebbed or flowed
with the tide, I hoped to obtain currents analogous to those of the brass
ball (161.).
189. I constantly obtained deflections at the galvanometer, but they were
very irregular, and were, in succession, referred to other causes than that
sought for. The different condition of the water as to purity on the two
sides of the river; the difference in temperature; slight differences in
the plates, in the solder used, in the more or less perfect contact made by
twisting or otherwise; all produced effects in turn: and though I
experimented on the water passing through the middle arches only; used
platina plates instead of copper; and took every other precaution, I could
not after three days obtain any satisfactory results.
190. Theoretically, it seems a necessary consequence, that where water is
flowing, there electric currents should be formed; thus, if a line be
imagined passing from Dover to Calais through the sea, and returning
through the land beneath the water to Dover, it traces out a circuit of
conducting matter, one part of which, when the water moves up or down the
channel, is cutting the magnetic curves of the earth, whilst the other is
relatively at rest. This is a repetition of the wire experiment (171.), but
with worse conductors. Still there is every reason to believe that electric
currents do run in the general direction of the circuit described, either
one way or the other, according as the passage of the waters is up or down
the channel. Where the lateral extent of the moving water is enormously
increased, it does not seem improbable that the effect should become
sensible; and the gulf stream may thus, perhaps, from electric currents
moving across it, by magneto-electric induction from the earth, exert a
sensible influence upon the forms of the lines of magnetic variation[A].
[A] Theoretically, even a ship or a boat when passing on the surface
of the water, in northern or southern latitudes, should have currents
of electricity running through it directly across the line of her
motion; or if the water is flowing past the ship at anchor, similar
currents should occur.
191. Though positive results have not yet been obtained by the action of
the earth upon water and aqueous fluids, yet, as the experiments are very
limited in their extent, and as such fluids do yield the current by
artificial magnets (23.), (for transference of the current is proof that it
may be produced (213.),) the supposition made, that the earth produces
these induced currents within itself (181.) in consequence of its diurnal
rotation, is still highly probable (222, 223.); and when it is considered
that the moving masses extend for thousands of miles across the magnetic
curves, cutting them in various directions within its mass, as well as at
the surface, it is possible the electricity may rise to considerable
intensity.
192. I hardly dare venture, even in the most hypothetical form, to ask
whether the Aurora Borealis and Australia may not be the discharge of
electricity, thus urged towards the poles of the earth, from whence it is
endeavouring to return by natural and appointed means above the earth to
the equatorial regions. The non-occurrence of it in very high latitudes is
not at all against the supposition; and it is remarkable that Mr. Fox, who
observed the deflections of the magnetic needle at Falmouth, by the Aurora
Borealis, gives that direction of it which perfectly agrees with the
present view. He states that all the variations at night were towards the
east[A], and this is what would happen if electric currents were setting
from south to north in the earth under the needle, or from north to south
in space above it.
[A] Philosophical Transactions, 1831, p. 202.
§ 6. _General remarks and illustrations of the Force and Direction of
Magneto-electric Induction._
193. In the repetition and variation of Arago's experiment by Messrs.
Babbage, Herschel, and Harris, these philosophers directed their attention
to the differences of force observed amongst the metals and other
substances in their action on the magnet. These differences were very
great[A], and led me to hope that by mechanical combinations of various
metals important results might be obtained (183.). The following
experiments were therefore made, with a view to obtain, if possible, any
such difference of the action of two metals,
[B] Philosophical Transactions, 1825, p. 472; 1831, p.78.
194. A piece of soft iron bonnet-wire covered with cotton was laid bare and
cleaned at one extremity, and there fastened by metallic contact with the
clean end of a copper wire. Both wires were then twisted together like the
strands of a rope, for eighteen or twenty inches; and the remaining parts
being made to diverge, their extremities were connected with the wires of
the galvanometer. The iron wire was about two feet long, the continuation
to the galvanometer being copper.
195. The twisted copper and iron (touching each other nowhere but at the
extremity) were then passed between the poles of a powerful magnet arranged
horse-shoe fashion (fig. 32.); but not the slightest effect was observed at
the galvanometer, although the arrangement seemed fitted to show any
electrical difference between the two metals relative to the action of the
magnet,
196. A soft iron cylinder was then covered with paper at the middle part,
and the twisted portion of the above compound wire coiled as a spiral
around it, the connexion with the galvanometer still being made at the ends
A and B. The iron cylinder was then brought in contact with the poles of a
powerful magnet capable of raising thirty pounds; yet no signs of
electricity appeared at the galvanometer. Every precaution was applied in
making and breaking contact to accumulate effect, but no indications of a
current could be obtained.
197. Copper and tin, copper and zinc, tin and zinc, tin and iron, and zinc
and iron, were tried against each other in a similar manner (194), but not
the slightest sign of electric currents could be procured.
198. Two flat spirals, one of copper and the other of iron, containing each
eighteen inches of wire, were connected with each other and with the
galvanometer, and then put face to face so as to be in contrary directions.
When brought up to the magnetic pole (53.). No electrical indications at
the galvanometer were observed. When one was turned round so that both were
in the same direction, the effect at the galvanometer was very powerful.
199. The compound helix of copper and iron wire formerly described (8.) was
arranged as a double helix, one of the helices being all iron and
containing two hundred and fourteen feet, the other all copper and
continuing two hundred and eight feet. The two similar ends AA of the
copper and iron helix were connected together, and the other ends BB of
each helix connected with the galvanometer; so that when a magnet was
introduced into the centre of the arrangement, the induced currents in the
iron and copper would tend to proceed in contrary directions. Yet when a
magnet was inserted, or a soft iron bar within made a magnet by contact
with poles, no effect at the needle was produced.
200. A glass tube about fourteen inches long was filled with strong
sulphuric acid. Twelve inches of the end of a clean copper wire were bent
up into a bundle and inserted into the tube, so as to make good superficial
contact with the acid, and the rest of the wire passed along the outside of
the tube and away to the galvanometer. A wire similarly bent up at the
extremity was immersed in the other end of the sulphuric acid, and also
connected with the galvanometer, so that the acid and copper wire were in
the same parallel relation to each other in this experiment as iron and
copper were in the first (194). When this arrangement was passed in a
similar manner between the poles of the magnet, not the slightest effect at
the galvanometer could be perceived.
201. From these experiments it would appear, that when metals of different
kinds connected in one circuit are equally subject in every circumstance to
magneto-electric induction, they exhibit exactly equal powers with respect
to the currents which either are formed, or tend to form, in them. The same
even appears to be the case with regard to fluids, and probably all other
substances.
202. Still it seemed impossible that these results could indicate the
relative inductive power of the magnet upon the different metals; for that
the effect should be in some relation to the conducting power seemed a
necessary consequence (139.), and the influence of rotating plates upon
magnets had been found to bear a general relation to the conducting power
of the substance used.
203. In the experiments of rotation (81.), the electric current is excited
and discharged in the same substance, be it a good or bad conductor; but in
the experiments just described the current excited in iron could not be
transmitted but through the copper, and that excited in copper had to pass
through iron: i.e. supposing currents of dissimilar strength to be formed
in the metals proportionate to their conducting power, the stronger current
had to pass through the worst conductor, and the weaker current through the
best.
204. Experiments were therefore made in which different metals insulated
from each other were passed between the poles of the magnet, their opposite
ends being connected with the same end of the galvanometer wire, so that
the currents formed and led away to the galvanometer should oppose each
other; and when considerable lengths of different wires were used, feeble
deflections were obtained.
205. To obtain perfectly satisfactory results a new galvanometer was
constructed, consisting of two independent coils, each containing eighteen
feet of silked copper wire. These coils were exactly alike in shape and
number of turns, and were fixed side by side with a small interval between
them, in which a double needle could be hung by a fibre of silk exactly as
in the former instrument (87.). The coils may be distinguished by the
letters KL, and when electrical currents were sent through them in the same
direction, acted upon the needle with the sum of their powers; when in
opposite directions, with the difference of their powers.
206. The compound helix (199. 8.) was now connected, the ends A and B of
the iron with A and B ends of galvanometer coil K, and the ends A and B of
the copper with B and A ends of galvanometer coil L, so that the currents
excited in the two helices should pass in opposite directions through the
coils K and L. On introducing a small cylinder magnet within the helices,
the galvanometer needle was powerfully deflected. On disuniting the iron
helix, the magnet caused with the copper helix alone still stronger
deflection in the same direction. On reuniting the iron helix, and
unconnecting the copper helix, the magnet caused a moderate deflection in
the contrary direction. Thus it was evident that the electric current
induced by a magnet in a copper wire was far more powerful than the current
induced by the same magnet in an equal iron wire.
207. To prevent any error that might arise from the greater influence, from
vicinity or other circumstances, of one coil on the needle beyond that of
the other, the iron and copper terminations were changed relative to the
galvanometer coils KL, so that the one which before carried the current
from the copper now conveyed that from the iron, and vice versa. But the
same striking superiority of the copper was manifested as before. This
precaution was taken in the rest of the experiments with other metals to be
described.
208. I then had wires of iron, zinc, copper, tin, and lead, drawn to the
same diameter (very nearly one twentieth of an inch), and I compared
exactly equal lengths, namely sixteen feet, of each in pairs in the
following manner: The ends of the copper wire were connected with the ends
A and B of galvanometer coil K, and the ends of the zinc wire with the
terminations A and B of the galvanometer coil L. The middle part of each
wire was then coiled six times round a cylinder of soft iron covered with
paper, long enough to connect the poles of Daniell's horse-shoe magnet
(56.) (fig. 33.), so that similar helices of copper and zinc, each of six
turns, surrounded the bar at two places equidistant from each other and
from the poles of the magnet; but these helices were purposely arranged so
as to be in contrary directions, and therefore send contrary currents
through the galvanometer coils K and L,
209. On making and breaking contact between the soft iron bar and the poles
of the magnet, the galvanometer was strongly affected; on detaching the
zinc it was still more strongly affected in the same direction. On taking
all the precautions before alluded to (207.), with others, it was
abundantly proved that the current induced by the magnet in copper was far
more powerful than in zinc.
210. The copper was then compared in a similar manner with tin, lead, and
iron, and surpassed them all, even more than it did zinc. The zinc was then
compared experimentally with the tin, lead, and iron, and found to produce
a more powerful current than any of them. Iron in the same manner proved
superior to tin and lead. Tin came next, and lead the last.
211. Thus the order of these metals is copper, zinc, iron, tin, and lead.
It is exactly their order with respect to conducting power for electricity,
and, with the exception of iron, is the order presented by the
magneto-rotation experiments of Messrs. Babbage, Herschel, Harris, &c. The
iron has additional power in the latter kind of experiments, because of its
ordinary magnetic relations, and its place relative to magneto-electric
action of the kind now under investigation cannot be ascertained by such
trials. In the manner above described it may be correctly ascertained[A].
[A] Mr. Christie, who being appointed reporter upon this paper, had it
in his hands before it was complete, felt the difficulty (202.); and
to satisfy his mind, made experiments upon iron and copper with the
large magnet(44.), and came to the same conclusions as I have arrived
at. The two sets of experiments were perfectly independent of each
other, neither of us being aware of the other's proceedings.
212. It must still be observed that in these experiments the whole effect
between different metals is not obtained; for of the thirty-four feet of
wire included in each circuit, eighteen feet are copper in both, being the
wire of the galvanometer coils; and as the whole circuit is concerned in
the resulting force of the current, tin's circumstance must tend to
diminish the difference which would appear between the metals if the
circuits were of the same substances throughout. In the present case the
difference obtained is probably not more than a half of that which would be
given if the whole of each circuit were of one metal.
213. These results tend to prove that the currents produced by
magneto-electric induction in bodies is proportional to their conducting
power. That they are _exactly_ proportional to and altogether dependent
upon the conducting power, is, I think, proved by the perfect neutrality
displayed when two metals or other substances, as acid, water, &c. &c.
(201. 186.), are opposed to each other in their action. The feeble current
which tends to be produced in the worse conductor, has its transmission
favoured in the better conductor, and the stronger current which tends to
form in the latter has its intensity diminished by the obstruction of the
former; and the forces of generation and obstruction are so perfectly
neutralize each other exactly. Now as the obstruction is inversely as the
balanced as to conducting power, the tendency to generate a current must
be directly as that power to produce this perfect equilibrium.
214. The cause of the equality of action under the various circumstances
described, where great extent of wire (183.) or wire and water (181.) were
connected together, which yet produced such different effects upon the
magnet, is now evident and simple.
215. The effects of a rotating substance upon a needle or magnet ought,
where ordinary magnetism has no influence, to be directly as the conducting
power of the substance; and I venture now to predict that such will be
found to be the case; and that in all those instances where non-conductors
have been supposed to exhibit this peculiar influence, the motion has been
due to some interfering cause of an ordinary kind; as mechanical
communication of motion through the parts of the apparatus, or otherwise
(as in the case Mr. Harris has pointed out[A]); or else to ordinary
magnetic attractions. To distinguish the effects of the latter from those
of the induced electric currents, I have been able to devise a most perfect
test, which shall be almost immediately described (243.).
[A] Philosophical Transactions, 1831. p. 68.
216. There is every reason to believe that the magnet or magnetic needle
will become an excellent measurer of the conducting power of substances
rotated near it; for I have found by careful experiment, that when a
constant current of electricity was sent successively through a series of
wires of copper, platina, zinc, silver, lead, and tin, drawn to the same
diameter; the deflection of the needle was exactly equal by them all. It
must be remembered that when bodies are rotated in a horizontal plane, the
magnetism of the earth is active upon them. As the effect is general to the
whole of the plate, it may not interfere in these cases; but in some
experiments and calculations may be of important consequence.
217. Another point which I endeavoured to ascertain, was, whether it was
essential or not that the moving part of the wire should, in cutting the
magnetic curves, pass into positions of greater or lesser magnetic force;
or whether, always intersecting curves of equal magnetic intensity, the
mere motion was sufficient for the production of the current. That the
latter is true, has been proved already in several of the experiments on
terrestrial magneto-electric induction. Thus the electricity evolved from
the copper plate (149.), the currents produced in the rotating globe (161,
&c.), and those passing through the moving wire (171.), are all produced
under circumstances in which the magnetic force could not but be the same
during the whole experiments.
218. To prove the point with an ordinary magnet, a copper disc was cemented
upon the end of a cylinder magnet, with paper intervening; the magnet and
disc were rotated together, and collectors (attached to the galvanometer)
brought in contact with the circumference and the central part of the
copper plate. The galvanometer needle moved as in former cases, and the
_direction_ of motion was the _same_ as that which would have resulted, if
the copper only had revolved, and the magnet been fixed. Neither was there
any apparent difference in the quantity of deflection. Hence, rotating the
magnet causes no difference in the results; for a rotatory and a stationary
magnet produce the same effect upon the moving copper.
219. A copper cylinder, closed at one extremity, was then put over the
magnet, one half of which it inclosed like a cap; it was firmly fixed, and
prevented from touching the magnet anywhere by interposed paper. The
arrangement was then floated in a narrow jar of mercury, so that the lower
edge of the copper cylinder touched the fluid metal; one wire of the
galvanometer dipped into this mercury, and the other into a little cavity
in the centre of the end of the copper cap. Upon rotating the magnet and
its attached cylinder, abundance of electricity passed through the
galvanometer, and in the same direction as if the cylinder had rotated
only, the magnet being still. The results therefore were the same as those
with the disc (218.).
220. That the metal of the magnet itself might be substituted for the
moving cylinder, disc, or wire, seemed an inevitable consequence, and yet
one which would exhibit the effects of magneto-electric induction in a
striking form. A cylinder magnet had therefore a little hole made in the
centre of each end to receive a drop of mercury, and was then floated pole
upwards in the same metal contained in a narrow jar. One wire from the
galvanometer dipped into the mercury of the jar, and the other into the
drop contained in the hole at the upper extremity of the axis. The magnet
was then revolved by a piece of string passed round it, and the
galvanometer-needle immediately indicated a powerful current of
electricity. On reversing the order of rotation, the electrical current was
reversed. The direction of the electricity was the same as if the copper
cylinder (219.) or a copper wire had revolved round the fixed magnet in the
same direction as that which the magnet itself had followed. Thus a
_singular independence_ of the magnetism and the bar in which it resides is
rendered evident.
221. In the above experiment the mercury reached about halfway up the
magnet; but when its quantity was increased until within one eighth of an
inch of the top, or diminished until equally near the bottom, still the
same effects and the _same direction_ of electrical current was obtained.
But in those extreme proportions the effects did not appear so strong as
when the surface of the mercury was about the middle, or between that and
an inch from each end. The magnet was eight inches and a half long, and
three quarters of an inch in diameter.
222. Upon inversion of the magnet, and causing rotation in the same
direction, i.e. always screw or always unscrew, then a contrary current of
electricity was produced. But when the motion of the magnet was continued
in a direction constant in relation to its _own axis_, then electricity of
the same kind was collected at both poles, and the opposite electricity at
the equator, or in its neighbourhood, or in the parts corresponding to it.
If the magnet be held parallel to the axis of the earth, with its unmarked
pole directed to the pole star, and then rotated so that the parts at its
southern side pass from west to east in conformity to the motion of the
earth; then positive electricity may be collected at the extremities of the
magnet, and negative electricity at or about the middle of its mass.
223. When the galvanometer was very sensible, the mere spinning of the
magnet in the air, whilst one of the galvanometer wires touched the
extremity, and the other the equatorial parts, was sufficient to evolve a
current of electricity and deflect the needle.
224. Experiments were then made with a similar magnet, for the purpose of
ascertaining whether any return of the electric current could occur at the
central or axial parts, they having the same angular velocity of rotation
as the other parts (259.) the belief being that it could not.
225. A cylinder magnet, seven inches in length, and three quarters of an
inch in diameter, had a hole pierced in the direction of its axis from one
extremity, a quarter of an inch in diameter, and three inches deep. A
copper cylinder, surrounded by paper and amalgamated at both extremities,
was introduced so as to be in metallic contact at the bottom of the hole,
by a little mercury, with the middle of the magnet; insulated at the sides
by the paper; and projecting about a quarter of an inch above the end of
the steel. A quill was put over the copper rod, which reached to the paper,
and formed a cup to receive mercury for the completion of the circuit. A
high paper edge was also raised round that end of the magnet and mercury
put within it, which however had no metallic connexion with that in the
quill, except through the magnet itself and the copper rod (fig. 34.). The
wires A and B from the galvanometer were dipped into these two portions of
mercury; any current through them could, therefore, only pass down the
magnet towards its equatorial parts, and then up the copper rod; or vice
versa.
226. When thus arranged and rotated screw fashion, the marked end of the
galvanometer needle went west, indicating that there was a current through
the instrument from A to B and consequently from B through the magnet and
copper rod to A (fig. 34.).
227. The magnet was then put into a jar of mercury (fig. 35.) as before
(219.); the wire A left in contact with the copper axis, but the wire B
dipped in the mercury of the jar, and therefore in metallic communication
with the equatorial parts of the magnet instead of its polar extremity. On
revolving the magnet screw fashion, the galvanometer needle was deflected
in the same direction as before, but far more powerfully. Yet it is evident
that the parts of the magnet from the equator to the pole were out of the
electric circuit.
228. Then the wire A was connected with the mercury on the extremity of the
magnet, the wire B still remaining in contact with that in the jar (fig.
36.), so that the copper axis was altogether out of the circuit. The magnet
was again revolved screw fashion, and again caused the same deflection of
the needle, the current being as strong as it was in the last trial (227.),
and much stronger than at first (226.).
229. Hence it is evident that there is no discharge of the current at the
centre of the magnet, for the current, now freely evolved, is up through
the magnet; but in the first experiment (226.) it was down. In fact, at
that time, it was only the part of the moving metal equal to a little disc
extending from the end of the wire B in the mercury to the wire A that was
efficient, i.e. moving with a different angular velocity to the rest of the
circuit (258.); and for that portion the direction of the current is
consistent with the other results.
230. In the two after experiments, the _lateral_ parts of the magnet or of
the copper rod are those which move relative to the other parts of the
circuit, i.e. the galvanometer wires; and being more extensive,
intersecting more curves, or moving with more velocity, produce the greater
effect. For the discal part, the direction of the induced electric current
is the same in all, namely, from the circumference towards the centre.
* * * * *
231. The law under which the induced electric current excited in bodies
moving relatively to magnets, is made dependent on the intersection of the
magnetic curves by the metal (114.) being thus rendered more precise and
definite (217. 220. 224.), seem now even to apply to the cause in the first
section of the former paper (26.); and by rendering a perfect reason for
the effects produced, take away any for supposing that peculiar condition,
which I ventured to call the electro-tonic state (60.).
232. When an electrical current is passed through a wire, that wire is
surrounded at every part by magnetic curves, diminishing in intensity
according to their distance from the wire, and which in idea may be likened
to rings situated in planes perpendicular to the wire or rather to the
electric current within it. These curves, although different in form, are
perfectly analogous to those existing between two contrary magnetic poles
opposed to each other; and when a second wire, parallel to that which
carries the current, is made to approach the latter (18.), it passes
through magnetic curves exactly of the same kind as those it would
intersect when carried between opposite magnetic poles (109.) in one
direction; and as it recedes from the inducing wire, it cuts the curves
around it in the same manner that it would do those between the same poles
if moved in the other direction.
233. If the wire NP (fig. 40.) have an electric current passed through it
in the direction from P to N, then the dotted ring may represent a magnetic
curve round it, and it is in such a direction that if small magnetic
needles lie placed as tangents to it, they will become arranged as in the
figure, _n_ and _s_ indicating north and south ends (14. _note_.).
234. But if the current of electricity were made to cease for a while, and
magnetic poles were used instead to give direction to the needles, and make
them take the same position as when under the influence of the current,
then they must be arranged as at fig. 41; the marked and unmarked poles
_ab_ above the wire, being in opposite directions to those _a'b'_ below. In
such a position therefore the magnetic curves between the poles _ab_ and
_a'b'_ have the same general direction with the corresponding parts of the
ring magnetic curve surrounding the wire NP carrying an electric current.
235. If the second wire _pn_ (fig. 40.) be now brought towards the
principal wire, carrying a current, it will cut an infinity of magnetic
curves, similar in direction to that figured, and consequently similar in
direction to those between the poles _ab_ of the magnets (fig. 41.), and it
will intersect these current curves in the same manner as it would the
magnet curves, if it passed from above between the poles downwards. Now,
such an intersection would, with the magnets, induce an electric current in
the wire from _p_ to _n_ (114.); and therefore as the curves are alike in
arrangement, the same effect ought to result from the intersection of the
magnetic curves dependent on the current in the wire NP; and such is the
case, for on approximation the induced current is in the opposite direction
to the principal current (19.).
236. If the wire _p'n'_ be carried up from below, it will pass in the
opposite direction between the magnetic poles; but then also the magnetic
poles themselves are reversed (fig. 41.), and the induced current is
therefore (114.) still in the same direction as before. It is also, for
equally sufficient and evident reasons, in the same direction, if produced
by the influence of the curves dependent upon the wire.
237. When the second wire is retained at rest in the vicinity the principal
wire, no current is induced through it, for it is intersecting no magnetic
curves. When it is removed from the principal wire, it intersects the
curves in the opposite direction to what it did before (235.); and a
current in the opposite direction is induced, which therefore corresponds
with the direction of the principal current (19.). The same effect would
take place if by inverting the direction of motion of the wire in passing
between either set of poles (fig. 41.), it were made to intersect the
curves there existing in the opposite direction to what it did before.
238. In the first experiments (10. 13.), the inducing wire and that under
induction were arranged at a fixed distance from each other, and then an
electric current sent through the former. In such cases the magnetic curves
themselves must be considered as moving (if I may use the expression)
across the wire under induction, from the moment at which they begin to be
developed until the magnetic force of the current is at its utmost;
expanding as it were from the wire outwards, and consequently being in the
same relation to the fixed wire under induction as if _it_ had moved in the
opposite direction across them, or towards the wire carrying the current.
Hence the first current induced in such cases was in the contrary direction
to the principal current (17. 235.). On breaking the battery contact, the
magnetic curves (which are mere expressions for arranged magnetic forces)
may be conceived as contracting upon and returning towards the failing
electrical current, and therefore move in the opposite direction across the
wire, and cause an opposite induced current to the first.
239. When, in experiments with ordinary magnets, the latter, in place of
being moved past the wires, were actually made near them (27. 36.), then a
similar progressive development of the magnetic curves may be considered as
having taken place, producing the effects which would have occurred by
motion of the wires in one direction; the destruction of the magnetic power
corresponds to the motion of the wire in the opposite direction.
240. If, instead of intersecting the magnetic curves of a straight wire
carrying a current, by approximating or removing a second wire (235.), a
revolving plate be used, being placed for that purpose near the wire, and,
as it were, amongst the magnetic curves, then it ought to have continuous
electric currents induced within it; and if a line joining the wire with
the centre of the plate were perpendicular to both, then the induced
current ought to be, according to the law (114.), directly across the
plate, from one side to the other, and at right angles to the direction of
the inducing current.
241. A single metallic wire one twentieth of an inch in diameter had an
electric current passed through it, and a small copper disc one inch and a
half in diameter revolved near to and under, but not in actual contact with
it (fig. 39). Collectors were then applied at the opposite edges of the
disc, and wires from them connected with the galvanometer. As the disc
revolved in one direction, the needle was deflected on one side: and when
the direction of revolution was reversed, the needle was inclined on the
other side, in accordance with the results anticipated.
242. Thus the reasons which induce me to suppose a particular state in the
wire (60.) have disappeared; and though it still seems to me unlikely that
a wire at rest in the neighbourhood of another carrying a powerful electric
current is entirely indifferent to it, yet I am not aware of any distinct
_facts_ which authorize the conclusion that it is in a particular state.
* * * * *
243. In considering the nature of the cause assigned in these papers to
account for the mutual influence of magnets and moving metals (120.), and
comparing it with that heretofore admitted, namely, the induction of a
feeble magnetism like that produced in iron, it occurred to me that a most
decisive experimental test of the two views could be applied (215.).
244. No other known power has like direction with that exerted between an
electric current and a magnetic pole; it is tangential, while all other
forces, acting at a distance, are direct. Hence, if a magnetic pole on one
side of a revolving plate follow its course by reason of its obedience to
the tangential force exerted upon it by the very current of electricity
which it has itself caused, a similar pole on the opposite side of the
plate should immediately set it free from this force; for the currents
which tend to be formed by the action of the two poles are in opposite
directions; or rather no current tends to be formed, or no magnetic curves
are intersected (114.); and therefore the magnet should remain at rest. On
the contrary, if the action of a north magnetic pole were to produce a
southness in the nearest part of the copper plate, and a diffuse northness
elsewhere (82.), as is really the case with iron; then the use of another
north pole on the opposite side of the same part of the plate should double
the effect instead of destroying it, and double the tendency of the first
magnet to move with the plate.
245. A thick copper plate (85.) was therefore fixed on a vertical axis, a
bar magnet was suspended by a plaited silk cord, so that its marked pole
hung over the edge of the plate, and a sheet of paper being interposed, the
plate was revolved; immediately the magnetic pole obeyed its motion and
passed off in the same direction. A second magnet of equal size and
strength was then attached to the first, so that its marked pole should
hang _beneath_ the edge of the copper plate in a corresponding position to
that above, and at an equal distance (fig. 37.). Then a paper sheath or
screen being interposed as before, and the plate revolved, the poles were
found entirely indifferent to its motion, although either of them alone
would have followed the course of rotation.
246. On turning one magnet round, so that _opposite_ poles were on each
side of the plate, then the mutual action of the poles and the moving metal
was a maximum.
247. On suspending one magnet so that its axis was level with the plate,
and either pole opposite its edge, the revolution of the plate caused no
motion of the magnet. The electrical currents dependent upon induction
would now tend to be produced in a vertical direction across the thickness
of the plate, but could not be so discharged, or at least only to so slight
a degree as to leave all effects insensible; but ordinary magnetic
induction, or that on an iron plate, would be equally if not more
powerfully developed in such a position (251.).
248. Then, with regard to the production of electricity in these
cases:--whenever motion was communicated by the plate to the magnets,
currents existed; when it was not communicated, they ceased. A marked pole
of a large bar magnet was put under the edge of the plate; collectors (86.)
applied at the axis and edge of the plate as on former occasions (fig.
38.), and these connected with the galvanometer; when the plate was
revolved, abundance of electricity passed to the instrument. The unmarked
pole of a similar magnet was then put over the place of the former pole, so
that contrary poles were above and below; on revolving the plate, the
electricity was more powerful than before. The latter magnet was then
turned end for end, so that marked poles were both above and below the
plate, and then, upon revolving it, scarcely any electricity was procured.
By adjusting the distance of the poles so as to correspond with their
relative force, they at last were brought so perfectly to neutralize each
other's inductive action upon the plate, that no electricity could be
obtained with the most rapid motion.
249. I now proceeded to compare the effect of similar and dissimilar poles
upon iron and copper, adopting for the purpose Mr. Sturgeon's very useful
form of Arago's experiment. This consists in a circular plate of metal
supported in a vertical plane by a horizontal axis, and weighted a little
at one edge or rendered excentric so as to vibrate like a pendulum. The
poles of the magnets are applied near the side and edges of these plates,
and then the number of vibrations, required to reduce the vibrating arc a
certain constant quantity, noted. In the first description of this
instrument[A] it is said that opposite poles produced the greatest
retarding effect, and similar poles none; and yet within a page of the
place the effect is considered as of the same kind with that produced in
iron.
[A] Edin. Phil. Journal, 1825, p. 124.
250. I had two such plates mounted, one of copper, one of iron. The copper
plate alone gave sixty vibrations, in the average of several experiments,
before the arc of vibration was reduced from one constant mark to another.
On placing opposite magnetic poles near to, and on each side of, the same
place, the vibrations were reduced to fifteen. On putting similar poles on
each side of it, they rose to fifty; and on placing two pieces of wood of
equal size with the poles equally near, they became fifty-two. So that,
when similar poles were used, the magnetic effect was little or none, (the
obstruction being due to the confinement of the air, rather,) whilst with
opposite poles it was the greatest possible. When a pole was presented to
the edge of the plate, no retardation occurred.
251. The iron plate alone made thirty-two vibrations, whilst the arc of
vibration diminished a certain quantity. On presenting a magnetic pole to
the edge of the plate (247.), the vibrations were diminished to eleven; and
when the pole was about half an inch from the edge, to five.
252. When the marked pole was put at the side of the iron plate at a
certain distance, the number of vibrations was only five. When the marked
pole of the second bar was put on the opposite side of the plate at the
same distance (250.), the vibrations were reduced to two. But when the
second pole was an unmarked one, yet occupying exactly the same position,
the vibrations rose to twenty-two. By removing the stronger of these two
opposite poles a little way from the plate, the vibrations increased to
thirty-one, or nearly the original number. But on removing it _altogether_,
they fell to between five and six.
253. Nothing can be more clear, therefore, than that with iron, and bodies
admitting of ordinary magnetic induction, _opposite_ poles on opposite
sides of the edge of the plate neutralize each other's effect, whilst
_similar_ poles exalt the action; a single pole end on is also sufficient.
But with copper, and substances not sensible to ordinary magnetic
impressions, _similar_ poles on opposite sides of the plate neutralize each
other; _opposite_ poles exalt the action; and a single pole at the edge or
end on does nothing.
254. Nothing can more completely show the thorough independence of the
effects obtained with the metals by Arago, and those due to ordinary
magnetic forces; and henceforth, therefore, the application of two poles to
various moving substances will, if they appear at all magnetically
affected, afford a proof of the nature of that affection. If opposite poles
produce a greater effect than one pole, the result will be due to electric
currents. If similar poles produce more effect than one, then the power is
_not_ electrical; it is not like that active in the metals and carbon when
they are moving, and in most cases will probably be found to be not even
magnetical, but the result of irregular causes not anticipated and
consequently not guarded against.
255. The result of these investigations tends to show that there are really
but very few bodies that are magnetic in the manner of iron. I have often
sought for indications of this power in the common metals and other
substances; and once in illustration of Arago's objection (82.), and in
hopes of ascertaining the existence of currents in metals by the momentary
approach of a magnet, suspended a disc of copper by a single fibre of silk
in an excellent vacuum, and approximated powerful magnets on the outside of
the jar, making them approach and recede in unison with a pendulum that
vibrated as the disc would do: but no motion could be obtained; not merely,
no indication of ordinary magnetic powers, but none or _any electric
current_ occasioned in the metal by the approximation and recession of the
magnet. I therefore venture to arrange substances in three classes as
regards their relation to magnets; first, those which are affected when at
rest, like iron, nickel, &c., being such as possess ordinary magnetic
properties; then, those which are affected when in motion, being conductors
of electricity in which are produced electric currents by the inductive
force of the magnet; and, lastly, those which are perfectly indifferent to
the magnet, whether at rest or in motion.
256. Although it will require further research, and probably close
investigation, both experimental and mathematical, before the exact mode of
action between a magnet and metal moving relatively to each other is
ascertained; yet many of the results appear sufficiently clear and simple
to allow of expression in a somewhat general manner.--If a terminated wire
move so as to cut a magnetic curve, a power is called into action which
tends to urge an electric current through it; but this current cannot be
brought into existence unless provision be made at the ends of the wire for
its discharge and renewal.
257. If a second wire move in the same direction as the first, the same
power is exerted upon it, and it is therefore unable to alter the condition
of the first: for there appear to be no natural differences among
substances when connected in a series, by which, when moving under the same
circumstances relative to the magnet, one tends to produce a more powerful
electric current in the whole circuit than another (201. 214.).
258. But if the second wire move with a different velocity, or in some
other direction, then variations in the force exerted take place; and if
connected at their extremities, an electric current passes through them.
259. Taking, then, a mass of metal or an endless wire, and referring to the
pole of the magnet as a centre of action, (which though perhaps not
strictly correct may be allowed for facility of expression, at present,) if
all parts move in the same direction, and with the same angular velocity,
and through magnetic curves of constant intensity, then no electric
currents are produced. This point is easily observed with masses subject to
the earth's magnetism, and may be proved with regard to small magnets; by
rotating them, and leaving the metallic arrangements stationary, no current
is produced.
260. If one part of the wire or metal cut the magnetic curves, whilst the
other is stationary, then currents are produced. All the results obtained
with the galvanometer are more or less of this nature, the galvanometer
extremity being the fixed part. Even those with the wire, galvanometer, and
earth (170.), may be considered so without any error in the result.
261. If the motion of the metal be in the same direction, but the angular
velocity of its parts relative to the pole of the magnet different, then
currents are produced. This is the case in Arago's experiment, and also in
the wire subject to the earth's induction (172.), when it was moved from
west to east.
262. If the magnet moves not directly to or from the arrangement, but
laterally, then the case is similar to the last.
263. If different parts move in opposite directions across the magnetic
curves, then the effect is a maximum for equal velocities.
264. All these in fact are variations of one simple condition, namely, that
all parts of the mass shall not move in the same direction across the
curves, and with the same angular velocity. But they are forms of
expression which, being retained in the mind, I have found useful when
comparing the consistency of particular phenomena with general results.
_Royal Institution,
December 21, 1831._