§ 1. _On the Induction of Electric Currents._ § 2. _On the Evolution of
Electricity from Magnetism._ § 3. _On a new Electrical Condition of
Matter._ § 4. _On_ Arago's _Magnetic Phenomena._
[Read November 24, 1831.]
1. The power which electricity of tension possesses of causing an opposite
electrical state in its vicinity has been expressed by the general term
Induction; which, as it has been received into scientific language, may
also, with propriety, be used in the same general sense to express the
power which electrical currents may possess of inducing any particular
state upon matter in their immediate neighbourhood, otherwise indifferent.
It is with this meaning that I purpose using it in the present paper.
2. Certain effects of the induction of electrical currents have already
been recognised and described: as those of magnetization; Ampère's
experiments of bringing a copper disc near to a flat spiral; his repetition
with electro-magnets of Arago's extraordinary experiments, and perhaps a
few others. Still it appeared unlikely that these could be all the effects
which induction by currents could produce; especially as, upon dispensing
with iron, almost the whole of them disappear, whilst yet an infinity of
bodies, exhibiting definite phenomena of induction with electricity of
tension, still remain to be acted upon by the induction of electricity in
motion.
3. Further: Whether Ampère's beautiful theory were adopted, or any other,
or whatever reservation were mentally made, still it appeared very
extraordinary, that as every electric current was accompanied by a
corresponding intensity of magnetic action at right angles to the current,
good conductors of electricity, when placed within the sphere of this
action, should not have any current induced through them, or some sensible
effect produced equivalent in force to such a current.
4. These considerations, with their consequence, the hope of obtaining
electricity from ordinary magnetism, have stimulated me at various times to
investigate experimentally the inductive effect of electric currents. I
lately arrived at positive results; and not only had my hopes fulfilled,
but obtained a key which appeared to me to open out a full explanation of
Arago's magnetic phenomena, and also to discover a new state, which may
probably have great influence in some of the most important effects of
electric currents.
5. These results I purpose describing, not as they were obtained, but in
such a manner as to give the most concise view of the whole.
§ 1. _Induction of Electric Currents._
6. About twenty-six feet of copper wire one twentieth of an inch in
diameter were wound round a cylinder of wood as a helix, the different
spires of which were prevented from touching by a thin interposed twine.
This helix was covered with calico, and then a second wire applied in the
same manner. In this way twelve helices were superposed, each containing an
average length of wire of twenty-seven feet, and all in the same direction.
The first, third, fifth, seventh, ninth, and eleventh of these helices were
connected at their extremities end to end, so as to form one helix; the
others were connected in a similar manner; and thus two principal helices
were produced, closely interposed, having the same direction, not touching
anywhere, and each containing one hundred and fifty-five feet in length of
wire.
7. One of these helices was connected with a galvanometer, the other with a
voltaic battery of ten pairs of plates four inches square, with double
coppers and well charged; yet not the slightest sensible reflection of the
galvanometer-needle could be observed.
8. A similar compound helix, consisting of six lengths of copper and six of
soft iron wire, was constructed. The resulting iron helix contained two
hundred and fourteen feet of wire, the resulting copper helix two hundred
and eight feet; but whether the current from the trough was passed through
the copper or the iron helix, no effect upon the other could be perceived
at the galvanometer.
9. In these and many similar experiments no difference in action of any
kind appeared between iron and other metals.
10. Two hundred and three feet of copper wire in one length were coiled
round a large block of wood; other two hundred and three feet of similar
wire were interposed as a spiral between the turns of the first coil, and
metallic contact everywhere prevented by twine. One of these helices was
connected with a galvanometer, and the other with a battery of one hundred
pairs of plates four inches square, with double coppers, and well charged.
When the contact was made, there was a sudden and very slight effect at the
galvanometer, and there was also a similar slight effect when the contact
with the battery was broken. But whilst the voltaic current was continuing
to pass through the one helix, no galvanometrical appearances nor any
effect like induction upon the other helix could be perceived, although the
active power of the battery was proved to be great, by its heating the
whole of its own helix, and by the brilliancy of the discharge when made
through charcoal.
11. Repetition of the experiments with a battery of one hundred and twenty
pairs of plates produced no other effects; but it was ascertained, both at
this and the former time, that the slight deflection of the needle
occurring at the moment of completing the connexion, was always in one
direction, and that the equally slight deflection produced when the contact
was broken, was in the other direction; and also, that these effects
occurred when the first helices were used (6. 8.).
12. The results which I had by this time obtained with magnets led me to
believe that the battery current through one wire, did, in reality, induce
a similar current through the other wire, but that it continued for an
instant only, and partook more of the nature of the electrical wave passed
through from the shock of a common Leyden jar than of the current from a
voltaic battery, and therefore might magnetise a steel needle, although it
scarcely affected the galvanometer.
13. This expectation was confirmed; for on substituting a small hollow
helix, formed round a glass tube, for the galvanometer, introducing a steel
needle, making contact as before between the battery and the inducing wire
(7. 10.), and then removing the needle before the battery contact was
broken, it was found magnetised.
14. When the battery contact was first made, then an unmagnetised needle
introduced into the small indicating helix (13.), and lastly the battery
contact broken, the needle was found magnetised to an equal degree
apparently as before; but the poles were of the contrary kind.
15. The same effects took place on using the large compound helices first
described (6. 8.).
16. When the unmagnetised needle was put into the indicating helix, before
contact of the inducing wire with the battery, and remained there until the
contact was broken, it exhibited little or no magnetism; the first effect
having been nearly neutralised by the second (13. 14.). The force of the
induced current upon making contact was found always to exceed that of the
induced current at breaking of contact; and if therefore the contact was
made and broken many times in succession, whilst the needle remained in the
indicating helix, it at last came out not unmagnetised, but a needle
magnetised as if the induced current upon making contact had acted alone on
it. This effect may be due to the accumulation (as it is called) at the
poles of the unconnected pile, rendering the current upon first making
contact more powerful than what it is afterwards, at the moment of breaking
contact.
17. If the circuit between the helix or wire under induction and the
galvanometer or indicating spiral was not rendered complete _before_ the
connexion between the battery and the inducing wire was completed or
broken, then no effects were perceived at the galvanometer. Thus, if the
battery communications were first made, and then the wire under induction
connected with the indicating helix, no magnetising power was there
exhibited. But still retaining the latter communications, when those with
the battery were broken, a magnet was formed in the helix, but of the
second kind (14.), i.e. with poles indicating a current in the same
direction to that belonging to the battery current, or to that always
induced by that current at its cessation.
18. In the preceding experiments the wires were placed near to each other,
and the contact of the inducing one with the buttery made when the
inductive effect was required; but as the particular action might be
supposed to be exerted only at the moments of making and breaking contact,
the induction was produced in another way. Several feet of copper wire were
stretched in wide zigzag forms, representing the letter W, on one surface
of a broad board; a second wire was stretched in precisely similar forms on
a second board, so that when brought near the first, the wires should
everywhere touch, except that a sheet of thick paper was interposed. One of
these wires was connected with the galvanometer, and the other with a
voltaic battery. The first wire was then moved towards the second, and as
it approached, the needle was deflected. Being then removed, the needle was
deflected in the opposite direction. By first making the wires approach and
then recede, simultaneously with the vibrations of the needle, the latter
soon became very extensive; but when the wires ceased to move from or
towards each other, the galvanometer-needle soon came to its usual
position.
19. As the wires approximated, the induced current was in the _contrary_
direction to the inducing current. As the wires receded, the induced
current was in the _same_ direction as the inducing current. When the wires
remained stationary, there was no induced current (54.).
20. When a small voltaic arrangement was introduced into the circuit
between the galvanometer (10.) and its helix or wire, so as to cause a
permanent deflection of 30° or 40°, and then the battery of one hundred
pairs of plates connected with the inducing wire, there was an
instantaneous action as before (11.); but the galvanometer-needle
immediately resumed and retained its place unaltered, notwithstanding the
continued contact of the inducing wire with the trough: such was the case
in whichever way the contacts were made (33.).
21. Hence it would appear that collateral currents, either in the same or
in opposite directions, exert no permanent inducing power on each other,
affecting their quantity or tension.
22. I could obtain no evidence by the tongue, by spark, or by heating fine
wire or charcoal, of the electricity passing through the wire under
induction; neither could I obtain any chemical effects, though the contacts
with metallic and other solutions were made and broken alternately with
those of the battery, so that the second effect of induction should not
oppose or neutralise the first (13. 16.).
23. This deficiency of effect is not because the induced current of
electricity cannot pass fluids, but probably because of its brief duration
and feeble intensity; for on introducing two large copper plates into the
circuit on the induced side (20.), the plates being immersed in brine, but
prevented from touching each other by an interposed cloth, the effect at
the indicating galvanometer, or helix, occurred as before. The induced
electricity could also pass through a voltaic trough (20.). When, however,
the quantity of interposed fluid was reduced to a drop, the galvanometer
gave no indication.
24. Attempts to obtain similar effects by the use of wires conveying
ordinary electricity were doubtful in the results. A compound helix similar
to that already described, containing eight elementary helices (6.), was
used. Four of the helices had their similar ends bound together by wire,
and the two general terminations thus produced connected with the small
magnetising helix containing an unmagnetised needle (13.). The other four
helices were similarly arranged, but their ends connected with a Leyden
jar. On passing the discharge, the needle was found to be a magnet; but it
appeared probable that a part of the electricity of the jar had passed off
to the small helix, and so magnetised the needle. There was indeed no
reason to expect that the electricity of a jar possessing as it does great
tension, would not diffuse itself through all the metallic matter
interposed between the coatings.
25. Still it does not follow that the discharge of ordinary electricity
through a wire does not produce analogous phenomena to those arising from
voltaic electricity; but as it appears impossible to separate the effects
produced at the moment when the discharge begins to pass, from the equal
and contrary effects produced when it ceases to pass (16.), inasmuch as
with ordinary electricity these periods are simultaneous, so there can be
scarcely any hope that in this form of the experiment they can be
perceived.
26. Hence it is evident that currents of voltaic electricity present
phenomena of induction somewhat analogous to those produced by electricity
of tension, although, as will be seen hereafter, many differences exist
between them. The result is the production of other currents, (but which
are only momentary,) parallel, or tending to parallelism, with the inducing
current. By reference to the poles of the needle formed in the indicating
helix (13. 14.) and to the deflections of the galvanometer-needle (11.), it
was found in all cases that the induced current, produced by the first
action of the inducing current, was in the contrary direction to the
latter, but that the current produced by the cessation of the inducing
current was in the same direction (19.). For the purpose of avoiding
periphrasis, I propose to call this action of the current from the voltaic
battery, _volta-electric induction_. The properties of the second wire,
after induction has developed the first current, and whilst the electricity
from the battery continues to flow through its inducing neighbour (10.
18.), constitute a peculiar electric condition, the consideration of which
will be resumed hereafter (60.). All these results have been obtained with
a voltaic apparatus consisting of a single pair of plates.
§ 2. _Evolution of Electricity from Magnetism._
27. A welded ring was made of soft round bar-iron, the metal being
seven-eighths of an inch in thickness, and the ring six inches in external
diameter. Three helices were put round one part of this ring, each
containing about twenty-four feet of copper wire one twentieth of an inch
thick; they were insulated from the iron and each other, and superposed in
the manner before described (6.), occupying about nine inches in length
upon the ring. They could be used separately or conjointly; the group may
be distinguished by the letter A (Pl. I. fig. 1.). On the other part of the
ring about sixty feet of similar copper wire in two pieces were applied in
the same manner, forming a helix B, which had the same common direction
with the helices of A, but being separated from it at each extremity by
about half an inch of the uncovered iron.
28. The helix B was connected by copper wires with a galvanometer three
feet from the ring. The helices of A were connected end to end so as to
form one common helix, the extremities of which were connected with a
battery of ten pairs of plates four inches square. The galvanometer was
immediately affected, and to a degree far beyond what has been described
when with a battery of tenfold power helices _without iron_ were used
(10.); but though the contact was continued, the effect was not permanent,
for the needle soon came to rest in its natural position, as if quite
indifferent to the attached electro-magnetic arrangement. Upon breaking the
contact with the batterry, the needle was again powerfully deflected, but
in the contrary direction to that induced in the first instance.
29. Upon arranging the apparatus so that B should be out of use, the
galvanometer be connected with one of the three wires of A (27.), and the
other two made into a helix through which the current from the trough (28.)
was passed, similar but rather more powerful effects were produced.
30. When the battery contact was made in one direction, the
galvanometer-needle was deflected on the one side; if made in the other
direction, the deflection was on the other side. The deflection on breaking
the battery contact was always the reverse of that produced by completing
it. The deflection on making a battery contact always indicated an induced
current in the opposite direction to that from the battery; but on breaking
the contact the deflection indicated an induced current in the same
direction as that of the battery. No making or breaking of the contact at B
side, or in any part of the galvanometer circuit, produced any effect at
the galvanometer. No continuance of the battery current caused any
deflection of the galvanometer-needle. As the above results are common to
all these experiments, and to similar ones with ordinary magnets to be
hereafter detailed, they need not be again particularly described.
31. Upon using the power of one hundred pairs of plates (10.) with this
ring, the impulse at the galvanometer, when contact was completed or
broken, was so great as to make the needle spin round rapidly four or five
times, before the air and terrestrial magnetism could reduce its motion to
mere oscillation.
32. By using charcoal at the ends of the B helix, a minute _spark_ could be
perceived when the contact of the battery with A was completed. This spark
could not be due to any diversion of a part of the current of the battery
through the iron to the helix B; for when the battery contact was
continued, the galvanometer still resumed its perfectly indifferent state
(28.). The spark was rarely seen on breaking contact. A small platina wire
could not be ignited by this induced current; but there seems every reason
to believe that the effect would be obtained by using a stronger original
current or a more powerful arrangement of helices.
33. A feeble voltaic current was sent through the helix B and the
galvanometer, so as to deflect the needle of the latter 30° or 40°, and
then the battery of one hundred pairs of plates connected with A; but after
the first effect was over, the galvanometer-needle resumed exactly the
position due to the feeble current transmitted by its own wire. This took
place in whichever way the battery contacts were made, and shows that here
again (20.) no permanent influence of the currents upon each other, as to
their quantity and tension, exists.
34. Another arrangement was then employed connecting the former experiments
on volta-electric induction (6-26.) with the present. A combination of
helices like that already described (6.) was constructed upon a hollow
cylinder of pasteboard: there were eight lengths of copper wire, containing
altogether 220 feet; four of these helices were connected end to end, and
then with the galvanometer (7.); the other intervening four were also
connected end to end, and the battery of one hundred pairs discharged
through them. In this form the effect on the galvanometer was hardly
sensible (11.), though magnets could be made by the induced current (13.).
But when a soft iron cylinder seven eighths of an inch thick, and twelve
inches long, was introduced into the pasteboard tube, surrounded by the
helices, then the induced current affected the galvanometer powerfully and
with all the phenomena just described (30.). It possessed also the power of
making magnets with more energy, apparently, than when no iron cylinder was
present.
35. When the iron cylinder was replaced by an equal cylinder of copper, no
effect beyond that of the helices alone was produced. The iron cylinder
arrangement was not so powerful as the ring arrangement already described
(27.).
36. Similar effects were then produced by _ordinary magnets_: thus the
hollow helix just described (34.) had all its elementary helices connected
with the galvanometer by two copper wires, each five feet in length; the
soft iron cylinder was introduced into its axis; a couple of bar magnets,
each twenty-four inches long, were arranged with their opposite poles at
one end in contact, so as to resemble a horse-shoe magnet, and then contact
made between the other poles and the ends of the iron cylinder, so as to
convert it for the time into a magnet (fig. 2.): by breaking the magnetic
contacts, or reversing them, the magnetism of the iron cylinder could be
destroyed or reversed at pleasure.
37. Upon making magnetic contact, the needle was deflected; continuing the
contact, the needle became indifferent, and resumed its first position; on
breaking the contact, it was again deflected, but in the opposite direction
to the first effect, and then it again became indifferent. When the
magnetic contacts were reversed the deflections were reversed.
38. When the magnetic contact was made, the deflection was such as to
indicate an induced current of electricity in the opposite direction to
that fitted to form a magnet, having the same polarity as that really
produced by contact with the bar magnets. Thus when the marked and unmarked
poles were placed as in fig. 3, the current in the helix was in the
direction represented, P being supposed to be the end of the wire going to
the positive pole of the battery, or that end towards which the zinc plates
face, and N the negative wire. Such a current would have converted the
cylinder into a magnet of the opposite kind to that formed by contact with
the poles A and B; and such a current moves in the opposite direction to
the currents which in M. Ampère's beautiful theory are considered as
constituting a magnet in the position figured[A].
[A] The relative position of an electric current and a magnet is by
most persons found very difficult to remember, and three or four helps
to the memory have been devised by M. Ampère and others. I venture to
suggest the following as a very simple and effectual assistance in
these and similar latitudes. Let the experimenter think he is looking
down upon a dipping needle, or upon the pole of the north, and then
let him think upon the direction of the motion of the hands of a
watch, or of a screw moving direct; currents in that direction round a
needle would make it into such a magnet as the dipping needle, or
would themselves constitute an electro-magnet of similar qualities; or
if brought near a magnet would tend to make it take that direction; or
would themselves be moved into that position by a magnet so placed; or
in M. Ampère's theory are considered as moving in that direction in
the magnet. These two points of the position of the dipping-needle and
the motion of the watch hands being remembered, any other relation of
the current and magnet can be at once deduced from it.
39. But as it might be supposed that in all the preceding experiments of
this section, it was by some peculiar effect taking place during the
formation of the magnet, and not by its mere virtual approximation, that
the momentary induced current was excited, the following experiment was
made. All the similar ends of the compound hollow helix (34.) were bound
together by copper wire, forming two general terminations, and these were
connected with the galvanometer. The soft iron cylinder (34.) was removed,
and a cylindrical magnet, three quarters of an inch in diameter and eight
inches and a half in length, used instead. One end of this magnet was
introduced into the axis of the helix (fig. 4.), and then, the
galvanometer-needle being stationary, the magnet was suddenly thrust in;
immediately the needle was deflected in the same direction as if the magnet
had been formed by either of the two preceding processes (34. 36.). Being
left in, the needle resumed its first position, and then the magnet being
withdrawn the needle was deflected in the opposite direction. These effects
were not great; but by introducing and withdrawing the magnet, so that the
impulse each time should be added to those previously communicated to the
needle, the latter could be made to vibrate through an arc of 180° or more.
40. In this experiment the magnet must not be passed entirely through the
helix, for then a second action occurs. When the magnet is introduced, the
needle at the galvanometer is deflected in a certain direction; but being
in, whether it be pushed quite through or withdrawn, the needle is
deflected in a direction the reverse of that previously produced. When the
magnet is passed in and through at one continuous motion, the needle moves
one way, is then suddenly stopped, and finally moves the other way.
41. If such a hollow helix as that described (34.) be laid east and west
(or in any other constant position), and a magnet be retained east and
west, its marked pole always being one way; then whichever end of the helix
the magnet goes in at, and consequently whichever pole of the magnet enters
first, still the needle is deflected the same way: on the other hand,
whichever direction is followed in withdrawing the magnet, the deflection
is constant, but contrary to that due to its entrance.
42. These effects are simple consequences of the _law_ hereafter to be
described (114).
43. When the eight elementary helices were made one long helix, the effect
was not so great as in the arrangement described. When only one of the
eight helices was used, the effect was also much diminished. All care was
taken to guard against tiny direct action of the inducing magnet upon the
galvanometer, and it was found that by moving the magnet in the same
direction, and to the same degree on the outside of the helix, no effect on
the needle was produced.
44. The Royal Society are in possession of a large compound magnet formerly
belonging to Dr. Gowin Knight, which, by permission of the President and
Council, I was allowed to use in the prosecution of these experiments: it
is at present in the charge of Mr. Christie, at his house at Woolwich,
where, by Mr. Christie's kindness, I was at liberty to work; and I have to
acknowledge my obligations to him for his assistance in all the experiments
and observations made with it. This magnet is composed of about 450 bar
magnets, each fifteen inches long, one inch wide, and half an inch thick,
arranged in a box so as to present at one of its extremities two external
poles (fig. 5.). These poles projected horizontally six inches from the
box, were each twelve inches high and three inches wide. They were nine
inches apart; and when a soft iron cylinder, three quarters of an inch in
diameter and twelve inches long, was put across from one to the other, it
required a force of nearly one hundred pounds to break the contact. The
pole to the left in the figure is the marked pole[A].
[A] To avoid any confusion as to the poles of the magnet, I shall
designate the pole pointing to the north as the marked pole; I may
occasionally speak of the north and south ends of the needle, but do
not mean thereby north and south poles. That is by many considered the
true north pole of a needle which points to the south; but in this
country it in often called the south pole.
45. The indicating galvanometer, in all experiments made with this magnet,
was about eight feet from it, not directly in front of the poles, but about
16° or 17° on one side. It was found that on making or breaking the
connexion of the poles by soft iron, the instrument was slightly affected;
but all error of observation arising from this cause was easily and
carefully avoided.
46. The electrical effects exhibited by this magnet were very striking.
When a soft iron cylinder thirteen inches long was put through the compound
hollow helix, with its ends arranged as two general terminations (39.),
these connected with the galvanometer, and the iron cylinder brought in
contact with the two poles of the magnet (fig. 5.), so powerful a rush of
electricity took place that the needle whirled round many times in
succession[A].
[A] A soft iron bar in the form of a lifter to a horse-shoe magnet,
when supplied with a coil of this kind round the middle of it,
becomes, by juxta-position with a magnet, a ready source of a brief
but determinate current of electricity.
47. Notwithstanding this great power, if the contact was continued, the
needle resumed its natural position, being entirely uninfluenced by the
position of the helix (30.). But on breaking the magnetic contact, the
needle was whirled round in the opposite direction with a force equal to
the former.
48. A piece of copper plate wrapped _once_ round the iron cylinder like a
socket, but with interposed paper to prevent contact, had its edges
connected with the wires of the galvanometer. When the iron was brought in
contact with the poles the galvanometer was strongly affected.
49. Dismissing the helices and sockets, the galvanometer wire was passed
over, and consequently only half round the iron cylinder (fig. 6.); but
even then a strong effect upon the needle was exhibited, when the magnetic
contact was made or broken.
50. As the helix with its iron cylinder was brought towards the magnetic
poles, but _without making contact_, still powerful effects were produced.
When the helix, without the iron cylinder, and consequently containing no
metal but copper, was approached to, or placed between the poles (44.), the
needle was thrown 80°, 90°, or more, from its natural position. The
inductive force was of course greater, the nearer the helix, either with or
without its iron cylinder, was brought to the poles; but otherwise the same
effects were produced, whether the helix, &c. was or was not brought into
contact with the magnet; i.e. no permanent effect on the galvanometer was
produced; and the effects of approximation and removal were the reverse of
each other (30.).
51. When a bolt of copper corresponding to the iron cylinder was
introduced, no greater effect was produced by the helix than without it.
But when a thick iron wire was substituted, the magneto-electric induction
was rendered sensibly greater.
52. The direction of the electric current produced in all these experiments
with the helix, was the same as that already described (38.) as obtained
with the weaker bar magnets.
53. A spiral containing fourteen feet of copper wire, being connected with
the galvanometer, and approximated directly towards the marked pole in the
line of its axis, affected the instrument strongly; the current induced in
it was in the reverse direction to the current theoretically considered by
M. Ampère as existing in the magnet (38.), or as the current in an
electro-magnet of similar polarity. As the spiral was withdrawn, the
induced current was reversed.
54. A similar spiral had the current of eighty pairs of 4-inch plates sent
through it so as to form an electro-magnet, and then the other spiral
connected with the galvanometer (58.) approximated to it; the needle
vibrated, indicating a current in the galvanometer spiral the reverse of
that in the battery spiral (18. 26.). On withdrawing the latter spiral, the
needle passed in the opposite direction.
55. Single wires, approximated in certain directions towards the magnetic
pole, had currents induced in them. On their removal, the currents were
inverted. In such experiments the wires should not be removed in directions
different to those in which they were approximated; for then occasionally
complicated and irregular effects are produced, the causes of which will be
very evident in the fourth part of this paper.
56. All attempts to obtain chemical effects by the induced current of
electricity failed, though the precautions before described (22.), and all
others that could be thought of, were employed. Neither was any sensation
on the tongue, or any convulsive effect upon the limbs of a frog, produced.
Nor could charcoal or fine wire be ignited (133.). But upon repeating the
experiments more at leisure at the Royal Institution, with an armed
loadstone belonging to Professor Daniell and capable of lifting about
thirty pounds, a frog was very _powerfully convulsed_ each time magnetic
contact was made. At first the convulsions could not be obtained on
breaking magnetic contact; but conceiving the deficiency of effect was
because of the comparative slowness of separation, the latter act was
effected by a blow, and then the frog was convulsed strongly. The more
instantaneous the union or disunion is effected, the more powerful the
convulsion. I thought also I could perceive the _sensation_ upon the tongue
and the _flash_ before the eyes; but I could obtain no evidence of chemical
decomposition.
57. The various experiments of this section prove, I think, most completely
the production of electricity from ordinary magnetism. That its intensity
should be very feeble and quantity small, cannot be considered wonderful,
when it is remembered that like thermo-electricity it is evolved entirely
within the substance of metals retaining all their conducting power. But an
agent which is conducted along metallic wires in the manner described;
which whilst so passing possesses the peculiar magnetic actions and force
of a current of electricity; which can agitate and convulse the limbs of a
frog; and which, finally, can produce a spark[A] by its discharge through
charcoal (32.), can only be electricity. As all the effects can be produced
by ferruginous electro-magnets (34.), there is no doubt that arrangements
like the magnets of Professors Moll, Henry, Ten Eyke, and others, in which
as many as two thousand pounds have been lifted, may be used for these
experiments; in which case not only a brighter spark may be obtained, but
wires also ignited, and, as the current can pass liquids (23.), chemical
action be produced. These effects are still more likely to be obtained when
the magneto-electric arrangements to be explained in the fourth section are
excited by the powers of such apparatus.
[A] For a mode of obtaining the spark from the common magnet which I
have found effectual, see the Philosophical Magazine for June 1832, p.
5. In the same Journal for November 1834, vol. v. p. 349, will be
found a method of obtaining the magneto-electric spark, still simpler
in its principle, the use of soft iron being dispensed with
altogether.--_Dec. 1838._
58. The similarity of action, almost amounting to identity, between common
magnets and either electro-magnets or volta-electric currents, is
strikingly in accordance with and confirmatory of M. Ampère's theory, and
furnishes powerful reasons for believing that the action is the same in
both cases; but, as a distinction in language is still necessary, I propose
to call the agency thus exerted by ordinary magnets, _magneto-electric_ or
_magnelectric_ induction (26).
59. The only difference which powerfully strikes the attention as existing
between volta-electric and magneto-electric induction, is the suddenness of
the former, and the sensible time required by the latter; but even in this
early state of investigation there are circumstances which seem to
indicate, that upon further inquiry this difference will, as a
philosophical distinction, disappear (68).[A]
[A] For important additional phenomena and developments of the
induction of electrical currents, see now the ninth series,
1048-1118.--_Dec. 1838._
§ 3. _New Electrical State or Condition of Matter._[A]
[A] This section having been read at the Royal Society and reported
upon, and having also, in consequence of a letter from myself to M.
Hachette, been noticed at the French Institute, I feel bound to let it
stand as part of the paper; but later investigations (intimated 73.
76. 77.) of the laws governing those phenomena, induce me to think
that the latter can be fully explained without admitting the
electro-tonic state. My views on this point will appear in the second
series of these researches.--M.F.
60. Whilst the wire is subject to either volta-electric or magneto-electric
induction, it appears to be in a peculiar state; for it resists the
formation of an electrical current in it, whereas, if in its common
condition, such a current would be produced; and when left uninfluenced it
has the power of originating a current, a power which the wire does not
possess under common circumstances. This electrical condition of matter has
not hitherto been recognised, but it probably exerts a very important
influence in many if not most of the phenomena produced by currents of
electricity. For reasons which will immediately appear (71.), I have, after
advising with several learned friends, ventured to designate it as the
_electro-ionic_ state.
61. This peculiar condition shows no known electrical effects whilst it
continues; nor have I yet been able to discover any peculiar powers
exerted, or properties possessed, by matter whilst retained in this state.
62. It shows no reaction by attractive or repulsive powers. The various
experiments which have been made with powerful magnets upon such metals, as
copper, silver, and generally those substances not magnetic, prove this
point; for the substances experimented upon, if electrical conductors, must
have acquired this state; and yet no evidence of attractive or repulsive
powers has been observed. I have placed copper and silver discs, very
delicately suspended on torsion balances in vacuo near to the poles of very
powerful magnets, yet have not been able to observe the least attractive or
repulsive force.
63. I have also arranged a fine slip of gold-leaf very near to a bar of
copper, the two being in metallic contact by mercury at their extremities.
These have been placed in vacuo, so that metal rods connected with the
extremities of the arrangement should pass through the sides of the vessel
into the air. I have then moved powerful magnetic poles, about this
arrangement, in various directions, the metallic circuit on the outside
being sometimes completed by wires, and sometimes broken. But I never could
obtain any sensible motion of the gold-leaf, either directed to the magnet
or towards the collateral bar of copper, which must have been, as far as
induction was concerned, in a similar state to itself.
64. In some cases it has been supposed that, under such circumstances,
attractive and repulsive forces have been exhibited, i.e. that such bodies
have become slightly magnetic. But the phenomena now described, in
conjunction with the confidence we may reasonably repose in M. Ampère's
theory of magnetism, tend to throw doubt on such cases; for if magnetism
depend upon the attraction of electrical currents, and if the powerful
currents at first excited, both by volta-electric and magneto-electric
induction, instantly and naturally cease (12. 28. 47.), causing at the same
time an entire cessation of magnetic effects at the galvanometer needle,
then there can be little or no expectation that any substances not
partaking of the peculiar relation in which iron, nickel, and one or two
other bodies, stand, should exhibit magneto-attractive powers. It seems far
more probable, that the extremely feeble permanent effects observed have
been due to traces of iron, or perhaps some other unrecognised cause not
magnetic.
65. This peculiar condition exerts no retarding or accelerating power upon
electrical currents passing through metal thus circumstanced (20. 33.).
Neither could any such power upon the inducing current itself be detected;
for when masses of metal, wires, helices, &c. were arranged in all possible
ways by the side of a wire or helix, carrying a current measured by the
galvanometer (20.), not the slightest permanent change in the indication of
the instrument could be perceived. Metal in the supposed peculiar state,
therefore, conducts electricity in all directions with its ordinary
facility, or, in other words, its conducting power is not sensibly altered
by it.
66. All metals take on the peculiar state. This is proved in the preceding
experiments with copper and iron (9.), and with gold, silver, tin, lead,
zinc, antimony, bismuth, mercury, &c. by experiments to be described in the
fourth part (132.), admitting of easy application. With regard to iron, the
experiments prove the thorough and remarkable independence of these
phenomena of induction, and the ordinary magnetical appearances of that
metal.
67. This state is altogether the effect of the induction exerted, and
ceases as soon as the inductive force is removed. It is the same state,
whether produced by the collateral passage of voltaic currents (26.), or
the formation of a magnet (34. 36.), or the mere approximation of a magnet
(39. 50.); and is a strong proof in addition to those advanced by M.
Ampère, of the identity of the agents concerned in these several
operations. It probably occurs, momentarily, during the passage of the
common electric spark (24.), and may perhaps be obtained hereafter in bad
conductors by weak electrical currents or other means (74. 76).
68. The state appears to be instantly assumed (12.), requiring hardly a
sensible portion of time for that purpose. The _difference_ of time between
volta-electric and magneto-electric induction, rendered evident by the
galvanometer (59.), may probably be thus explained. When a voltaic current
is sent through one of two parallel wires, as those of the hollow helix
(34.), a current is produced in the other wire, as brief in its continuance
as the time required for a single action of this kind, and which, by
experiment, is found to be inappreciably small. The action will seem still
more instantaneous, because, as there is an accumulation of power in the
poles of the battery before contact, the first rush of electricity in the
wire of communication is greater than that sustained after the contact is
completed; the wire of induction becomes at the moment electro-tonic to an
equivalent degree, which the moment after sinks to the state in which the
continuous current can sustain it, but in sinking, causes an opposite
induced current to that at first produced. The consequence is, that the
first induced wave of electricity more resembles that from the discharge of
an electric jar, than it otherwise would do.
69. But when the iron cylinder is put into the same helix (31.), previous
to the connexion being made with the battery, then the current from the
latter may be considered as active in inducing innumerable currents of a
similar kind to itself in the iron, rendering it a magnet. This is known by
experiment to occupy time; for a magnet so formed, even of soft iron, does
not rise to its fullest intensity in an instant, and it may be because the
currents within the iron are successive in their formation or arrangement.
But as the magnet can induce, as well as the battery current, the combined
action of the two continues to evolve induced electricity, until their
joint effect is at a maximum, and thus the existence of the deflecting
force is prolonged sufficiently to overcome the inertia of the galvanometer
needle.
70. In all those cases where the helices or wires are advanced towards or
taken from the magnet (50. 55.), the direct or inverted current of induced
electricity continues for the time occupied in the advance or recession;
for the electro-tonic state is rising to a higher or falling to a lower
degree during that time, and the change is accompanied by its corresponding
evolution of electricity; but these form no objections to the opinion that
the electro-tonic state is instantly assumed.
71. This peculiar state appears to be a state of tension, and may be
considered as _equivalent_ to a current of electricity, at least equal to
that produced either when the condition is induced or destroyed. The
current evolved, however, first or last, is not to be considered a measure
of the degree of tension to which the electro-tonic state has risen; for as
the metal retains its conducting powers unimpaired (65.), and as the
electricity evolved is but for a moment, (the peculiar state being
instantly assumed and lost (68.),) the electricity which may be led away by
long wire conductors, offering obstruction in their substance proportionate
to their small lateral and extensive linear dimensions, can be but a very
small portion of that really evolved within the mass at the moment it
assumes this condition. Insulated helices and portions of metal instantly
assumed the state; and no traces of electricity could be discovered in
them, however quickly the contact with the electrometer was made, after
they were put under induction, either by the current from the battery or
the magnet. A single drop of water or a small piece of moistened paper (23.
56.) was obstacle sufficient to stop the current through the conductors,
the electricity evolved returning to a state of equilibrium through the
metal itself, and consequently in an unobserved manner.
72. The tension of this state may therefore be comparatively very great.
But whether great or small, it is hardly conceivable that it should exist
without exerting a reaction upon the original inducing current, and
producing equilibrium of some kind. It might be anticipated that this would
give rise to a retardation of the original current; but I have not been
able to ascertain that this is the case. Neither have I in any other way as
yet been able to distinguish effects attributable to such a reaction.
73. All the results favour the notion that the electro-tonic state relates
to the particles, and not to the mass, of the wire or substance under
induction, being in that respect different to the induction exerted by
electricity of tension. If so, the state may be assumed in liquids when no
electrical current is sensible, and even in non-conductors; the current
itself, when it occurs, being as it were a contingency due to the existence
of conducting power, and the momentary propulsive force exerted by the
particles during their arrangement. Even when conducting power is equal,
the currents of electricity, which as yet are the only indicators of this
state, may be unequal, because of differences as to numbers, size,
electrical condition, &c. &c. in the particles themselves. It will only be
after the laws which govern this new state are ascertained, that we shall
be able to predict what is the true condition of, and what are the
electrical results obtainable from, any particular substance.
74. The current of electricity which induces the electro-tonic state in a
neighbouring wire, probably induces that state also in its own wire; for
when by a current in one wire a collateral wire is made electro-tonic, the
latter state is not rendered any way incompatible or interfering with a
current of electricity passing through it (62.). If, therefore, the current
were sent through the second wire instead of the first, it does not seem
probable that its inducing action upon the second would be less, but on the
contrary more, because the distance between the agent and the matter acted
upon would be very greatly diminished. A copper bolt had its extremities
connected with a galvanometer, and then the poles of a battery of one
hundred pairs of plates connected with the bolt, so as to send the current
through it; the voltaic circuit was then suddenly broken, and the
galvanometer observed for any indications of a return current through the
copper bolt due to the discharge of its supposed electro-tonic state. No
effect of the kind was obtained, nor indeed, for two reasons, ought it to
be expected; for first, as the cessation of induction and the discharge of
the electro-tonic condition are simultaneous, and not successive, the
return current would only be equivalent to the neutralization of the last
portion of the inducing current, and would not therefore show any
alteration of direction; or assuming that time did intervene, and that the
latter current was really distinct from the former, its short, sudden
character (12. 26.) would prevent it from being thus recognised.
75. No difficulty arises, I think, in considering the wire thus rendered
electro-tonic by its own current more than by any external current,
especially when the apparent non-interference of that state with currents
is considered (62. 71.). The simultaneous existence of the conducting and
electro-tonic states finds an analogy in the manner in which electrical
currents can be passed through magnets, where it is found that both the
currents passed, and those of the magnets, preserve all their properties
distinct from each other, and exert their mutual actions.
76. The reason given with regard to metals extends also to fluids and all
other conductors, and leads to the conclusion that when electric currents
are passed through them they also assume the electro-tonic state. Should
that prove to be the case, its influence in voltaic decomposition, and the
transference of the elements to the poles, can hardly be doubted. In the
electro-tonic state the homogeneous particles of matter appear to have
assumed a regular but forced electrical arrangement in the direction of the
current, which if the matter be undecomposable, produces, when relieved, a
return current; but in decomposable matter this forced state may be
sufficient to make an elementary particle leave its companion, with which
it is in a constrained condition, and associate with the neighbouring
similar particle, in relation to which it is in a more natural condition,
the forced electrical arrangement being itself discharged or relieved, at
the same time, as effectually as if it had been freed from induction. But
as the original voltaic current is continued, the electro-tonic state may
be instantly renewed, producing the forced arrangement of the compound
particles, to be as instantly discharged by a transference of the
elementary particles of the opposite kind in opposite directions, but
parallel to the current. Even the differences between common and voltaic
electricity, when applied to effect chemical decomposition, which Dr.
Wollaston has pointed out[A], seem explicable by the circumstances
connected with the induction of electricity from these two sources (25.).
But as I have reserved this branch of the inquiry, that I might follow out
the investigations contained in the present paper, I refrain (though much
tempted) from offering further speculations.
[A] Philosophical Transactions, 1801, p. 247.
77. Marianini has discovered and described a peculiar affection of the
surfaces of metallic discs, when, being in contact with humid conductors, a
current of electricity is passed through them; they are then capable of
producing a reverse current of electricity, and Marianini has well applied
the effect in explanation of the phenomena of Ritter's piles[A]. M.A. de la
Rive has described a peculiar property acquired by metallic conductors,
when being immersed in a liquid as poles, they have completed, for some
time, the voltaic circuit, in consequence of which, when separated from the
battery and plunged into the same fluid, they by themselves produce an
electric current[B]. M.A. Van Beek has detailed cases in which the
electrical relation of one metal in contact with another has been preserved
after separation, and accompanied by its corresponding chemical effects[C].
These states and results appear to differ from the electro-tonic state and
its phenomena; but the true relation of the former to the latter can only
be decided when our knowledge of all these phenomena has been enlarged.
[A] Annales de Chimie, xxxviii. 5.
[B] Ibid. xxviii. 190.
[C] Ibid. xxxviii. 49.
78. I had occasion in the commencement of this paper (2.) to refer to an
experiment by Ampère, as one of those dependent upon the electrical
induction of currents made prior to the present investigation, and have
arrived at conclusions which seem to imply doubts of the accuracy of the
experiment (62. &c.); it is therefore due to M. Ampère that I should attend
to it more distinctly. When a disc of copper (says M. Ampère) was suspended
by a silk thread and surrounded by a helix or spiral, and when the charge
of a powerful voltaic battery was sent through the spiral, a strong magnet
at the same time being presented to the copper disc, the latter turned at
the moment to take a position of equilibrium, exactly as the spiral itself
would have turned had it been free to move. I have not been able to obtain
this effect, nor indeed any motion; but the cause of my failure in the
_latter_ point may be due to the momentary existence of the current not
allowing time for the inertia of the plate to be overcome (11. 12.). M.
Ampère has perhaps succeeded in obtaining motion from the superior delicacy
and power of his electro-magnetical apparatus, or he may have obtained only
the motion due to cessation of action. But all my results tend to invert
the sense of the proposition stated by M. Ampère, "that a current of
electricity tends to put the electricity of conductors near which it passes
in motion in the same direction," for they indicate an opposite direction
for the produced current (26. 53.); and they show that the effect is
momentary, and that it is also produced by magnetic induction, and that
certain other extraordinary effects follow thereupon.
79. The momentary existence of the phenomena of induction now described is
sufficient to furnish abundant reasons for the uncertainty or failure of
the experiments, hitherto made to obtain electricity from magnets, or to
effect chemical decomposition or arrangement by their means[A].
[A] The Lycée, No. 36, for January 1st, has a long and rather
premature article, in which it endeavours to show anticipations by
French philosophers of my researches. It however mistakes the
erroneous results of MM. Fresnel and Ampère for true ones, and then
imagines my true results are like those erroneous ones. I notice it
here, however, for the purpose of doing honour to Fresnel in a much
higher degree than would have been merited by a feeble anticipation of
the present investigations. That great philosopher, at the same time
with myself and fifty other persons, made experiments which the
present paper proves could give no expected result. He was deceived
for the moment, and published his imaginary success; but on more
carefully repeating his trials, he could find no proof of their
accuracy; and, in the high and pure philosophic desire to remove error
as well as discover truth, he recanted his first statement. The
example of Berzelius regarding the first Thorina is another instance
of this fine feeling; and as occasions are not rare, it would be to
the dignity of science if such examples were more frequently
followed.--February 10th, 1832.
80. It also appears capable of explaining fully the remarkable phenomena
observed by M. Arago between metals and magnets when neither are moving
(120.), as well as most of the results obtained by Sir John Herschel,
Messrs. Babbage, Harris, and others, in repeating his experiments;
accounting at the same time perfectly for what at first appeared
inexplicable; namely, the non-action of the same metals and magnets when at
rest. These results, which also afford the readiest means of obtaining
electricity from magnetism, I shall now proceed to describe.
§ 4. _Explication of Arago's Magnetic Phenomena._
81. If a plate of copper be revolved close to a magnetic needle, or magnet,
suspended in such a way that the latter may rotate in a plane parallel to
that of the former, the magnet tends to follow the motion of the plate; or
if the magnet be revolved, the plate tends to follow its motion; and the
effect is so powerful, that magnets or plates of many pounds weight may be
thus carried round. If the magnet and plate be at rest relative to each
other, not the slightest effect, attractive or repulsive, or of any kind,
can be observed between them (62.). This is the phenomenon discovered by M.
Arago; and he states that the effect takes place not only with all metals,
but with solids, liquids, and even gases, i.e. with all substances (130.).
82. Mr. Babbage and Sir John Herschel, on conjointly repeating the
experiments in this country[A], could obtain the effects only with the
metals, and with carbon in a peculiar state (from gas retorts), i.e. only
with excellent conductors of electricity. They refer the effect to
magnetism induced in the plate by the magnet; the pole of the latter
causing an opposite pole in the nearest part of the plate, and round this a
more diffuse polarity of its own kind (120.). The essential circumstance in
producing the rotation of the suspended magnet is, that the substance
revolving below it shall acquire and lose its magnetism in sensible time,
and not instantly (124.). This theory refers the effect to an attractive
force, and is not agreed to by the discoverer, M. Arago, nor by M. Ampère,
who quote against it the absence of all attraction when the magnet and
metal are at rest (62. 126.), although the induced magnetism should still
remain; and who, from experiments made with a long dipping needle, conceive
the action to be always repulsive (125.).
[A] Philosophical Transactions, 1825, p. 467.
83. Upon obtaining electricity from magnets by the means already described
(36 46.), I hoped to make the experiment of M. Arago a new source of
electricity; and did not despair, by reference to terrestrial
magneto-electric induction, of being able to construct a new electrical
machine. Thus stimulated, numerous experiments were made with the magnet of
the Royal Society at Mr. Christie's house, in all of which I had the
advantage of his assistance. As many of these were in the course of the
superseded by more perfect arrangements, I shall consider myself at liberty
investigation to rearrange them in a manner calculated to convey most
readily what appears to me to be a correct view of the nature of the
phenomena.
84. The magnet has been already described (44.). To concentrate the poles,
and bring them nearer to each other, two iron or steel bars, each about six
or seven inches long, one inch wide, and half an inch thick, were put
across the poles as in fig. 7, and being supported by twine from slipping,
could be placed as near to or far from each other as was required.
Occasionally two bars of soft iron were employed, so bent that when
applied, one to each pole, the two smaller resulting poles were vertically
over each other, either being uppermost at pleasure.
85. A disc of copper, twelve inches in diameter, and about one fifth of an
inch in thickness, fixed upon a brass axis, was mounted in frames so as to
allow of revolution either vertically or horizontally, its edge being at
the same time introduced more or less between the magnetic poles (fig. 7.).
The edge of the plate was well amalgamated for the purpose of obtaining a
good but moveable contact, and a part round the axis was also prepared in a
similar manner.
86. Conductors or electric collectors of copper and lead were constructed
so as to come in contact with the edge of the copper disc (85.), or with
other forms of plates hereafter to be described (101.). These conductors
were about four inches long, one third of an inch wide, and one fifth of an
inch thick; one end of each was slightly grooved, to allow of more exact
adaptation to the somewhat convex edge of the plates, and then amalgamated.
Copper wires, one sixteenth of an inch in thickness, attached, in the
ordinary manner, by convolutions to the other ends of these conductors,
passed away to the galvanometer.
87. The galvanometer was roughly made, yet sufficiently delicate in its
indications. The wire was of copper covered with silk, and made sixteen or
eighteen convolutions. Two sewing-needles were magnetized and fixed on to a
stem of dried grass parallel to each other, but in opposite directions, and
about half an inch apart; this system was suspended by a fibre of unspun
silk, so that the lower needle should be between the convolutions of the
multiplier, and the upper above them. The latter was by much the most
powerful magnet, and gave terrestrial direction to the whole; fig. 8.
represents the direction of the wire and of the needles when the instrument
was placed in the magnetic meridian: the ends of the wires are marked A and
B for convenient reference hereafter. The letters S and N designate the
south and north ends of the needle when affected merely by terrestrial
magnetism; the end N is therefore the marked pole (44.). The whole
instrument was protected by a glass jar, and stood, as to position and
distance relative to the large magnet, under the same circumstances as
before (45.).
88. All these arrangements being made, the copper disc was adjusted as in
fig. 7, the small magnetic poles being about half an inch apart, and the
edge of the plate inserted about half their width between them. One of the
galvanometer wires was passed twice or thrice loosely round the brass axis
of the plate, and the other attached to a conductor (86.), which itself was
retained by the hand in contact with the amalgamated edge of the disc at
the part immediately between the magnetic poles. Under these circumstances
all was quiescent, and the galvanometer exhibited no effect. But the
instant the plate moved, the galvanometer was influenced, and by revolving
the plate quickly the needle could be deflected 90° or more.
89. It was difficult under the circumstances to make the contact between
the conductor and the edge of the revolving disc uniformly good and
extensive; it was also difficult in the first experiments to obtain a
regular velocity of rotation: both these causes tended to retain the needle
in a continual state of vibration; but no difficulty existed in
ascertaining to which side it was deflected, or generally, about what line
it vibrated. Afterwards, when the experiments were made more carefully, a
permanent deflection of the needle of nearly 45° could be sustained.
90. Here therefore was demonstrated the production of a permanent current
of electricity by ordinary magnets (57.).
91. When the motion of the disc was reversed, every other circumstance
remaining the same, the galvanometer needle was deflected with equal power
as before; but the deflection was on the opposite side, and the current of
electricity evolved, therefore, the reverse of the former.
92. When the conductor was placed on the edge of the disc a little to the
right or left, as in the dotted positions fig. 9, the current of
electricity was still evolved, and in the same direction as at first (88.
91.). This occurred to a considerable distance, i.e. 50° or 60° on each
side of the place of the magnetic poles. The current gathered by the
conductor and conveyed to the galvanometer was of the same kind on both
sides of the place of greatest intensity, but gradually diminished in force
from that place. It appeared to be equally powerful at equal distances from
the place of the magnetic poles, not being affected in that respect by the
direction of the rotation. When the rotation of the disc was reversed, the
direction of the current of electricity was reversed also; but the other
circumstances were not affected.
93. On raising the plate, so that the magnetic poles were entirely hidden
from each other by its intervention, (a. fig. 10,) the same effects were
produced in the same order, and with equal intensity as before. On raising
it still higher, so as to bring the place of the poles to c, still the
effects were produced, and apparently with as much power as at first.
94. When the conductor was held against the edge as if fixed to it, and
with it moved between the poles, even though but for a few degrees, the
galvanometer needle moved and indicated a current of electricity, the same
as that which would have been produced if the wheel had revolved in the
same direction, the conductor remaining stationary.
95. When the galvanometer connexion with the axis was broken, and its wires
made fast to two conductors, both applied to the edge of the copper disc,
then currents of electricity were produced, presenting more complicated
appearances, but in perfect harmony with the above results. Thus, if
applied as in fig. 11, a current of electricity through the galvanometer
was produced; but if their place was a little shifted, as in fig. 12, a
current in the contrary direction resulted; the fact being, that in the
first instance the galvanometer indicated the difference between a strong
current through A and a weak one through B, and in the second, of a weak
current through A and a strong one through B (92.), and therefore produced
opposite deflections.
96. So also when the two conductors were equidistant from the magnetic
poles, as in fig. 13, no current at the galvanometer was perceived,
whichever way the disc was rotated, beyond what was momentarily produced by
irregularity of contact; because equal currents in the same direction
tended to pass into both. But when the two conductors were connected with
one wire, and the axis with the other wire, (fig. 14,) then the
galvanometer showed a current according with the direction of rotation
(91.); both conductors now acting consentaneously, and as a single
conductor did before (88.).
97. All these effects could be obtained when only one of the poles of the
magnet was brought near to the plate; they were of the same kind as to
direction, &c., but by no means so powerful.
98. All care was taken to render these results independent of the earth's
magnetism, or of the mutual magnetism of the magnet and galvanometer
needles. The contacts were made in the magnetic equator of the plate, and
at other parts; the plate was placed horizontally, and the poles
vertically; and other precautions were taken. But the absence of any
interference of the kind referred to, was readily shown by the want of all
effect when the disc was removed from the poles, or the poles from the
disc; every other circumstance remaining the same.
99. The _relation of the current_ of electricity produced, to the magnetic
pole, to the direction of rotation of the plate, &c. &c., may be expressed
by saying, that when the unmarked pole (44. 84.) is beneath the edge of the
plate, and the latter revolves horizontally, screw-fashion, the electricity
which can be collected at the edge of the plate nearest to the pole is
positive. As the pole of the earth may mentally be considered the unmarked
pole, this relation of the rotation, the pole, and the electricity evolved,
is not difficult to remember. Or if, in fig. 15, the circle represent the
copper disc revolving in the direction of the arrows, and _a_ the outline
of the unmarked pole placed beneath the plate, then the electricity
collected at _b_ and the neighbouring parts is positive, whilst that
collected at the centre _c_ and other parts is negative (88.). The currents
in the plate are therefore from the centre by the magnetic poles towards
the circumference.
100. If the marked pole be placed above, all other things remaining the
same, the electricity at _b_, fig. 15, is still positive. If the marked
pole be placed below, or the unmarked pole above, the electricity is
reversed. If the direction of revolution in any case is reversed, the
electricity is also reversed.
101. It is now evident that the rotating plate is merely another form of
the simpler experiment of passing a piece of metal between the magnetic
poles in a rectilinear direction, and that in such cases currents of
electricity are produced at right angles to the direction of the motion,
and crossing it at the place of the magnetic pole or poles. This was
sufficiently shown by the following simple experiment: A piece of copper
plate one fifth of an inch thick, one inch and a half wide, and twelve
inches long, being amalgamated at the edges, was placed between the
magnetic poles, whilst the two conductors from the galvanometer were held
in contact with its edges; it was then drawn through between the poles of
the conductors in the direction of the arrow, fig. 16; immediately the
galvanometer needle was deflected, its north or marked end passed eastward,
indicating that the wire A received negative and the wire B positive
electricity; and as the marked pole was above, the result is in perfect
accordance with the effect obtained by the rotatory plate (99.).
102. On reversing the motion of the plate, the needle at the galvanometer
was deflected in the opposite direction, showing an opposite current.
103. To render evident the character of the electrical current existing in
various parts of the moving copper plate, differing in their relation to
the inducing poles, one collector (86.) only was applied at the part to be
examined near to the pole, the other being connected with the end of the
plate as the most neutral place: the results are given at fig. 17-20, the
marked pole being above the plate. In fig. 17, B received positive
electricity; but the plate moving in the same direction, it received on the
opposite side, fig. 18, negative electricity: reversing the motion of the
latter, as in fig. 20, B received positive electricity; or reversing the
motion of the first arrangement, that of fig. 17 to fig. 19, B received
negative electricity.
104. When the plates were previously removed sideways from between the
magnets, as in fig. 21, so as to be quite out of the polar axis, still the
same effects were produced, though not so strongly.
105. When the magnetic poles were in contact, and the copper plate was
drawn between the conductors near to the place, there was but very little
effect produced. When the poles were opened by the width of a card, the
effect was somewhat more, but still very small.
106. When an amalgamated copper wire, one eighth of an inch thick, was
drawn through between the conductors and poles (101.), it produced a very
considerable effect, though not so much as the plates.
107. If the conductors were held permanently against any particular parts
of the copper plates, and carried between the magnetic poles with them,
effects the same as those described were produced, in accordance with the
results obtained with the revolving disc (94.).
108. On the conductors being held against the ends of the plates, and the
latter then passed between the magnetic poles, in a direction transverse to
their length, the same effects were produced (fig. 22.). The parts of the
plates towards the end may be considered either as mere conductors, or as
portions of metal in which the electrical current is excited, according to
their distance and the strength of the magnet; but the results were in
perfect harmony with those before obtained. The effect was as strong as
when the conductors were held against the sides of the plate (101.).
109. When a mere wire, connected with the galvanometer so as to form a
complete circuit, was passed through between the poles, the galvanometer
was affected; and upon moving the wire to and fro, so as to make the
alternate impulses produced correspond with the vibrations of the needle,
the latter could be increased to 20° or 30° on each side the magnetic
meridian.
110. Upon connecting the ends of a plate of metal with the galvanometer
wires, and then carrying it between the poles from end to end (as in fig.
23.), in either direction, no effect whatever was produced upon the
galvanometer. But the moment the motion became transverse, the needle was
deflected.
111. These effects were also obtained from _electro-magnetic poles_,
resulting from the use of copper helices or spirals, either alone or with
iron cores (34. 54.). The directions of the motions were precisely the
same; but the action was much greater when the iron cores were used, than
without.
112. When a flat spiral was passed through edgewise between the poles, a
curious action at the galvanometer resulted; the needle first went strongly
one way, but then suddenly stopped, as if it struck against some solid
obstacle, and immediately returned. If the spiral were passed through from
above downwards, or from below upwards, still the motion of the needle was
in the same direction, then suddenly stopped, and then was reversed. But on
turning the spiral half-way round, i.e. edge for edge, then the directions
of the motions were reversed, but still were suddenly interrupted and
inverted as before. This double action depends upon the halves of the
spiral (divided by a line passing through its centre perpendicular to the
direction of its motion) acting in opposite directions; and the reason why
the needle went to the same side, whether the spiral passed by the poles in
the one or the other direction, was the circumstance, that upon changing
the motion, the direction of the wires in the approaching half of the
spiral was changed also. The effects, curious as they appear when
witnessed, are immediately referable to the action of single wires (40.
109.).
113. Although the experiments with the revolving plate, wires, and plates
of metal, were first successfully made with the large magnet belonging to
the Royal Society, yet they were all ultimately repeated with a couple of
bar magnets two feet long, one inch and a half wide, and half an inch
thick; and, by rendering the galvanometer (87.) a little more delicate,
with the most striking results. Ferro-electro-magnets, as those of Moll,
Henry, &c. (57.), are very powerful. It is very essential, when making
experiments on different substances, that thermo-electric effects (produced
by contact of the fingers, &c.) be avoided, or at least appreciated and
accounted for; they are easily distinguished by their permanency, and their
independence of the magnets, or of the direction of the motion.
114. The relation which holds between the magnetic pole, the moving wire or
metal, and the direction of the current evolved, i.e. _the law_ which
governs the evolution of electricity by magneto-electric induction, is very
simple, although rather difficult to express. If in fig. 24, PN represent a
horizontal wire passing by a marked magnetic pole, so that the direction of
its motion shall coincide with the curved line proceeding from below
upwards; or if its motion parallel to itself be in a line tangential to the
curved line, but in the general direction of the arrows; or if it pass the
pole in other directions, but so as to cut the magnetic curves[A] in the
same general direction, or on the same side as they would be cut by the
wire if moving along the dotted curved line;--then the current of
electricity in the wire is from P to N. If it be carried in the reverse
directions, the electric current will be from N to P. Or if the wire be in
the vertical position, figured P' N', and it be carried in similar
directions, coinciding with the dotted horizontal curve so far, as to cut
the magnetic curves on the same side with it, the current will be from P'
to N'. If the wire be considered a tangent to the curved surface of the
cylindrical magnet, and it be carried round that surface into any other
position, or if the magnet itself be revolved on its axis, so as to bring
any part opposite to the tangential wire,--still, if afterwards the wire be
moved in the directions indicated, the current of electricity will be from
P to N; or if it be moved in the opposite direction, from N to P; so that
as regards the motions of the wire past the pole, they may be reduced to
two, directly opposite to each other, one of which produces a current from
P to N, and the other from N to P.
[A] By magnetic curves, I mean the lines of magnetic forces, however
modified by the juxtaposition of poles, which would be depicted by
iron filings; or those to which a very small magnetic needle would
form a tangent.
115. The same holds true of the unmarked pole of the magnet, except that if
it be substituted for the one in the figure, then, as the wires are moved
in the direction of the arrows, the current of electricity would be from N
to P, and when they move in the reverse direction, from P to N.
116. Hence the current of electricity which is excited in metal when moving
in the neighbourhood of a magnet, depends for its direction altogether upon
the relation of the metal to the resultant of magnetic action, or to the
magnetic curves, and may be expressed in a popular way thus; Let AB (fig.
25.) represent a cylinder magnet, A being the marked pole, and B the
unmarked pole; let PN be a silver knife-blade, resting across the magnet
with its edge upward, and with its marked or notched side towards the pole
A; then in whatever direction or position this knife be moved edge
foremost, either about the marked or the unmarked pole, the current of
electricity produced will be from P to N, provided the intersected curves
proceeding from A abut upon the notched surface of the knife, and those
from B upon the unnotched side. Or if the knife be moved with its back
foremost, the current will be from N to P in every possible position and
direction, provided the intersected curves abut on the same surfaces as
before. A little model is easily constructed, by using a cylinder of wood
for a magnet, a flat piece for the blade, and a piece of thread connecting
one end of the cylinder with the other, and passing through a hole in the
blade, for the magnetic curves: this readily gives the result of any
possible direction.
117. When the wire under induction is passing by an electromagnetic pole,
as for instance one end of a copper helix traversed by the electric current
(34.), the direction of the current in the approaching wire is the same
with that of the current in the parts or sides of the spirals nearest to
it, and in the receding wire the reverse of that in the parts nearest to
it.
118. All these results show that the power of inducing electric currents is
circumferentially exerted by a magnetic resultant or axis of power, just as
circumferential magnetism is dependent upon and is exhibited by an electric
current.
119. The experiments described combine to prove that when a piece of metal
(and the same may be true of all conducting matter (213.) ) is passed
either before a single pole, or between the opposite poles of a magnet, or
near electro-magnetic poles, whether ferruginous or not, electrical
currents are produced across the metal transverse to the direction of
motion; and which therefore, in Arago's experiments, will approximate
towards the direction of radii. If a single wire be moved like the spoke of
a wheel near a magnetic pole, a current of electricity is determined
through it from one end towards the other. If a wheel be imagined,
constructed of a great number of these radii, and this revolved near the
pole, in the manner of the copper disc (85.), each radius will have a
current produced in it as it passes by the pole. If the radii be supposed
to be in contact laterally, a copper disc results, in which the directions
of the currents will be generally the same, being modified only by the
coaction which can take place between the particles, now that they are in
metallic contact.
120. Now that the existence of these currents is known, Arago's phenomena
may be accounted for without considering them as due to the formation in
the copper, of a pole of the opposite kind to that approximated, surrounded
by a diffuse polarity of the same kind (82.); neither is it essential that
the plate should acquire and lose its state in a finite time; nor on the
other hand does it seem necessary that any repulsive force should be
admitted as the cause of the rotation (82.).
121. The effect is precisely of the same kind as the electromagnetic
rotations which I had the good fortune to discover some years ago[A].
According to the experiments then made which have since been abundantly
confirmed, if a wire (PN fig. 26.) be connected with the positive and
negative ends of a voltaic buttery, so that the positive electricity shall
pass from P to N, and a marked magnetic pole N be placed near the wire
between it and the spectator, the pole will move in a direction tangential
to the wire, i.e. towards the right, and the wire will move tangentially
towards the left, according to the directions of the arrows. This is
exactly what takes place in the rotation of a plate beneath a magnetic
pole; for let N (fig. 27.) be a marked pole above the circular plate, the
latter being rotated in the direction of the arrow: immediately currents of
positive electricity set from the central parts in the general direction of
the radii by the pole to the parts of the circumference _a_ on the other
side of that pole (99. 119.), and are therefore exactly in the same
relation to it as the current in the wire (PN, fig. 26.), and therefore the
pole in the same manner moves to the right hand.
[A] Quarterly Journal of Science, vol. xii. pp. 74. 186. 416. 283.
122. If the rotation of the disc be reversed, the electric currents are
reversed (91.), and the pole therefore moves to the left hand. If the
contrary pole be employed, the effects are the same, i.e. in the same
direction, because currents of electricity, the reverse of those described,
are produced, and by reversing both poles and currents, the visible effects
remain unchanged. In whatever position the axis of the magnet be placed,
provided the same pole be applied to the same side of the plate, the
electric current produced is in the same direction, in consistency with the
law already stated (114, &c.); and thus every circumstance regarding the
direction of the motion may be explained.
123. These currents are _discharged or return_ in the parts of the plate on
each side of and more distant from the place of the pole, where, of course,
the magnetic induction is weaker; and when the collectors are applied, and
a current of electricity is carried away to the galvanometer (88.), the
deflection there is merely a repetition, by the same current or part of it,
of the effect of rotation in the magnet over the plate itself.
124. It is under the point of view just put forth that I have ventured to
say it is not necessary that the plate should acquire and lose its state in
a finite time (120.); for if it were possible for the current to be fully
developed the instant _before_ it arrived at its state of nearest
approximation to the vertical pole of the magnet, instead of opposite to or
a little beyond it, still the relative motion of the pole and plate would
be the same, the resulting force being in fact tangential instead of
direct.
125. But it is possible (though not necessary for the rotation) that _time_
may be required for the development of the maximum current in the plate, in
which case the resultant of all the forces would be in advance of the
magnet when the plate is rotated, or in the rear of the magnet when the
latter is rotated, and many of the effects with pure electro-magnetic poles
tend to prove this is the case. Then, the tangential force may be resolved
into two others, one parallel to the plane of rotation, and the other
perpendicular to it; the former would be the force exerted in making the
plate revolve with the magnet, or the magnet with the plate; the latter
would be a repulsive force, and is probably that, the effects of which M.
Arago has also discovered (82.).
126. The extraordinary circumstance accompanying this action, which has
seemed so inexplicable, namely, the cessation of all phenomena when the
magnet and metal are brought to rest, now receives a full explanation
(82.); for then the electrical currents which cause the motion cease
altogether.
127. All the effects of solution of metallic continuity, and the consequent
diminution of power described by Messrs. Babbage and Herschel[A], now
receive their natural explanation, as well also as the resumption of power
when the cuts were filled up by metallic substances, which, though
conductors of electricity, were themselves very deficient in the power of
influencing magnets. And new modes of cutting the plate may be devised,
which shall almost entirely destroy its power. Thus, if a copper plate
(81.) be cut through at about a fifth or sixth of its diameter from the
edge, so as to separate a ring from it, and this ring be again fastened on,
but with a thickness of paper intervening (fig. 29.), and if Arago's
experiment be made with this compound plate so adjusted that the section
shall continually travel opposite the pole, it is evident that the magnetic
currents will be greatly interfered with, and the plate probably lose much
of its effect[B].
[A] Philosophical Transactions, 1825, p. 481.
[B] This experiment has actually been made by Mr. Christie, with the
results here described, and is recorded in the Philosophical
Transactions for 1827, p. 82.
An elementary result of this kind was obtained by using two pieces of thick
copper, shaped as in fig. 28. When the two neighbouring edges were
amalgamated and put together, and the arrangement passed between the poles
of the magnet, in the direction parallel to these edges, a current was
urged through the wires attached to the outer angles, and the galvanometer
became strongly affected; but when a single film of paper was interposed,
and the experiment repeated, no sensible effect could be produced.
128. A section of this kind could not interfere much with the induction of
magnetism, supposed to be of the nature ordinarily received by iron.
129. The effect of rotation or deflection of the needle, which M. Arago
obtained by ordinary magnets, M. Ampère succeeded in procuring by
electro-magnets. This is perfectly in harmony with the results relative to
volta-electric and magneto-electric induction described in this paper. And
by using flat spirals of copper wire, through which electric currents were
sent, in place of ordinary magnetic poles (Ill.), sometimes applying a
single one to one side of the rotating plate, and sometimes two to opposite
sides, I obtained the induced currents of electricity from the plate
itself, and could lead them away to, and ascertain their existence by, the
galvanometer.
130. The cause which has now been assigned for the rotation in Arago's
experiment, namely, the production of electrical currents, seems abundantly
sufficient in all cases where the metals, or perhaps even other conductors,
are concerned; but with regard to such bodies as glass, resins, and, above
all, gases, it seems impossible that currents of electricity, capable of
producing these effects, should be generated in them. Yet Arago found that
the effects in question were produced by these and by all bodies tried
(81.). Messrs. Babbage and Herschel, it is true, did not observe them with
any substance not metallic, except carbon, in a highly conducting state
(82.). Mr. Harris has ascertained their occurrence with wood, marble,
freestone and annealed glass, but obtained no effect with sulphuric acid
and saturated solution of sulphate of iron, although these are better
conductors of electricity than the former substances.
131. Future investigations will no doubt explain these difficulties, and
decide the point whether the retarding or dragging action spoken of is
always simultaneous with electric currents.[A] The existence of the action
in metals, only whilst the currents exist, i.e. whilst motion is given (82.
88.), and the explication of the repulsive action observed by M. Arago (82.
125.), are powerful reasons for referring it to this cause; but it may be
combined with others which occasionally act alone.
[A] Experiments which I have since made convince me that this
particular action is always due to the electrical currents formed; and
they supply a test by which it may be distinguished from the action of
ordinary magnetism, or any other cause, including those which are
mechanical or irregular, producing similar effects (254.)
132. Copper, iron, tin, zinc, lead, mercury, and all the metals tried,
produced electrical currents when passed between the magnetic poles: the
mercury was put into a glass tube for the purpose. The dense carbon
deposited in coal gas retorts, also produced the current, but ordinary
charcoal did not. Neither could I obtain any sensible effects with brine,
sulphuric acid, saline solutions, &c., whether rotated in basins, or
inclosed in tubes and passed between the poles.
133. I have never been able to produce any sensation upon the tongue by the
wires connected with the conductors applied to the edges of the revolving
plate (88.) or slips of metal (101.). Nor have I been able to heat a fine
platina wire, or produce a spark, or convulse the limbs of a frog. I have
failed also to produce any chemical effects by electricity thus evolved
(22. 56).
134. As the electric current in the revolving copper plate occupies but a
small space, proceeding by the poles and being discharged right and left at
very small distances comparatively (123.); and as it exists in a thick mass
of metal possessing almost the highest conducting power of any, and
consequently offering extraordinary facility for its production and
discharge; and as, notwithstanding this, considerable currents may be drawn
off which can pass through narrow wires, forty, fifty, sixty, or even one
hundred feet long; it is evident that the current existing in the plate
itself must be a very powerful one, when the rotation is rapid and the
magnet strong. This is also abundantly proved by the obedience and
readiness with which a magnet ten or twelve pounds in weight follows the
motion of the plate and will strongly twist up the cord by which it is
suspended.
135. Two rough trials were made with the intention of constructing
_magneto-electric machines_. In one, a ring one inch and a half broad and
twelve inches external diameter, cut from a thick copper plate, was mounted
so as to revolve between the poles of the magnet and represent a plate
similar to those formerly used (101.), but of interminable length; the
inner and outer edges were amalgamated, and the conductors applied one to
each edge, at the place of the magnetic poles. The current of electricity
evolved did not appear by the galvanometer to be stronger, if so strong, as
that from the circular plate (88.).
136. In the other, small thick discs of copper or other metal, half an inch
in diameter, were revolved rapidly near to the poles, but with the axis of
rotation out of the polar axis; the electricity evolved was collected by
conductors applied as before to the edges (86.). Currents were procured,
but of strength much inferior to that produced by the circular plate.
137. The latter experiment is analogous to those made by Mr. Barlow with a
rotating iron shell, subject to the influence of the earth[A]. The effects
obtained by him have been referred by Messrs. Babbage and Herschel to the
same cause as that considered as influential in Arago's experiment[B]; but
it would be interesting to know how far the electric current which might be
produced in the experiment would account for the deflexion of the needle.
The mere inversion of a copper wire six or seven times near the poles of
the magnet, and isochronously with the vibrations of the galvanometer
needle connected with it, was sufficient to make the needle vibrate through
an arc of 60° or 70°. The rotation of a copper shell would perhaps decide
the point, and might even throw light upon the more permanent, though
somewhat analogous effects obtained by Mr. Christie.
[A] Philosophical Transactions, 1825. p. 317.
[B] Ibid. 1825. p. 485.
138. The remark which has already been made respecting iron (66.), and the
independence of the ordinary magnetical phenomena of that substance and the
phenomena now described of magneto-electric induction in that and other
metals, was fully confirmed by many results of the kind detailed in this
section. When an iron plate similar to the copper one formerly described
(101.) was passed between the magnetic poles, it gave a current of
electricity like the copper plate, but decidedly of less power; and in the
experiments upon the induction of electric currents (9.), no difference in
the kind of action between iron and other metals could be perceived. The
power therefore of an iron plate to drag a magnet after it, or to intercept
magnetic action, should be carefully distinguished from the similar power
of such metals as silver, copper, &c. &c., inasmuch as in the iron by far
the greater part of the effect is due to what may be called ordinary
magnetic action. There can be no doubt that the cause assigned by Messrs.
Babbage and Herschel in explication of Arago's phenomena is the true one,
when iron is the metal used.
139. The very feeble powers which were found by those philosophers to
belong to bismuth and antimony, when moving, of affecting the suspended
magnet, and which has been confirmed by Mr. Harris, seem at first
disproportionate to their conducting powers; whether it be so or not must
be decided by future experiment (73.)[A]. These metals are highly
crystalline, and probably conduct electricity with different degrees of
facility in different directions; and it is not unlikely that where a mass
is made up of a number of crystals heterogeneously associated, an effect
approaching to that of actual division may occur (127.); or the currents of
electricity may become more suddenly deflected at the confines of similar
crystalline arrangements, and so be more readily and completely discharged
within the mass.
[A] I have since been able to explain these differences, and prove,
with several metals, that the effect is in the order of the conducting
power; for I have been able to obtain, by magneto-electric induction,
currents of electricity which are proportionate in strength to the
conducting power of the bodies experimented with (211.).
§. _Royal Institution, November 1831._
_Note._--In consequence of the long period which has intervened between the
reading and printing of the foregoing paper, accounts of the experiments
have been dispersed, and, through a letter of my own to M. Hachette, have
reached France and Italy. That letter was translated (with some errors),
and read to the Academy of Sciences at Paris, 26th December, 1831. A copy
of it in _Le Temps_ of the 28th December quickly reached Signor Nobili,
who, with Signor Antinori, immediately experimented upon the subject, and
obtained many of the results mentioned in my letter; others they could not
obtain or understand, because of the brevity of my account. These results
by Signori Nobili and Antinori have been embodied in a paper dated 31st
January 1832, and printed and published in the number of the _Antologia_
dated November 1831 (according at least to the copy of the paper kindly
sent me by Signor Nobili). It is evident the work could not have been then
printed; and though Signor Nobili, in his paper, has inserted my letter as
the text of his experiments, yet the circumstance of back date has caused
many here, who have heard of Nobili's experiments by report only, to
imagine his results were anterior to, instead of being dependent upon,
mine.
I may be allowed under these circumstances to remark, that I experimented
on this subject several years ago, and have published results. (See
Quarterly Journal of Science for July 1825, p. 338.) The following also is
an extract from my note-book, dated November 28, 1825: "Experiments on
induction by connecting wire of voltaic battery:--a battery of four
troughs, ten pairs of plates, each arranged side by side--the poles
connected by a wire about four feet long, parallel to which was another
similar wire separated from it only by two thicknesses of paper, the ends
of the latter were attached to a galvanometer:--exhibited no action, &c.
&c. &c.--Could not in any way render any induction evident from the
connecting wire." The cause of failure at that time is now evident
(79.).--M.F. April, 1832.