EXPERIMENTAL RESEARCHES IN ELECTRICITY
by
MICHAEL FARADAY, D.C.L. F.R.S.
Fullerian Profesor of Chemistry in the Royal Institution.
Corresponding Member, etc. of the Royal and Imperial Academies of
Science of Paris, Petersburgh, Florence, Copenhagen, Berlin,
Gottingen, Modena, Stockholm, Palermo, etc. etc.
In Two Volumes
VOL. I.
Second Edition
Reprinted from the PHILOSOPHICAL TRANSACTIONS of 1831-1838.
London:
Richard and John Edward Taylor,
Printers and Publishers to the University of London,
Red Lion Court, Fleet Street
1849
PREFACE
I have been induced by various circumstances to collect in One Volume the
Fourteen Series of Experimental Researches in Electricity, which have
appeared in the Philosophical Transactions during the last seven years: the
chief reason has been the desire to supply at a moderate price the whole of
these papers, with an Index, to those who may desire to have them.
The readers of the volume will, I hope, do me the justice to remember that
it was not written as a _whole_, but in parts; the earlier portions rarely
having any known relation at the time to those which might follow. If I had
rewritten the work, I perhaps might have considerably varied the form, but
should not have altered much of the real matter: it would not, however,
then have been considered a faithful reprint or statement of the course and
results of the whole investigation, which only I desired to supply.
I may be allowed to express my great satisfaction at finding, that the
different parts, written at intervals during seven years, harmonize so well
as they do. There would have been nothing particular in this, if the parts
had related only to matters well-ascertained before any of them were
written: - but as each professes to contain something of original discovery,
or of correction of received views, it does surprise even my partiality,
that they should have the degree of consistency and apparent general
accuracy which they seem to me to present.
I have made some alterations in the text, but they have been altogether of
a typographical or grammatical character; and even where greatest, have
been intended to explain the sense, not to alter it. I have often added
Notes at the bottom of the page, as to paragraphs 59, 360, 439, 521, 552,
555, 598, 657, 883, for the correction of errors, and also the purpose of
illustration: but these are all distinguished from the Original Notes of
the Researches by the date of _Dec. 1838_.
The date of a scientific paper containing any pretensions to discovery is
frequently a matter of serious importance, and it is a great misfortune
that there are many most valuable communications, essential to the history
and progress of science, with respect to which this point cannot now be
ascertained. This arises from the circumstance of the papers having no
dates attached to them individually, and of the journals in which they
appear having such as are inaccurate, i.e. dates of a period earlier than
that of publication. I may refer to the note at the end of the First
Series, as an illustration of the kind of confusion thus produced. These
circumstances have induced me to affix a date at the top of every other
page, and I have thought myself justified in using that placed by the
Secretary of the Royal Society on each paper as it was received. An author
has no right, perhaps, to claim an earlier one, unless it has received
confirmation by some public act or officer.
Before concluding these lines I would beg leave to make a reference or two;
first, to my own Papers on Electro-magnetic Rotations in the Quarterly
Journal of Science, 1822. xii. 74. 186. 283. 416, and also to my Letter on
Magneto-electric Induction in the Annales de Chimie, li. p. 404. These
might, as to the matter, very properly have appeared in this volume, but
they would have interfered with it as a simple reprint of the "Experimental
Researches" of the Philosophical Transactions.
Then I wish to refer, in relation to the Fourth Series on a new law of
Electric Conduction, to Franklin's experiments on the non-conduction of
ice, which have been very properly separated and set forth by Professor
Bache (Journal of the Franklin Institute, 1836. xvii. 183.). These, which I
did not at all remember as to the extent of the effect, though they in no
way anticipate the expression of the law I state as to the general effect
of liquefaction on electrolytes, still should never be forgotten when
speaking of that law as applicable to the case of water.
There are two papers which I am anxious to refer to, as corrections or
criticisms of parts of the Experimental Researches. The first of these is
one by Jacobi (Philosophical Magazine, 1838. xiii. 401.), relative to the
possible production of a spark on completing the junction of the two metals
of a single pair of plates (915.). It is an excellent paper, and though I
have not repeated the experiments, the description of them convinces me
that I must have been in error. The second is by that excellent
philosopher, Marianini (Memoria della Societa Italiana di Modena, xxi.
205), and is a critical and experimental examination of Series viii, and of
the question whether metallic contact is or is not _productive_ of a part
of the electricity of the voltaic pile. I see no reason as yet to alter the
opinion I have given; but the paper is so very valuable, comes to the
question so directly, and the point itself is of such great importance,
that I intend at the first opportunity renewing the inquiry, and, if I can,
rendering the proofs either on the one side or the other undeniable to all.
Other parts of these researches have received the honour of critical
attention from various philosophers, to all of whom I am obliged, and some
of whose corrections I have acknowledged in the foot notes. There are, no
doubt, occasions on which I have not felt the force of the remarks, but
time and the progress of science will best settle such cases; and, although
I cannot honestly say that I _wish_ to be found in error, yet I do
fervently hope that the progress of science in the hands of its many
zealous present cultivators will be such, as by giving us new and other
developments, and laws more and more general in their applications, will
even make me think that what is written and illustrated in these
experimental researches, belongs to the by-gone parts of science.
MICHAEL FARADAY.
Royal Institution,
March, 1839.
CONTENTS.
Par.
Series I. §. 1. Induction of electric currents 6
§. 2. Evolution of electricity from magnetism 27
§. 3. New electrical state or condition of matter 60
§. 4. Explication of Arago's magnetic phenomena 81
Series II. §. 5. Terrestrial magneto-electric induction 140
§. 6. Force and direction of magneto-electric
induction generally 193
Series III. §. 7. Identity of electricities from different
sources 265
- - - - i Voltaic electricity 268
- - - - ii Ordinary electricity 284
- - - - iii Magneto-electricity 343
- - - - iv Thermo-electricity 349
- - - - v Animal electricity 351
§. 8. Relation by measure of common and voltaic
electricity 361
- - Note respecting Ampère's inductive results
after 379
Series IV. §. 9. New law of electric conduction 380
§. 10. On conducting power generally 418
Series V. §. 11. Electro-chemical decomposition 450
- - ¶ 1. New conditions of electro-chemical
decomposition 453
- - ¶ 2. Influence of water in such decomposition 472
- - ¶ 3. Theory of electro-chemical decomposition 477
Series VI. §. 12. Power of platina, &c. to induce combination 564
Series VII. §. 11.* Electro-chemical decomposition continued
(nomenclature) 661
- - ¶ 4. Some general conditions of
Electro-chemical decomposition 669
- - ¶ 5. Volta-electrometer 704
- - ¶ 6. Primary and secondary results 742
- - ¶ 7. Definite nature and extent of
electro-chemical forces 783
- - - - Electro-chemical equivalents 822
§. 13. Absolute quantity of Electricity in the
molecules of matter 852
Series VIII. §. 14. Electricity of the voltaic pile 875
- - ¶ 1. Simple voltaic circles 875
- - ¶ 2. Electrolytic intensity 966
- - ¶ 3. Associated voltaic circles; or battery 989
- - ¶ 4. Resistance of an electrolyte to
decomposition 1007
- - ¶ 5. General remarks on the active battery 1034
Series IX. §. 15. Induction of a current on itself 1048
- - Inductive action of currents generally 1101
Series X. §. 16. Improved voltaic battery 1119
§. 17. Practical results with the voltaic battery 1136
Series XI. §. 18. On static induction 1161
- - ¶ 1. Induction an action of contiguous
particles 1161
- - ¶ 2. Absolute charge of matter 1169
- - ¶ 3. Electrometer and inductive apparatus 1179
- - ¶ 4. Induction in curved lines 1215
- - - - Conduction by glass, lac, sulphur, &c. 1283
- - ¶ 5. Specific inductive capacity 1252
- - ¶ 6. General results as to the nature of
induction 1295
- - - - Differential inductometer 1307
Series XII. - - ¶ 7. Conduction or conductive discharge 1320
- - ¶ 8. Electrolytic discharge 1343
- - ¶ 9. Disruptive discharge 1359
- - - - - - Insulation 1362
- - - - - - as spark 1406
- - - - - - as brush 1425
- - - - - - positive and negative 1465
Series XIII. - - - - - - as glow 1526
- - - - - - dark 1544
- - ¶ 10. Convection; or carrying discharge 1562
- - ¶ 11. Relation of a vacuum to electrical
phenomena 1613
§. 19. Nature of the electric current 1617
- - - - its transverse forces 1653
Series XIV. §. 20. Nature of the electric force or forces 1667
§. 21. Relation of the electric and magnetic
forces 1709
§. 22. Note on electrical excitation 1737
Index
Notes
EXPERIMENTAL RESEARCHES
IN
ELECTRICITY.
FIRST SERIES.
§ 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].