§ 15. _On the influence by induction of an Electric Current on itself:--and
on the inductive action of Electric Currents generally._
Received December 18, 1834,--Read January 29, 1835.
1048. The following investigations relate to a very remarkable inductive
action of electric currents, or of the different parts of the same current
(74.), and indicate an immediate connexion between such inductive action
and the direct transmission of electricity through conducting bodies, or
even that exhibited in the form of a spark.
1049. The inquiry arose out of a fact communicated to me by Mr. Jenkin,
which is as follows. If an ordinary wire of short length be used as the
medium of communication between the two plates of an electromotor
consisting of a single pair of metals, no management will enable the
experimenter to obtain an electric shock from this wire; but if the wire
which surrounds an electro-magnet be used, a shock is felt each time the
contact with the electromotor is broken, provided the ends of the wire be
grasped one in each hand.
1050. Another effect is observed at the same time, which has long been
known to philosophers, namely, that a bright electric spark occurs at the
place of disjunction.
1051. A brief account of these results, with some of a corresponding
character which I had observed in using long wires, was published in the
Philosophical Magazine for 1834[A]; and I added to them some observations
on their nature. Further investigations led me to perceive the inaccuracy
of my first notions, and ended in identifying these effects with the
phenomena of induction which I had been fortunate enough to develop in the
First Series of these Experimental Researches (1.-59.)[B]. Notwithstanding
this identity, the extension and the results supply, lead me to believe
that they will be found worthy of the attention of the Royal Society.
[A] Vol. v. pp. 349, 444.
[B] Philosophical Transactions, 1832, p. 126.
1052. The _electromotor_ used consisted of a cylinder of zinc introduced
between the two parts of a double cylinder of copper, and preserved from
metallic contact in the usual way by corks. The zinc cylinder was eight
inches high and four inches in diameter. Both it and the copper cylinder
were supplied with stiff wires, surmounted by cups containing mercury; and
it was at these cups that the contacts of wires, helices, or
electro-magnets, used to complete the circuit, were made or broken. These
cups I will call G and E throughout the rest of this paper (1079.).
1053. Certain _helices_ were constructed, some of which it will be
necessary to describe. A pasteboard tube had four copper wires, one
twenty-fourth of an inch in thickness, wound round it, each forming a helix
in the same direction from end to end: the convolutions of each wire were
separated by string, and the superposed helices prevented from touching by
intervening calico. The lengths of the wires forming the helices were 48,
49.5, 48, and 45 feet. The first and third wires were united together so as
to form one consistent helix of 96 feet in length; and the second and
fourth wires were similarly united to form a second helix, closely
interwoven with the first, and 94.5 feet in length. These helices may be
distinguished by the numbers i and ii. They were carefully examined by a
powerful current of electricity and a galvanometer, and found to have no
communication with each other.
1054. Another helix was constructed upon a similar pasteboard tube, two
lengths of the same copper wire being used, each forty-six feet long. These
were united into one consistent helix of ninety-two feet, which therefore
was nearly equal in value to either of the former helices, but was not in
close inductive association with them. It may be distinguished by the
number iii.
1055. A fourth helix was constructed of very thick copper wire, being
one-fifth of an inch in diameter; the length of wire used was seventy-nine
feet, independent of the straight terminal portions.
1056. The principal _electro-magnet_ employed consisted of a cylindrical
bar of soft iron twenty-five inches long, and one inch and three quarters
in diameter, bent into a ring, so that the ends nearly touched, and
surrounded by three coils of thick copper wire, the similar ends of which
were fastened together; each of these terminations was soldered to a copper
rod, serving as a conducting continuation of the wire. Hence any electric
current sent through the rods was divided in the helices surrounding the
ring, into three parts, all of which, however, moved in the same direction.
The three wires may therefore be considered as representing one wire, of
thrice the thickness of the wire really used.
1057. Other electro-magnets could be made at pleasure by introducing a soft
iron rod into any of the helices described (1053, &c.).
1058. The _galvanometer_ which I had occasion to use was rough in its
construction, having but one magnetic needle, and not at all delicate in
its indications.
1059. The effects to be considered _depend on the conductor_ employed to
complete the communication between the zinc and copper plates of the
electromotor; and I shall have to consider this conductor under four
different forms: as the helix of an electro-magnet (1056); as an ordinary
helix (1053, &c.); as a _long_ extended wire, having its course such that
the parts can exert little or no mutual influence; and as a _short_ wire.
In all cases the conductor was of copper.
1060. The peculiar effects are best shown by the _electro-magnet_ (1056.).
When it was used to complete the communication at the electromotor, there
was no sensible spark on _making_ contact, but on _breaking_ contact there
was a very large and bright spark, with considerable combustion of the
mercury. Then, again, with respect to the shock: if the hands were
moistened in salt and water, and good contact between them and the wires
retained, no shock could be felt upon _making_ contact at the electromotor,
but a powerful one on _breaking_ contact.
1061. When the _helix_ i or iii (1053, &c.) was used as the connecting
conductor, there was also a good spark on breaking contact, but none
(sensibly) on making contact. On trying to obtain the shock from these
helices, I could not succeed at first. By joining the similar ends of i and
ii so as to make the two helices equivalent to one helix, having wire of
double thickness, I could just obtain the sensation. Using the helix of
thick wire (1055.) the shock was distinctly obtained. On placing the tongue
between two plates of silver connected by wires with the parts which the
hands had heretofore touched (1064.), there was a powerful shock on
_breaking_ contact, but none on _making_ contact.
1062. The power of producing these phenomena exists therefore in the simple
helix, as in the electro-magnet, although by no means in the same high
degree.
1063. On putting a bar of soft iron into the helix, it became an
electro-magnet (1057.), and its power was instantly and greatly raised. On
putting a bar of copper into the helix, no change was produced, the action
being that of the helix alone. The two helices i and ii, made into one
helix of twofold length of wire, produced a greater effect than either i or
ii alone.
1064. On descending from the helix to the mere _long wire_, the following
effects were obtained, A copper wire, 0.18 of an inch in diameter, and 132
feet in length, was laid out upon the floor of the laboratory, and used as
the connecting conductor (1059.); it gave no sensible spark on making
contact, but produced a bright one on breaking contact, yet not so bright
as that from the helix (1061.) On endeavouring to obtain the electric shock
at the moment contact was broken, I could not succeed so as to make it pass
through the hands; but by using two silver plates fastened by small wires
to the extremity of the principal wire used, and introducing the tongue
between those plates, I succeeded in obtaining powerful shocks upon the
tongue and gums, and could easily convulse a flounder, an eel, or a frog.
None of these effects could be obtained directly from the electromotor,
i.e. when the tongue, frog, or fish was in a similar, and therefore
comparative manner, interposed in the course of the communication between
the zinc and copper plates, separated everywhere else by the acid used to
excite the combination, or by air. The bright spark and the shock, produced
only on breaking contact, are therefore effects of the same kind as those
produced in a higher degree by the helix, and in a still higher degree by
the electro-magnet.
1065. In order to compare an extended wire with a helix, the helix i,
containing ninety-six feet, and ninety-six feet of the same-sized wire
lying on the floor of the laboratory, were used alternately as conductors:
the former gave a much brighter spark at the moment of disjunction than the
latter. Again, twenty-eight feet of copper wire were made up into a helix,
and being used gave a good spark on disjunction at the electromotor; being
then suddenly pulled out and again employed, it gave a much smaller spark
than before, although nothing but its spiral arrangement had been changed.
1066. As the superiority of a helix over a wire is important to the
philosophy of the effect, I took particular pains to ascertain the fact
with certainty. A wire of copper sixty-seven feet long was bent in the
middle so as to form a double termination which could be communicated with
the electromotor; one of the halves of this wire was made into a helix and
the other remained in its extended condition. When these were used
alternately as the connecting wire, the helix half gave by much the
strongest spark. It even gave a stronger spark than when it and the
extended wire were used conjointly as a double conductor.
1067. When a _short wire_ is used, _all_ these effects disappear. If it be
only two or three inches long, a spark can scarcely be perceived on
breaking the junction. If it be ten or twelve inches long and moderately
thick, a small spark may be more easily obtained. As the length is
increased, the spark becomes proportionately brighter, until from extreme
length the resistance offered by the metal as a conductor begins to
interfere with the principal result.
1068. The effect of elongation was well shown thus: 114 feet of copper
wire, one-eighteenth of an inch in diameter, were extended on the floor and
used as a conductor; it remained cold, but gave a bright spark on breaking
contact. Being crossed so that the two terminations were in contact near
the extremities, it was again used as a conductor, only twelve inches now
being included in the circuit: the wire became very hot from the greater
quantity of electricity passing through it, and yet the spark on breaking
contact was scarcely visible. The experiment was repeated with a wire
one-ninth of an inch in diameter and thirty-six feet long with the same
results.
1069. That the effects, and also the action, in all these forms of the
experiment are identical, is evident from the manner in which the former
can be gradually raised from that produced by the shortest wire to that of
the most powerful electro-magnet: and this capability of examining what
will happen by the most powerful apparatus, and then experimenting for the
same results, or reasoning from them, with the weaker arrangements, is of
great advantage in making out the true principles of the phenomena.
1070. The action is evidently dependent upon the wire which serves as a
conductor; for it varies as that wire varies in its length or arrangement.
The shortest wire may be considered as exhibiting the full effect of spark
or shock which the electromotor can produce by its own direct power; all
the additional force which the arrangements described can excite being due
to some affection of the current, either permanent or momentary, in the
wire itself. That it is a _momentary_ effect, produced only at the instant
of breaking contact, will be fully proved (1089. 1100.).
1071. No change takes place in the quantity or intensity of the current
during the time the latter is _continued_, from the moment after contact is
made, up to that previous to disunion, except what depends upon the
increased obstruction offered to the passage of the electricity by a long
wire as compared to a short wire. To ascertain this point with regard to
_quantity_, the helix i (1053.) and the galvanometer (1055.) were both made
parts of the metallic circuit used to connect the plates of a small
electromotor, and the deflection at the galvanometer was observed; then a
soft iron core was put into the helix, and as soon as the momentary effect
was over, and the needle had become stationary, it was again observed, and
found to stand exactly at the same division as before. Thus the quantity
passing through the wire when the current was continued was the same either
with or without the soft iron, although the peculiar effects occurring at
the moment of disjunction were very different in degree under such
variation of circumstances.
1072. That the quality of _intensity_ belonging to the constant current did
not vary with the circumstances favouring the peculiar results under
consideration, so as to yield an explanation of those results, was
ascertained in the following manner. The current excited by an electromotor
was passed through short wires, and its intensity tried by subjecting
different substances to its electrolyzing power (912. 966. &c.); it was
then passed through the wires of the powerful electro-magnet (1056.), and
again examined with respect to its intensity by the same means and found
unchanged. Again, the constancy of the _quantity_ passed in the above
experiment (1071.) adds further proof that the intensity could not have
varied; for had it been increased upon the introduction of the soft iron,
there is every reason to believe that the quantity passed in a given time
would also have increased.
1073. The fact is, that under many variations of the experiments, the
permanent current _loses_ in force as the effects upon breaking contact
become _exalted_. This is abundantly evident in the comparative experiments
with long and short wires (1068.); and is still more strikingly shown by
the following variation. Solder an inch or two in length of fine platina
wire (about one-hundredth of an inch in diameter) on to one end of the long
communicating wire, and also a similar length of the same platina wire on
to one end of the short communication; then, in comparing the effects of
these two communications, make and break contact between the platina
terminations and the mercury of the cup G or E (1079.). When the short wire
is used, the platina will be _ignited by the constant current_, because of
the quantity of electricity, but the spark on breaking contact will be
hardly visible; on using the longer communicating wire, which by
obstructing will diminish the current, the platina will remain cold whilst
the current passes, but give a bright spark at the moment it ceases: thus
the strange result is obtained of a diminished spark and shock from the
strong current, and increased effects from the weak one. Hence the spark
and shock at the moment of disjunction, although resulting from great
intensity and quantity, of the current _at that moment_, are no direct
indicators or measurers of the intensity or quantity of the constant
current previously passing, and by which they are ultimately produced.
* * * * *
1074. It is highly important in using the spark as an indication, by its
relative brightness, of these effects, to bear in mind certain
circumstances connected with its production and appearance (958.). An
ordinary electric spark is understood to be the bright appearance of
electricity passing suddenly through an interval of air, or other badly
conducting matter. A voltaic spark is sometimes of the same nature, but,
generally, is due to the ignition and even combustion of a minute portion
of a good conductor; and that is especially the case when the electromotor
consists of but one or few pairs of plates. This can be very well observed
if either or both of the metallic surfaces intended to touch be solid and
pointed. The moment they come in contact the current passes; it heats,
ignites, and even burns the touching points, and the appearance is as if
the spark passed on making contact, whereas it is only a case of ignition
by the current, contact being previously made, and is perfectly analogous
to the ignition of a fine platina wire connecting the extremities of a
voltaic battery.
1075. When mercury constitutes one or both of the surfaces used, the
brightness of the spark is greatly increased. But as this effect is due to
the action on, and probable combustion of, the metal, such sparks must only
be compared with other sparks also taken from mercurial surfaces, and not
with such as may be taken, for instance, between surfaces of platina or
gold, for then the appearances are far less bright, though the same
quantity of electricity be passed. It is not at all unlikely that the
commonly occurring circumstance of combustion may affect even the duration
of the light; and that sparks taken between mercury, copper, or other
combustible bodies, will continue for a period sensibly longer than those
passing between platina or gold.
1076. When the end of a short clean copper wire, attached to one plate of
an electromotor, is brought down carefully upon a surface of mercury
connected with the other plate, a spark, almost continuous, can be
obtained. This I refer to a succession of effects of the following nature:
first, contact,--then ignition of the touching points,--recession of the
mercury from the mechanical results of the heat produced at the place of
contact, and the electro-magnetic condition of the parts at the moment[A],
--breaking of the contact and the production of the peculiar intense effect
dependent thereon,--renewal of the contact by the returning surface of the
undulating mercury,--and then a repetition of the same series of effects,
and that with such rapidity as to present the appearance of a continued
discharge. If a long wire or an electro-magnet be used as the connecting
conductor instead of a short wire, a similar appearance may be produced by
tapping the vessel containing the mercury and making it vibrate; but the
sparks do not usually follow each other so rapidly as to produce an
apparently continuous spark, because of the time required, when the long
wire or electro-magnet is used, both for the full development of the
current (1101. 1106.) and for its complete cessation.
[A] Quarterly Journal of Science, vol. xii, p. 420.
1077. Returning to the phenomena in question, the first thought that arises
in the mind is, that the electricity circulates with something like
_momentum or inertia_ in the wire, and that thus a long wire produces
effects at the instant the current is stopped, which a short wire cannot
produce. Such an explanation is, however, at once set aside by the fact,
that the same length of wire produces the effects in very different
degrees, according as it is simply extended, or made into a helix, or forms
the circuit of an electro-magnet (1069.). The experiments to be adduced
(1089.) will still more strikingly show that the idea of momentum cannot
apply.
1078. The bright spark at the electromotor, and the shock in the arms,
appeared evidently to be due to _one_ current in the long wire, divided
into two parts by the double channel afforded through the body and through
the electromotor; for that the spark was evolved at the place of
disjunction with the electromotor, not by any direct action of the latter,
but by a force immediately exerted in the wire of communication, seemed to
be without doubt (1070.). It followed, therefore, that by using a better
conductor in place of the human body, the _whole_ of this extra current
might be made to pass at that place; and thus be separated from that which
the electromotor could produce by its immediate action, and its _direction_
be examined apart from any interference of the original and originating
current. This was found to be true; for on connecting the ends of the
principal wire together by a cross wire two or three feet in length,
applied just where the hands had felt the shock, the whole of the extra
current passed by the new channel, and then no better spark than one
producible by a short wire was obtained on disjunction at the electromotor.
1079. The _current_ thus separated was examined by galvanometers and
decomposing apparatus introduced into the course of this wire. I will
always speak of it as the current in the cross wire or wires, so that no
mistake, as to its place or origin, may occur. In the wood-cut, Z and C
represent the zinc and copper plates of the electromotor; G and E the cups
of mercury where contact is made or broken (1052.); A and B the
terminations of D, the long wire, the helix or the electro-magnet, used to
complete the circuit; N and P are the cross wires, which can either be
brought into contact at _x_, or else have a galvanometer (1058.) or an
electrolyzing apparatus (312. 316.) interposed there.
[Illustration]
The production of the _shock_ from the current in the cross wire, whether D
was a long extended wire, or a helix, or an electro-magnet, has been
already described (1060. 1061. 1064.).
1080. The _spark_ of the cross-wire current could be produced at _x_ in the
following manner: D was made an electro-magnet; the metallic extremities at
_x_ were held close together, or rubbed lightly against each other, whilst
contact was broken at G or E. When the communication was perfect at _x_,
little or no spark appeared at G or E. When the condition of vicinity at
_x_ was favourable for the result required, a bright spark would pass there
at the moment of disjunction, _none_ occurring at G and E: this spark was
the luminous passage of the extra current through the cross-wires. When
there was no contact or passage of current at _x_, then the spark appeared
at G or E, the extra current forcing its way through the electromotor
itself. The same results were obtained by the use of the helix or the
extended wire at D in place of the electro-magnet.
1081. On introducing a fine platina wire at _x_, and employing the
electro-magnet at D, no visible effects occurred as long as contact was
continued; but on breaking contact at G or E, the fine wire was instantly
ignited and fused. A longer or thicker wire could be so adjusted at _x_ as
to show ignition, without fusion, every time the contact was broken at G
or E.
1082. It is rather difficult to obtain this effect with helices or wires,
and for very simple reasons: with the helices i, ii, or iii, there was such
retardation of the electric current, from the length of wire used, that a
full inch of platina wire one-fiftieth of an inch in diameter could be
retained ignited at the cross-wires during the _continuance of contact_, by
the portion of electricity passing through it. Hence it was impossible to
distinguish the particular effects at the moments of making or breaking
contact from this constant effect. On using the thick wire helix (1055.),
the same results ensued.
1083. Proceeding upon the known fact that electric currents of great
quantity but low intensity, though able to ignite thick wires, cannot
produce that effect upon thin ones, I used a very fine platina wire at _x_,
reducing its diameter until a spark appeared at G or E, when contact was
broken there. A quarter of an inch of such wire might be introduced at _x_
without being ignited by the _continuance_ of contact at G or E; but when
contact was broken at either place, this wire became red-hot; proving, by
this method, the production of the induced current at that moment.
1084. _Chemical decomposition_ was next effected by the cross-wire current,
an electro-magnet being used at D, and a decomposing apparatus, with
solution of iodide of potassium in paper (1079.), employed at _x_. The
conducting power of the connecting system A B D was sufficient to carry all
the primary current, and consequently no chemical action took place at _x_
during the _continuance_ of contact at G and E; but when contact was
broken, there was instantly decomposition at _x_. The iodine appeared
against the wire N, and not against the wire P; thus demonstrating that the
current through the cross-wires, when contact was broken, was in the
_reverse direction_ to that marked by the arrow, or that which the
electromotor would have sent through it.
1085. In this experiment a bright spark occurs at the place of disjunction,
indicating that only a small part of the extra current passed the apparatus
at _x_, because of the small conducting power of the latter.
1086. I found it difficult to obtain the chemical effects with the simple
helices and wires, in consequence of the diminished inductive power of
these arrangements, and because of the passage of a strong constant current
at _x_ whenever a very active electromotor was used (1082).
1087. The most instructive set of results was obtained, however, when the
_galvanometer_ was introduced at _x_. Using an electro-magnet at D, and
continuing contact, a current was then indicated by the deflection,
proceeding from P to N, in the direction of the arrow; the cross-wire
serving to carry one part of the electricity excited by the electromotor,
and that part of the arrangement marked A B D, the other and far greater
part, as indicated by the arrows. The magnetic needle was then forced back,
by pins applied upon opposite sides of its two extremities, to its natural
position when uninfluenced by a current; after which, contact being
_broken_ at G or E, it was deflected strongly in the opposite direction;
thus showing, in accordance with the chemical effects (1084), that the
extra current followed a course in the cross-wires _contrary_ to that
indicated by the arrow, i. e. contrary to the one produced by the direct
action of the electromotor[A].
[A] It was ascertained experimentally, that if a strong current was
passed through the galvanometer only, and the needle restrained in one
direction as above in its natural position, when the current was
stopped, no vibration of the needle in the opposite direction took
place.
1088. With the _helix_ only (1061.), these effects could scarcely be
observed, in consequence of the smaller inductive force of this
arrangement, the opposed action from induction in the galvanometer wire
itself, the mechanical condition and tension of the needle from the effect
of blocking (1087.) whilst the current due to continuance of contact was
passing round it; and because of other causes. With the _extended wire_
(1064.) all these circumstances had still greater influence, and therefore
allowed less chance of success.
1089. These experiments, establishing as they did, by the quantity,
intensity, and even direction, a distinction between the primary or
generating current and the extra current, led me to conclude that the
latter was identical with the induced current described (6. 26. 74.) in the
First Series of these Researches; and this opinion I was soon able to bring
to proof, and at the same times obtained not the partial (1078.) but entire
separation of one current from the other.
1090. The double helix (1053.) was arranged so that it should form the
connecting wire between the plates of the electromotor, in being out of the
current, and its ends unconnected. In this condition it acted very well,
and gave a good spark at the time and place of disjunction. The opposite
ends of ii were then connected together so as to form an endless wire, i
remaining unchanged: but now _no spark_, or one scarcely sensible, could be
obtained from the latter at the place of disjunction. Then, again, the ends
of ii were held so nearly together that any current running round that
helix should be rendered visible as a spark; and in this manner a spark was
obtained from ii when the junction of i with the electromotor was broken,
in place of appearing at the disjoined extremity of i itself.
1091. By introducing a galvanometer or decomposing apparatus into the
circuit formed by the helix ii, I could easily obtain the deflections and
decomposition occasioned by the induced current due to the breaking contact
at helix i, or even to that occasioned by making contact of that helix with
the electromotor; the results in both cases indicating the contrary
directions of the two induced currents thus produced (26.).
1092. All these effects, except those of decomposition, were reproduced by
two extended long wires, not having the form of helices, but placed close
to each other; and thus it was proved that the _extra current_ could be
removed from the wire carrying the original current to a neighbouring wire,
and was at the same time identified, in direction and every other respect,
with the currents producible by induction (1089.). The case, therefore, of
the bright spark and shock on disjunction may now be stated thus: If a
current be established in a wire, and another wire, forming a complete
circuit, be placed parallel to the first, at the moment the current in the
first is stopped it induces a current in the _same_ direction in the
second, the first exhibiting then but a feeble spark; but if the second
wire be away, disjunction of the first wire induces a current in itself in
the same direction, producing a strong spark. The strong spark in the
single long wire or helix, at the moment of disjunction, is therefore the
equivalent of the current which would be produced in a neighbouring wire if
such second current were permitted.
1093. Viewing the phenomena as the results of the induction of electrical
currents, many of the principles of action, in the former experiments,
become far more evident and precise. Thus the different effects of short
wires, long wires, helices, and electro-magnets (1069.) may be
comprehended. If the inductive action of a wire a foot long upon a
collateral wire also a foot in length, be observed, it will be found very
small; but if the same current be sent through a wire fifty feet long, it
will induce in a neighbouring wire of fifty feet a far more powerful
current at the moment of making or breaking contact, each successive foot
of wire adding to the sum of action; and by parity of reasoning, a similar
effect should take place when the conducting wire is also that in which the
induced current is formed (74.): hence the reason why a long wire gives a
brighter spark on breaking contact than a short one (1068.), although it
carries much less electricity.
1094. If the long wire be made into a helix, it will then be still more
effective in producing sparks and shocks on breaking contact; for by the
mutual inductive action of the convolutions each aids its neighbour, and
will be aided in turn, and the sum of effect will be very greatly
increased.
1095. If an electro-magnet be employed, the effect will be still more
highly exalted; because the iron, magnetized by the power of the continuing
current, will lose its magnetism at the moment the current ceases to pass,
and in so doing will tend to produce an electric current in the wire around
it (37. 38.), in conformity with that which the cessation of current in the
helix itself also tends to produce.
1096. By applying the laws of the induction of electric currents formerly
developed (6. &c.), various new conditions of the experiments could be
devised, which by their results should serve as tests of the accuracy of
the view just given. Thus, if a long wire be doubled, so that the current
in the two halves shall have opposite actions, it ought not to give a
sensible spark at the moment of disjunction: and this proved to be the
case, for a wire forty feet long, covered with silk, being doubled and tied
closely together to within four inches of the extremities, when used in
that state, gave scarcely a perceptible spark; but being opened out and the
parts separated, it gave a very good one. The two helices i and ii being
joined at their similar ends, and then used at their other extremities to
connect the plates of the electromotor, thus constituted one long helix, of
which one half was opposed in direction to the other half: under these
circumstances it gave scarcely a sensible spark, even when the soft iron
core was within, although containing nearly two hundred feet of wire. When
it was made into one consistent helix of the same length of wire it gave a
very bright spark.
1097. Similar proofs can be drawn from the mutual inductive action of two
separate currents (1110.); and it is important for the general principles
that the consistent action of two such currents should be established.
Thus, two currents going in the same direction should, if simultaneously
stopped, aid each other by their relative influence; or if proceeding in
contrary directions, should oppose each other under similar circumstances.
I endeavoured at first to obtain two currents from two different
electromotors, and passing them through the helices i and ii, tried to
effect the disjunctions mechanically at the same moment. But in this I
could not succeed; one was always separated before the other, and in that
case produced little or no spark, its inductive power being employed in
throwing a current round the remaining complete circuit (1090.): the
current which was stopped last always gave a bright spark. If it were ever
to become needful to ascertain whether two junctions were accurately broken
at the same moment, these sparks would afford a test for the purpose,
having an infinitesimal degree of perfection.
1098. I was able to prove the points by other expedients. Two short thick
wires were selected to serve as terminations, by which contact could be
made or broken with the electromotor. The compound helix, consisting of i
and ii (1053.), was adjusted so that the extremities of the two helices
could be placed in communication with the two terminal wires, in such a
manner that the current moving through the thick wires should be divided
into two equal portions in the two helices, these portions travelling,
according to the mode of connexion, either in the same direction or in
contrary directions at pleasure. In this manner two streams could be
obtained, both of which could be stopped simultaneously, because the
disjunction could be broken at G or F by removing a single wire. When the
helices were in contrary directions, there was scarcely a sensible spark at
the place of disjunction; but when they were in accordance there was a very
bright one.
1099. The helix i was now used constantly, being sometimes associated, as
above, with helix ii in an according direction, and sometimes with helix
iii, which was placed at a little distance. The association i and ii, which
presented two currents able to affect each other by induction, because of
their vicinity, gave a brighter spark than the association i and iii, where
the two streams could not exert their mutual influence; but the difference
was not so great as I expected.
1100. Thus all the phenomena tend to prove that the effects are due to an
inductive action, occurring at the moment when the principal current is
stopped. I at one time thought they were due to an action continued during
the _whole time_ of the current, and expected that a steel magnet would
have an influence according to its position in the helix, comparable to
that of a soft iron bar, in assisting the effect. This, however, is not the
case; for hard steel, or a magnet in the helix, is not so effectual as soft
iron; nor does it make any difference how the magnet is placed in the
helix, and for very simple reasons, namely, that the effect does not depend
upon a permanent state of the core, but a _change of state_; and that the
magnet or hard steel cannot sink through such a difference of state as soft
iron, at the moment contact ceases, and therefore cannot produce an equal
effect in generating a current of electricity by induction (34. 37.).
* * * * *
1101. As an electric current acts by induction with equal energy at the
moment of its commencement as at the moment of its cessation (10. 26.), but
in a contrary direction, the reference of the effects under examination to
an inductive action, would lead to the conclusion that corresponding
effects of an opposite nature must occur in a long wire, a helix, or an
electro-magnet, every time that _contact is made with_ the electromotor.
These effects will tend to establish a resistance for the first moment in
the long conductor, producing a result equivalent to the reverse of a shock
or a spark. Now it is very difficult to devise means fit for the
recognition of such negative results; but as it is probable that some
positive effect is produced at the time, if we knew what to expect, I think
the few facts bearing upon this subject with which I am acquainted are
worth recording.
1102. The electro-magnet was arranged with an electrolyzing apparatus at
_x_, as before described (1084.), except that the intensity of the chemical
action at the electromotor was increased until the electric current was
just able to produce the feeblest signs of decomposition whilst contact was
continued at G and E (1079.); (the iodine of course appearing against the
end of the cross wire P;) the wire N was also separated from A at _r_, so
that contact there could be made or broken at pleasure. Under these
circumstances the following set of actions was repeated several times:
contact was broken at _r_, then broken at G, next made at _r_, and lastly
renewed at G; thus any current from N to P due to _breaking_ of contact was
avoided, but any additional force to the current from P to N due to
_making_ contact could be observed. In this way it was found, that a much
greater decomposing effect (causing the evolution of iodine against P)
could be obtained by a few completions of contact than by the current which
could pass in a much longer time if the contact was _continued_. This I
attribute to the act of induction in the wire ABD at the moment of contact
rendering that wire a worse conductor, or rather retarding the passage of
the electricity through it for the instant, and so throwing a greater
quantity of the electricity which the electromotor could produce, through
the cross wire passage NP. The instant the induction ceased, ABD resumed
its full power of carrying a constant current of electricity, and could
have it highly increased, as we know by the former experiments (1060.) by
the opposite inductive action brought into activity at the moment contact
at Z or C was _broken_.
1103. A galvanometer was then introduced at _x_, and the deflection of the
needle noted whilst contact was continued at G and E: the needle was then
blocked as before in one direction (1087.), so that it should not return
when the current ceased, but remain in the position in which the current
could retain it. Contact at G or E was broken, producing of course no
visible effect; it was then renewed, and the needle was instantly
deflected, passing from the blocking pins to a position still further from
its natural place than that which the constant current could give, and thus
showing, by the temporary excess of current in this cross communication,
the temporary retardation in the circuit ABD.
1104. On adjusting a platina wire at _x_ (1081.) so that it should not be
ignited by the current passing through it whilst contact at G and E was
_continued_, and yet become red-hot by a current somewhat more powerful, I
was readily able to produce its ignition upon _making contact_, and again
upon _breaking contact_. Thus the momentary retardation in ABD on making
contact was again shown by this result, as well also as the opposite result
upon breaking contact. The two ignitions of the wire at _x_ were of course
produced by electric currents moving in opposite directions.
1105. Using the _helix_ only, I could not obtain distinct deflections at
_x_, due to the extra effect on making contact, for the reasons already
mentioned (1088.). By using a very fine platina wire there (1083.), I did
succeed in obtaining the igniting effect for making contact in the same
manner, though by no means to the same degree, as with the electro-magnet
(1104).
1106. We may also consider and estimate the effect on _making contact_, by
transferring the force of induction from the wire carrying the original
current to a lateral wire, as in the cases described (1090.); and we then
are sure, both by the chemical and galvanometrical results (1091.), that
the forces upon making and breaking contact, like action and reaction, are
equal in their strength but contrary in their direction. If, therefore, the
effect on making contact resolves itself into a mere retardation of the
current at the first moment of its existence, it must be, in its degree,
equivalent to the high exaltation of that same current at the moment
contact is broken.
1107. Thus the case, under the circumstances, is, that the intensity and
quantity of electricity moving in a current are smaller when the current
commences or is increased, and greater when it diminishes or ceases, than
they would be if the inductive action occurring at these moments did not
take place; or than they are in the original current wire if the inductive
action be transferred from that wire to a collateral one (1090.).
1108. From the facility of transference to neighbouring wires, and from the
effects generally, the inductive forces appear to be lateral, i.e. exerted
in a direction perpendicular to the direction of the originating and
produced currents: and they also appear to be accurately represented by the
magnetic curves, and closely related to, if not identical with, magnetic
forces.
1109. There can be no doubt that the current in one part of a wire can act
by induction upon other parts of the _same_ wire which are lateral to the
first, i.e. in the same vertical section (74.), or in the parts which are
more or less oblique to it (1112.), just as it can act in producing a
current in a neighbouring wire or in a neighbouring coil of the same wire.
It is this which gives the appearance of the current acting upon itself:
but all the experiments and all analogy tend to show that the elements (if
I may so say) of the currents do not act upon themselves, and so cause the
effect in question, but produce it by exciting currents in conducting
matter which is lateral to them.
1110. It is possible that some of the expressions I have used may seem to
imply, that the inductive action is essentially the action of one current
upon another, or of one element of a current upon another element of the
same current. To avoid any such conclusion I must explain more distinctly
my meaning. If an endless wire be taken, we have the means of generating a
current in it which shall run round the circuit without adding any
electricity to what was previously in the wire. As far as we can judge, the
electricity which appears as a current is the same as that which before was
quiescent in the wire; and though we cannot as yet point out the essential
condition of difference of the electricity at such times, we can easily
recognize the two states. Now when a current acts by induction upon
conducting matter lateral to it, it probably acts upon the electricity in
that conducting matter whether it be in the form of a _current_ or
_quiescent_, in the one case increasing or diminishing the current
according to its direction, in the other producing a current, and the
_amount_ of the inductive action is probably the same in both cases. Hence,
to say that the action of induction depended upon the mutual relation of
two or more currents, would, according to the restricted sense in which the
term current is understood at present (283. 517. 667.), be an error.
1111. Several of the effects, as, for instances, those with helices(1066.),
with according or counter currents (1097. 1098.), and those on the
production of lateral currents (1090.), appeared to indicate that a current
could produce an effect of induction in a neighbouring wire more readily
than in its own carrying wire, in which case it might be expected that some
variation of result would be produced if a bundle of wires were used as a
conductor instead of a single wire. In consequence the following
experiments were made. A copper wire one twenty-third of an inch in
diameter was cut into lengths of five feet each, and six of these being
laid side by side in one bundle, had their opposite extremities soldered to
two terminal pieces of copper. This arrangement could be used as a
discharging wire, but the general current could be divided into six
parallel streams, which might be brought close together, or, by the
separation of the wires, be taken more or less out of each other's
influence. A somewhat brighter spark was, I think, obtained on breaking
contact when the six wires were close together than when held asunder.
1112. Another bundle, containing twenty of these wires, was eighteen feet
long: the terminal pieces were one-fifth of an inch in diameter, and each
six inches long. This was compared with nineteen feet in length of copper
wire one-fifth of an inch in diameter. The bundle gave a smaller spark on
breaking contact than the latter, even when its strands were held together
by string: when they were separated, it gave a still smaller spark. Upon
the whole, however, the diminution of effect was not such as I expected:
and I doubt whether the results can be considered as any proof of the truth
of the supposition which gave rise to them.
1113. The inductive force by which two elements of one current (1109.
1110.) act upon each other, appears to diminish as the line joining them
becomes oblique to the direction of the current and to vanish entirely when
it is parallel. I am led by some results to suspect that it then even
passes into the repulsive force noticed by Ampère[A]; which is the cause of
the elevations in mercury described by Sir Humphry Davy[B], and which again
is probably directly connected with the quality of intensity.
[A] Recueil d'Observations Electro-Dynamiques, p. 285.
[B] Philosophical Transactions, 1823, p. 155.
1114. Notwithstanding that the effects appear only at the making and
breaking of contact, (the current remaining unaffected, seemingly, in the
interval,) I cannot resist the impression that there is some connected and
correspondent effect produced by this lateral action of the elements of the
electric stream during the time of its continuance (60. 242.). An action of
this kind, in fact, is evident in the magnetic relations of the parts of
the current. But admitting (as we may do for the moment) the magnetic
forces to constitute the power which produces such striking and different
results at the commencement and termination of a current, still there
appears to be a link in the chain of effects, a wheel in the physical
mechanism of the action, as yet unrecognised. If we endeavour to consider
electricity and magnetism as the results of two forces of a physical agent,
or a peculiar condition of matter, exerted in determinate directions
perpendicular to each other, then, it appears to me, that we must consider
these two states or forces as convertible into each other in a greater or
smaller degree; i.e. that an element of an electric current has not a
determinate electric force and a determinate magnetic force constantly
existing in the same ratio, but that the two forces are, to a certain
degree, convertible by a process or change of condition at present unknown
to us. How else can a current of a given intensity and quantity be able, by
its direct action, to sustain a state which, when allowed to react, (at the
cessation of the original current,) shall produce a second current, having
an intensity and quantity far greater than the generating one? This cannot
result from a direct reaction of the electric force; and if it result from
a change of electrical into magnetic force, and a reconversion back again,
it will show that they differ in something more than mere direction, as
regards _that agent_ in the conducting wire which constitutes their
immediate cause.
1115. With reference to the appearance, at different times, of the contrary
effects produced by the making and breaking contact, and their separation
by an intermediate and indifferent state, this separation is probably more
apparent than real. If the conduction of electricity be effected by
vibrations (283.), or by any other mode in which opposite forces are
successively and rapidly excited and neutralized, then we might expect a
peculiar and contrary development of force at the commencement and
termination of the periods during which the conducting action should last
(somewhat in analogy with the colours produced at the outside of an
imperfectly developed solar spectrum): and the intermediate actions,
although not sensible in the same way, may be very important and, for
instance, perhaps constitute the very essence of conductibility. It is by
views and reasons such as these, which seem to me connected with the
fundamental laws and facts of electrical science, that I have been induced
to enter, more minutely than I otherwise should have done, into the
experimental examination of the phenomena described in this paper.
1116. Before concluding, I may briefly remark, that on using a voltaic
battery of fifty pairs of plates instead of a single pair (1052.), the
effects were exactly of the same kind. The spark on making contact, for the
reasons before given, was very small (1101. 1107.); that on breaking
contact, very excellent and brilliant. The _continuous_ discharge did not
seem altered in character, whether a short wire or the powerful
electro-magnet were used as a connecting discharger.
1117. The effects produced at the commencement and end of a current, (which
are separated by an interval of time when that current is supplied from a
voltaic apparatus,) must occur at the same moment when a common electric
discharge is passed through a long wire. Whether, if happening accurately
at the same moment, they would entirely neutralize each other, or whether
they would not still give some definite peculiarity to the discharge, is a
matter remaining to be examined; but it is very probable that the peculiar
character and pungency of sparks drawn from a long wire depend in part upon
the increased intensity given at the termination of the discharge by the
inductive action then occurring.
1118. In the wire of the helix of magneto-electric machines, (as, for
instance, in Mr. Saxton's beautiful arrangement,) an important influence of
these principles of action is evidently shown. From the construction of the
apparatus the current is permitted to move in a complete metallic circuit
of great length during the first instants of its formation: it gradually
rises in strength, and is then suddenly stopped by the breaking of the
metallic circuit; and thus great intensity is given _by induction_ to the
electricity, which at that moment passes (1064. 1060.). This intensity is
not only shown by the brilliancy of the spark and the strength of the
shock, but also by the necessity which has been experienced of
well-insulating the convolutions of the helix, in which the current is
formed: and it gives to the current a force at these moments very far above
that which the apparatus could produce if the principle which forms the
subject of this paper were not called into play.
_Royal Institution,
December 8th, 1834._