The object of these articles is to lay down in the simplest and most
intelligible way the principles which are concerned in the mechanical
production of electric currents. Every one knows now that electric
lights are produced from powerful currents of electricity generated in
a machine containing magnets and coils of wire, and driven by a steam
engine, or gas engine, or water-wheel. But of the thousands who have
heard that a steam engine can thus provide us with electric currents,
how many are there who comprehend the action of the generator or
dynamo-electric machine? How many, of engineers even, can explain where
the electricity comes from, or how the mechanical power is converted
into electrical energy, or what the magnetism of the iron magnets has
to do with it all? Take any one of the dynamo-electric machines of
the present date--the Siemens, the Gramme, the Brush, or the Edison
machine--of each of these there exist descriptions excellent in their
way, and sufficient for men already versed in the technicalities of
electric science. But to those who have not served an apprenticeship to
the technicalities--to all but professed electricians--the action
of these machines is almost an unknown mystery. As, however, an
understanding of the how and the why of the dynamo-electric machine or
generator is the very A B C of electrical engineering, an exposition
of the fundamental principles of the mechanical production of electric
currents demands an important place in the current science of the day.
It will be our endeavor to expound these principles in the plainest
terms, while at the same time sacrificing nothing in point of scientific
accuracy or of essential detail.
The modern dynamo-electric machine or generator may be regarded as
a combination of iron bars and copper wires, certain parts of the
machinery being fixed, while other parts are driven round by the
application of mechanical forces. How the movement of copper wires and
iron bars in this peculiar arrangement can generate electric currents is
the point which we are proposing to make clear. Friction has nothing
to do with the matter. The old-fashioned spark-producing "electrical
machine" of our youthful days, in which a glass cylinder or disk was
rotated by a handle while a rubber of silk pressed against it, has
nothing in common with the dynamo-electric generator, except that in
both something turns upon an axis as a grindstone or the barrel of
a barrel-organ may do. In the modern "dynamo" we cannot help having
friction at the bearings and contact pieces, it is true, but there
should be no other friction. The moving coils of wire or "armatures"
should rotate freely without touching the iron pole-pieces of the fixed
portion of the machine. In fact friction would be fatal to the action of
the "dynamo." How then does it act? We will proceed to explain without
further delay. There are, however, three fundamental principles to be
borne in mind if we would follow the explanation clearly from step to
step, and these three principles must be laid down at the very outset.
1. The first principle is that the existence of the energy of electric
currents, and also the energy of magnetic attractions, must be sought
for not so much _in the wire_ that carries the current, or _in the bar_
of steel or iron that we call a magnet, as _in the space that surrounds_
the wire or the bar.
2. The second fundamental principle is that the electric current is, in
one sense, quite as much a _magnetic_ fact as an electrical fact; and
that the wire which carries a current through it has magnetic properties
(so long as the current flows) and can attract bits of iron to itself as
a steel magnet does.
3. The third principle to be borne in mind is that to do work of any
kind, whether mechanical or electrical, requires the expenditure of
energy to a certain amount. The steam engine cannot work without its
coal, nor the laborer without his food; nor will a flame go on burning
without its fuel of some kind or other. Neither can an electric current
go on flowing, nor an electric light keep on shedding forth its beams,
without a constant supply of energy from some source or other.
[Illustration: Fig. 1.]
The last of these three principles, involving the relation of electric
currents to the work they can do and to the energy expended in their
production, will be treated of separately and later. Meantime we resume
the task of showing how such currents can be produced mechanically, and
how magnetism comes in in the process.
[Illustration: Fig. 2]
Surrounding every magnet there is a "field" or region in which the
magnetic forces act. Any small magnet, such for example as a compass
needle, when brought into this field of force, exhibits a tendency to
set itself in a certain direction. It turns so as to point with its
north pole toward the south pole of the magnet, and with its south pole
toward the north pole of the magnet; or if it cannot do both these
things at once, it takes up an intermediate position under the joint
action of the separate forces and sets in along a certain line. Such
lines of force run through the magnetic "field" from one pole of the
magnet to the other in curves. If we define a line of force as being the
line along which a free north-seeking magnetic pole would be urged, then
these lines will run from the north pole of the magnet round to the
south pole, and pass through the substance of the magnet itself. In Fig.
1 a rough sketch is given of the lines of magnetic force as they emerge
from the poles of a bar magnet in tufts. The arrow heads show the
direction in which a free north pole would move. These lines of forces
are no fiction of the imagination, like the lines of latitude and
longitude on the globe; they exist and can be rendered visible by the
simplest of expedients. When iron filings are sprinkled upon a card or
a sheet of glass below which a magnet is placed, the filings set
themselves--especially if aided by a gentle tap--along the lines of
force. Fig. 2 is a reproduction from nature of this very experiment, and
surpasses any attempt to draw the lines of force artificially. It
is impossible to magnetize a magnet without also in this fashion
magnetizing the space surrounding the magnet; and the space thus filled
with the lines of force possesses properties which ordinary unmagnetic
space does not possess. These lines give us definite information about
the magnetic condition of the space where they are. Their direction
shows us the direction of the magnetic forces, and their density shows
us the strength of the magnetic forces; for where the force is strongest
there we have the lines of force most numerous and most strongly
delineated in the scattered filings. To complete this first
consideration of the magnetic field surrounding a magnet, we will take a
look at Fig. 3, which reproduces the lines of filings as they settle in
the field of force opposite the end of a bar magnet. The repulsion of
the north pole of the magnet upon the north poles of other magnets would
be, of course, in lines diverging radially from the magnet pole.
[Illustration: Fig. 3]
We will next consider the space surrounding a wire through which a
current of electricity is flowing. This wire has magnetic properties so
long as the current continues, and will, like a magnet, act on a compass
needle. But the needle never tries to point toward the wire; its
tendency is always to set itself broadside to the current and at right
angles to it. The "field" of a current flowing up a straight wire is,
in fact, not unlike the sketch shown in Fig. 4, where instead of tufted
groups we have a sort of magnetic whirl to represent the lines of force.
The lines of force of the galvanic field are, indeed, circles or curves
which inclose the conducting wire, and their number is proportional
to the strength of the current. In the figure, where the current is
supposed to be flowing up the wire (shown by the dark arrows), the
little arrows show the direction in which a free north pole would be
urged round the wire;[1] a south pole would, of course, be urged round
the wire in the contrary direction. Now, though when we look at the
telegraph wires, or at any wire carrying a current of electricity, we
cannot _see_ these whirls of magnetic force in the surrounding space,
there is no doubt that they exist there, and that a great part of the
energy spent in starting an electric current is spent in producing these
magnetic whirls in the surrounding space. There is, however, one way of
showing the existence of these lines of force; similar, indeed, to
that adopted for showing the lines of force in the field surrounding a
magnet. Pass the conducting wire up through a hole in a card or a plate
of glass, as shown in Fig. 5, and sprinkle filings over the surface.
They will, when the glass is gently tapped, arrange themselves in
concentric circles, the smallest and innermost being the best defined
because the magnetic force is strongest there. Fig 6 is an actual
reproduction of the circular lines produced in this fashion by iron
filings in the field of force surrounding an electric current.
[Footnote 1: It will not be out of place here to recall Ampere's
ingenious rule for remembering the direction in which a current urges
the pole of a magnetic needle. "Suppose a man swimming in the wire with
the current, and that he turns so as to face the needle, then the north
pole of the needle will be deflected toward his left hand."]
[Illustration: Fig. 4]
This experimental evidence must suffice to establish two of the three
fundamental points stated at the outset, for they prove conclusively
that the electric current may be treated as a magnetic phenomenon, and
that both in the case of the pole of a magnet, and in that of the wire
which carries a current, a portion, at any rate, of the energy of the
magnetic forces exists outside the magnet or the current, and must be
sought in the surrounding space.
[Illustration: Fig. 5]
[Illustration: Fig. 6]
Having grasped these two points, the next step in our argument is to
establish the relation between the current and the magnet, and to show
how one may produce the other.
[Illustration: Fig. 7]
If we wind a piece of copper wire into a helix or spiral, as in Fig. 7,
and pass a current of electricity through it, the magnetic whirls in the
surrounding space are modified, and the lines of force are no longer
small circles wrapping round the conducting wire. For now the lines of
force of adjacent strands of the coil merge into one another, and run
continuously through the helix from one end to the other. Compare this
figure with Fig. 1, and the similarity in the arrangement of the lines
of force is obvious. The front end of the helix acts, in fact, like the
north pole of a magnet, and the further end like the south pole. If a
small bar of iron be now pushed into the interior of this helix, the
lines of force will run through it and magnetize it, converting it into
an _electro-magnet_. The magnetic "field" of such an electro-magnet is
shown in Fig. 8, which is reproduced from the actual figure made by iron
filings. To magnetize the iron bar of the electro-magnet as strongly as
possible the wire should be coiled many times round, and the current
should be as strong as possible. This mode of making an iron rod or bar
into a powerful magnet is adopted in every dynamo-electric machine. For,
as will be presently explained, very powerful magnets are required, and
these magnets are most effectively made by sending the electric currents
through spiral coils of wire wound (as in Fig. 8) round the bars that
are to be made into magnets.
[Illustration: Fig. 8]
The reader will at this point probably be ready to jump to the
conclusion that magnets and currents are alike surrounded by a sort of
magnetic atmosphere, and such a view may help those to whom the subject
is fresh to realize how such actions as we have been describing can be
communicated from one magnet to another, or from a current to a magnet.
Nevertheless such a conclusion would be both premature and inaccurate.
Even in the most perfect vacuum these actions still go on, and the lines
of force can still be traced. It is probably more correct to conclude
that these magnetic actions are propagated through space not by special
magnetic atmospheres, but by there being movements and pressures and
tensions in the _ether_ which is believed to pervade all space as a
very thin medium more attenuated than the lightest gas, and which when
subjected to electro-magnetic forces assumes a peculiar state, and
gives rise to the actions which have been detailed in the preceding
paragraphs.
[Illustration: Fig. 9.]
The next point to be studied is the magnetic property of a single loop
of the wire through which an electric current flows. Fig. 9 represents
a single voltaic cell containing the usual plates of zinc and copper
dipping into acid to generate a current in the old-fashioned way. This
current flows from the zinc plate through the liquid to the copper
plate, and from thence it flows round the wire ring or circuit back to
the zinc plate. Here the lines of magnetic force in the surrounding
space are no longer only whirls like those drawn in Fig. 4 and 6, for
they react on one another and become nearly parallel where they pass
through the middle of the ring. The thick arrows show the direction of
the electric current, the fine arrows are the lines of magnetic force,
and show the paths along which a free north pole would be urged. All the
front face, where the arrow-heads are, will be like the north pole of a
magnet. All the other face of the ring will be like the south pole of a
magnet. Our ring resembles a flat magnet, one face all north pole the
other face all south pole. Such a magnet is sometimes called a "magnetic
shell."[1]
[Footnote 1: The rule for telling which face of the magnetic shell (or
of the loop circuit) is north and which south in its magnetic properties
is the following: If as you look at the circuit the current is flowing
in the same apparent direction as the hands of a clock move, then the
face you are looking at is a south pole. If the current flows the
opposite way round to the hands of a clock, then it is the north pole
face that you are looking at.]
Since the circuit through which the current is flowing has these
magnetic properties, it can attract other magnets or repel them
according to circumstances.
[Illustration: Fig. 10.]
If a magnet be placed near the circuit, so that its north pole, N, is
opposite that side of the circuit which acts as a south pole, the magnet
and the circuit will attract one another. The lines of force that
radiate from the end of the magnet, curve round and coalesce with
some of those of the circuit. It was shown by the late Professor
Clerk-Maxwell, that every portion of a circuit is acted upon by a force
urging it in such a direction as to make it inclose within its embrace
the greatest possible number of lines of force. This proposition, which
has been termed "Maxwell's Rule," is very important, because it can be
so readily applied to so many cases, and will enable one so easily
to think out the actual reaction in any particular case. The rule is
illustrated by the sketch shown in Fig. 10, where a bar magnet has been
placed with its north pole opposite the south face of the circuit of
the cell. The lines of force of the magnet are drawn into the ring and
coalesce with those due to the current. According to Faraday's mode of
regarding the actions in the magnetic field there is a tendency for the
lines of force to shorten themselves. This would occur if either the
magnet were pulled into the circuit, or the circuit were moved up toward
the magnet. Each attracts the other, and whichever of them is free to
move will move in obedience to the attraction. And the motion will in
either case be such as to increase the total number of lines of force
that pass through the circuit. Lest it should be thought that Fig. 10 is
fanciful or overdrawn, we reproduce an actual magnetic "field" made in
the manner described in the preceding article. Fig. 11 is a kind of
sectional view of Fig. 10, the circuit being represented merely by two
circular spots or holes above and below the middle line, the current
flowing toward the spectator through the lower spot, and passing in
front of the figure to the upper hole, where it flows down. Into this
circuit the pole, N, is attracted, the tendency being to draw as many
lines of force as possible into the embrace of the circuit.
[Illustration: Fig. 11.]
So far as the reasoning about these mutual actions of magnets and
currents is concerned, it would therefore appear that the lines of force
are the really important feature to be understood and studied. All our
reasons about the attractions of magnets could be equally well thought
out if there were no corporeal magnets there at all, only collections
of lines of force. Bars of iron and steel may be regarded as convenient
conductors of the lines of force; and the poles of magnets are simply
the places where the lines of force run out of the metal into the air or
_vice versa_. Electric currents also may be reasoned about, and their
magnetic actions foretold quite irrespective of the copper wire that
acts as a conductor; for here there are not even any poles; the lines
of force or magnetic whirls are wholly outside the metal. There is an
important difference, however, to be observed between the case of the
lines of force of the current, and that of the lines of force of the
magnet. The lines of force of the magnet are the magnet so far as
magnetic forces are concerned; for a piece of soft iron laid along the
lines of force thereby becomes a magnet and remains a magnet as long
as the lines of force pass through it. But the lines of force crossing
through a circuit are not the same thing as the current of electricity
that flows round the circuit. You may take a I loop of wire and put the
poles of magnets on each side of it so that the lines of force pass
through in great numbers from one face to the other, but if you have
them there even for months and years the mere presence of these lines
of force will not create an electric current even of the feeblest kind.
There must be _motion_ to induce a current of electricity to flow in a
wire circuit.
Faraday's great discovery was, in fact, that when the pole of a magnet
is moved into, or moved out of, a coil of wire, the motion produces,
while it lasts, currents of electricity in the coil. Such currents are
known as "induced currents;" and the action is called magneto-electric
"induction." The momentary current produced by plunging the magnet pole
into the wire coil or circuit is found to be in the opposite direction
to that in which a current must be sent if it were desired to attract
the magnet pole into the coil. If the reader will look back to Fig. 10
he will see that a north magnet pole is being attracted in from behind
into a circuit round which, as he views it, the current flows in an
opposite sense to that in which the hands of a clock move round. Now,
compare this figure with Fig. 12, which represents the generation of a
momentary induced current by the act of moving the north pole, N, toward
a wire ring, which is in this case connected with a little detecter
galvanometer, G. The momentary current flows round the circuit (as seen
by the spectator from the front) in the _same_ sense as the movement of
the hands of a clock. The induced current which results from the motion
is found, then, to be in a direction exactly opposed to that of the
current that would itself produce the same movement of the magnet pole.
If the north pole, instead of being moved toward or into the circuit,
were moved away from the circuit, this motion will also induce a
transient current to flow round the wire, but this time the current will
be in the same sense as that in Fig. 10, in the opposite sense to that
in Fig. 12. Pulling the magnet pole away sets up a current in the
reverse direction to that set up by pushing the pole nearer. In both
cases the currents only last while the motion lasts.
[Illustration: Fig. 12.]
Now in the first article it was pointed out that the lines of force of
the magnet indicate not only the direction, but the strength of the
magnetic forces. The stronger the pole of the magnet is, the greater
will be the _number of lines of force_ that radiate from its poles. The
strength of the current that flows round a circuit is also proportional
to the number of lines of force which are thereby caused to pass (as in
Fig. 9) through the circuit. The stronger the current, the more numerous
the lines of force that thread themselves through the circuit. When a
magnet is moved near a circuit near it, it is found that any alteration
in the number of lines of force that cross the circuit is accompanied
by the production of a current. Referring once more to Fig. 10, we will
call the direction of the current round the circuit in that figure the
_positive_ direction; and to define this direction we may remark that if
we were to view the circuit from such a point as to look along the lines
of force in their own direction, the direction of the current round
the circuit will appear to be the same as that of the hands of a clock
moving round a dial. If the magnet, N S, be now drawn away from the
circuit so that fewer of its lines of force passed through the circuit,
experiment shows the result that the current flowing in circuit will be
for the moment increased in strength, the _increase_ in strength being
proportional to the rate of _decrease_ in the number of lines of force.
So, on the other hand, if the magnet were pushed up toward the circuit,
the current in the circuit would be momentarily reduced in strength, the
decrease in strength in the current being proportional to the rate of
increase in the number of lines of force.
Similar considerations apply to the case of the simple circuit and the
magnet shown in Fig. 12. In this circuit there is no current flowing so
long as the magnet is at rest; but if the magnet be moved up toward
the circuit so as to _increase_ the number of lines of force that pass
through the circuit, there will be a momentary "inverse" current induced
in the circuit and it will flow in the _negative_ direction. While if
the magnet were moved away the _decrease_ in the number of lines of
force would result in a transient "direct" current, or one flowing in
the _positive_ direction.
It would be possible to deduce these results from an abstract
consideration of the matter from the point of view of the principle of
conservation of energy. But we prefer to reserve this point until a
general notion of the action of dynamo-electric machines has been given.
The following principles or generalized statements follow as a matter of
the very simplest consequence from the foregoing considerations:
(a) To induce a current in a coil of wire by means of a magnet there
must be relative motion between coil and magnet.
(b) Approach of a magnet to a coil or of a coil to a magnet induces
currents in the opposite direction to that induced by recession.
(c) The stronger the magnet the stronger will be the induced currents in
the coils.
(d) The more rapid the motion the stronger will be the momentary
current induced in the coils (but the time it lasts will, of course, be
shorter).
(e) The greater the number of turns in the coil the stronger will be the
total current induced in it by the movement of the magnet.
These points are of vital importance in the action of dynamo electric
generators. It remains, however, yet to be shown how these transient and
momentary induction currents can be so directed and manipulated as to be
made to combine into a steady and continuous supply. To bring a magnet
pole up toward a coil of wire is a process which can only last a very
limited time; and its recession from the coil also cannot furnish a
continuous current since it is a process of limited duration. In the
earliest machines in which the principle of magneto-electric induction
was applied, the currents produced were of this momentary kind,
alternating in direction. Coils of wire fixed to a rotating axis were
moved past the pole of a magnet. While the coil was approaching the
lines of force were increasing, and a momentary inverse current was set
up, which was immediately succeeded by a momentary direct current as the
coil receded from the pole. Such machines on a small scale are still to
be found in opticians' shops for the purpose of giving people shocks. On
a large scale alternate current machines are still employed for certain
purposes in electric lighting, as, for example, for use with the
Jablochkoff candle. Large alternate-current machines have been devised
by Wilde, Gramme, Siemens, De Meritens, and others.--_Engineering_.
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