THE ELECTRICITY OF MAGNETISM.
We have already seen how electricity was first produced by the
simple method of rubbing one body on another, then by the less
obvious means of chemical union, and next by the finer agency of
heat. In all these, it will be observed, a substantial contact is
necessary. We have now to consider a still more subtle process of
generation, not requiring actual contact, which, as might be
expected, was discovered later, that, mainly through the medium of
magnetism.
The curious mineral which has the property of attracting iron was
known to the Chinese several thousand years ago, and certainly to
the Greeks in the times of Thales, who, as in the case of the
rubbed amber, ascribed the property to its possession of a soul.
Lodestone, a magnetic oxide of iron (FE3O4), is found in various
parts of China, especially at T'szchou in Southern Chihli, which
was formerly known as the "City of the Magnet." It was called by
the Chinese the love-stone or thsu-chy, and the stone that
snatches iron or ny-thy-chy, and perchance its property of
pointing out the north and south direction was discovered by
dropping a light piece of the stone, if not a sewing needle made
of it, on the surface of still water. At all events, we read in
Pere Du Halde's Description de la Chine, that sometime in or about
the year 2635 B.C. the great Emperor Hoang-ti, having lost his way
in a fog whilst pursuing the rebellious Prince Tchiyeou on the
plains of Tchou-lou, constructed a chariot which showed the
cardinal points, thus enabling him to overtake and put the prince
to death.
A magnetic car preceded the Emperors of China in ceremonies of
state during the fourth century of our era. It contained a genius
in a feather dress who pointed to the south, and was doubtless
moved by a magnet floating in water or turning on a pivot. This
rude appliance was afterwards refined into the needle compass for
guiding mariners on the sea, and assisting the professors of feng-
shui or geomancy in their magic rites.
Magnetite was also found at Heraclea in Lydia, and at Magnesium on
the Meander or Magnesium at Sipylos, all in Asia Minor. It was
called the "Heraclean Stone" by the people, but came at length to
bear the name of "Magnet" after the city of Magnesia or the
mythical shepherd Magnes, who was said to have discovered it by
the attraction of his iron crook.
The ancients knew that it had the power of communicating its
attractive property to iron, for we read in Plato's "Ion" that a
number of iron rings can be supported in a chain by the Heraclean
Stone. Lucretius also describes an experiment in which iron
filings are made to rise up and "rave" in a brass basin by a
magnet held underneath. We are told by other writers that images
of the gods and goddesses were suspended in the air by lodestone
in the ceilings of the temples of Diana of Ephesus, of Serapis at
Alexandria, and others. It is surprising, however, that neither
the Greeks nor Romans, with all their philosophy, would seem to
have discovered its directive property.
During the dark ages pieces of Lodestone mounted as magnets were
employed in the "black arts." A small natural magnet of this kind
is shown in figure 25, where L is the stone shod with two iron
"pole-pieces," which are joined by a "keeper" A or separable
bridge of iron carrying a hook for supporting weights.
Apparently it was not until the twelfth century that the compass
found its way into Europe from the East. In the Landnammabok of
Ari Frode, the Norse historian, we read that Flocke Vildergersen,
a renowned viking, sailed from Norway to discover Iceland in the
year 868, and took with him two ravens as guides, for in those
days the "seamen had no lodestone (that is, no lidar stein, or
leading stone) in the northern countries." The Bible, a poem of
Guiot de Provins, minstrel at the court of Barbarossa, which was
written in or about the year 890, contains the first mention of
the magnet in the West. Guiot relates how mariners have an "art
which cannot deceive" of finding the position of the polestar,
that does not move. After touching a needle with the magnet, "an
ugly brown stone which draws iron to itself," he says they put the
needle on a straw and float it on water so that its point turns to
the hidden star, and enables them to keep their course. Arab
traders had probably borrowed the floating needle from the
Chinese, for Bailak Kibdjaki, author of the Merchant's Treasure,
written in the thirteenth century, speaks of its use in the Syrian
sea. The first Crusaders were probably instrumental in bringing it
to France, at all events Jacobus de Vitry (1204-15) and Vincent de
Beauvais (1250) mention its use, De Beauvais calling the poles of
the needle by the Arab words aphron and zohran.
Ere long the needle was mounted on a pivot and provided with a
moving card showing the principal directions. The variation of the
needle from the true north and south was certainly known in China
during the twelfth, and in Europe during the thirteenth century.
Columbus also found that the variation changed its value as he
sailed towards America on his memorable voyage of 1492. Moreover,
in 1576, Norman, a compass maker in London, showed that the north-
seeking end of the needle dipped below the horizontal.
In these early days it was supposed that lodestone in the pole-
star, that is to say, the "lodestar" of the poets or in mountains
of the far north, attracted the trembling needle; but in the year
1600, Dr. Gilbert, the founder of electric science, demonstrated
beyond a doubt that the whole earth was a great magnet. A magnet,
as is well known, has, like an electric battery, always two poles
or centres of attraction, which are situated near its extremities.
Sometimes, indeed, when the magnet is imperfect, there are
"consequent poles" of weaker force between them. One of the poles
is called the "north," and the other the "south," because if the
magnet were freely pivotted like a compass needle, the former
would turn to the north and the latter to the south.
Either pole will attract iron, but soft or annealed iron does not
retain the magnetism nearly so well as steel. Hence a boy's test
for the steel of his knife is only efficacious when the blade
itself becomes magnetic after being touched with the magnet. A
piece of steel is readily magnetised by stroking it from end to
end in one direction with the pole of a magnet, and in this way
compass needles and powerful bar magnets can be made.
The poles attract iron at a distance by "induction," just as a
charge of electricity, be it positive or negative, will attract a
neutral pith ball; and Dr. Gilbert showed that a north pole always
repels another north pole and attracts a south pole, while, on the
other hand, a south pole always repels a south pole and attracts a
north pole. This can be proved by suspending a magnetic needle
like a pithball, and approaching another towards it, as
illustrated in figure 26, where the north pole N attracts the
south S. Obviously there are two opposite kinds of magnetic poles,
as of electricity, which always appear together, and like magnetic
poles repel, unlike magnetic poles attract each other.
It follows that the magnetic pole of the compass needle which
turns to the north must be unlike the north and like the south
magnetic pole of the earth. Instead of calling it the "north," it
would be less confusing to call it the "north-seeking" pole of the
needle.
Gilbert made a "terella," or miniature of the earth, as a magnet,
and not only demonstrated how the compass needle sets along the
lines joining the north and south magnetic poles, but explained
the variation and the dip. He imagined that the magnetic poles
coincided with the geographical poles, but, as a matter of fact,
they do not, and, moreover, they are slowly moving round the
geographical poles, hence the declination of the needle, that is
to say its angle of divergence from the true meridian or north and
south line, is gradually changing. The north magnetic pole of the
earth was actually discovered by Sir John Ross north of British
America, on the coast of Boothia (latitude 70 degrees 5' N,
longitude 96 degrees 46' W), where, as foreseen, the needle
entirely lost its directive property and stood upright, or, so to
speak, on its head. The south magnetic pole lies in the Prince
Albert range of Victona Land, and was almost reached by Sir James
Clark Ross.
The magnetism of the earth is such as might be produced by a
powerful magnet inside, but its origin is unknown, although there
is reason to believe that masses of lodestone or magnetic iron
exist in the crust. Coulomb found that not only iron, but all
substances are more or less magnetic, and Faraday showed in 1845
that while some are attracted by a magnet others are repelled. The
former he called paramagnetic and the latter diamagnetic bodies.
The following is a list of these.--
Paramagnetic Diamagnetic
Iron Bismuth
Nickel Phosphorus
Cobalt Antimony
Aluminium Zinc
Manganese Mercury
Chromium Lead
Cerium Silver
Titanium Copper
Platinum Water
Many ores and Alcohol
salts of the Tellurium
above metals Selenium
Oxygen Sulphur
Thallium
Hydrogen
Air
We have theories of magnetism that reduce it to a phenomenon of
electricity, though we are ignorant of the real nature of both. If
we take a thin bar magnet and break it in two, we find that we
have now two shorter magnets, each with its "north" and "south"
poles, that is to say, poles of the same kind as the south and
north--magnetic poles of the earth. If we break each of these
again, we get four smaller magnets, and we can repeat the process
as often as we like. It is supposed, therefore, that every atom of
the bar is a little magnet in itself having its two opposite
poles, and that in magnetising the bar we have merely partially
turned all these atoms in one direction, that is to say, with
their north poles pointing one way and their south poles the other
way, as shown in figure 27. The polarity of the bar only shows
itself at the ends, where the molecular poles are, so to speak,
free.
There are many experiments which support this view. For example,
if we heat a magnet red hot it loses its magnetism, perhaps
because the heat has disarranged the particles and set the
molecular poles in all directions. Again, if we magnetise a piece
of soft iron we can destroy its magnetism by striking it so as to
agitate its atoms and throw them out of line. In steel, which is
iron with a small admixture of carbon, the atoms are not so free
as in soft iron, and hence, while iron easily loses its magnetism,
steel retains it, even under a shock, but not under a cherry red-
heat. Nevertheless, if we put the atoms of soft iron under a
strain by bending it, we shall find it retain its magnetism more
like a bit of steel.
It has been found, too, that the atoms show an indisposition to be
moved by the magnetising force which is known as HYSTERESIS. They
have a certain inertia, which can be overcome by a slight shock,
as though they had a difficulty of turning in the ranks to take up
their new positions. Even if this molecular theory is true,
however, it does not help us to explain why a molecule of matter
is a tiny magnet. We have only pushed the mystery back to the
atom. Something more is wanted, and electricians look for it in
the constitution of the atom, and in the luminiferous ether which
is believed to surround the atoms of matter, and to propagate not
merely the waves of light, but induction from one electrified body
to another.
We know in proof of this ethereal action that the space around a
magnet is magnetic. Thus, if we lay a horse-shoe magnet on a table
and sprinkle iron filings round it, they will arrange themselves
in curving lines between the poles, as shown in figure 28. Each
filing has become a little magnet, and these set themselves end to
end as the molecules in the metal are supposed to do. The "field"
about the magnet is replete with these lines, which follow certain
curves depending on the arrangement of the poles. In the horse-
shoe magnet, as seen, they chiefly issue from one pole and sweep
round to the other. They are never broken, and apparently they are
lines of stress in the circumambient ether. A pivoted magnet tends
to range itself along these lines, and thus the compass guides the
sailor on the ocean by keeping itself in the line between the
north and south magnetic poles of the earth. Faraday called them
lines of magnetic force, and said that the stronger the magnet the
more of these lines pass through a given space. Along them
"magnetic induction" is supposed to be propagated, and a magnet is
thus enabled to attract iron or any other magnetic substance. The
pole induces an opposite pole to itself in the nearest part of the
induced body and a like pole in the remote part. Consequently, as
unlike poles attract and like repel, the soft iron is attracted by
the inducing pole much as a pithball is attracted by an electric
charge.
The resemblances of electricity and magnetism did not escape
attention, and the derangement of the compass needle by the
lightning flash, formerly so disastrous at sea, pointed to an
intimate connection between them, which was ultimately disclosed
by Professor Oersted, of Copenhagen, in the year 1820. Oersted was
on the outlook for the required clue, and a happy chance is said
to have rewarded him. His experiment is shown in figure 29, where
a wire conveying a current of electricity flowing in the direction
of the arrow is held over a pivoted magnetic needle so that the
current flows from south to north. The needle will tend to set
itself at right angles to the wire, its north or north-seeking
pole moving towards the west. If the direction of the current is
reversed, the needle is deflected in the opposite direction, its
north pole moving towards the east. Further, if the wire is held
below the needle, in the first place, the north pole will turn
towards the east, and if the current be reversed it will move
towards the west.
The direction of a current can thus be told with the aid of a
compass needle. When the wire is wound many times round the needle
on a bobbin, the whole forms what is called a galvanoscope, as
shown in figure 30, where N is the needle and B the bobbin. When a
proper scale is added to the needle by which its deflections can
be accurately read, the instrument becomes a current measurer or
galvanometer, for within certain limits the deflection of the
needle is proportional to the strength of the current in the wire.
A rule commonly given for remembering the movement of the needle
is as follows:--Imagine yourself laid along the wire so that the
current flows from your feet to your head; then if you face the
needle you will see its north pole go to the left and its south
pole to the right. I find it simpler to recollect that if the
current flows from your head to your feet a north pole will move
round you from left to right in front. Or, again, if a current
flows from north to south, a north pole will move round it like
the sun round the earth.
The influence of the current on the needle implies a magnetic
action, and if we dust iron filings around the wire we shall find
they cling to it in concentric layers, showing that circular lines
of magnetic force enclose it like the water waves caused by a
stone dropped into a pond. Figure 31 represents the section of a
wire carrying a current, with the iron filings arranged in circles
round it. Since a magnetic pole tends to move in the direction of
the lines of force, we now see why a north or south pole tends to
move ROUND a current, and why a compass needle tries to set itself
at right angles to a current, as in the original experiment of
Oersted. The needle, having two opposite poles, is pulled in
opposite directions by the lines, and being pivoted, sets itself
tangentically to them. Were it free and flexible, it would curve
itself along one of the lines. Did it consist of a single pole, it
would revolve round the wire.
Action and re-action are equal and opposite, hence if the needle
is fixed and the wire free the current will move round the magnet;
and if both are free they will circle round each other. Applying
the above rule we shall find that when the north pole moves from
left to right the current moves from right to left. Ampere of
Paris, following Oersted, promptly showed that two parallel wires
carrying currents attracted each other when the currents flowed in
the same direction, and repelled each other when they flowed in
opposite directions. Thus, in figure 32, if A and B are the two
parallel wires, and A is mounted on pivots and free to move in
liquid "contacts" of mercury, it will be attracted or repelled by
B according as the two currents flow in the same or in opposite
directions. If the wires cross each other at right angles there is
no attraction or repulsion. If they cross at an acute angle, they
will tend to become parallel like two compass needles, when the
currents are in one direction, and to open to a right angle and
close up the other way when the currents are in opposite
directions, always tending to arrange themselves parallel and
flowing in the same direction. These effects arise from the
circular lines of force around the wire. When the currents are
similar the lines act as unlike magnetic poles and attract, but
when the currents are dissimilar the lines act as like magnetic
poles and repel each other.
Another important discovery of Ampere is that a circular current
behaves like a magnet; and it has been suggested by him that the
atoms are magnets because each has a circular current flowing
round it. A series of circular currents, such as the spiral S in
figure 33 gives, when connected to a battery C Z, is in fact a
skeleton ELECTRO-MAGNET having its north and south poles at the
extremities. If a rod or core of soft iron I be suspended by
fibres from a support, it will be sucked towards the middle of the
coil as into a vortex, by the circular magnetic lines of every
spire or turn of the coil. Such a combination is sometimes called
a solenoid, and is useful in practice.
When the core gains the interior of the coil it becomes a
veritable electromagnet, as found by Arago, having a north pole at
one end and a south pole at the other. Figure 34 illustrates a
common poker magnetised in the same way, and supporting nails at
both ends. The poker has become the core of the electromagnet. On
reversing the direction of the current through the spiral we
reverse the poles of the core, for the poker being of soft or
wrought iron, does not retain its magnetism like steel. If we stop
the current altogether it ceases to be a magnet, and the nails
will drop away from it.
Ampere's experiment in figure 32 has shown us that two currents,
more or less parallel, influence each other; but in 1831 Professor
Faraday of the Royal Institution, London, also found that when a
current is started and stopped in a wire, it induces a momentary
and opposite current in a parallel wire. Thus, if a current is
STARTED in the wire B (fig. 32) in direction of the arrow, it will
induce or give rise to a momentary current in the wire A, flowing
in a contrary direction to itself. Again, if the current in B be
STOPEED, a momentary current is set up in the wire A in a
direction the same as that of the exciting current in B. While the
current in B is quietly flowing there is no induced current in A;
and it is only at the start or the stoppage of the inducing or
PRIMARY current that the induced or SECONDARY current is set up.
Here again we have the influence of the magnetic field around the
wire conveying a current.
This is the principle of the "induction coil" so much employed in
medical electricity, and of the "transformer" or "converter" used
in electric illumination. It consists essentially, as shown in
figure 35, of two coils of wire, one enclosing the other, and both
parallel or concentric. The inner or primary coil P C is of short
thick wire of low resistance, and is traversed by the inducing
current of a battery B. To increase its inductive effect a core of
soft iron I C occupies its middle. The outer or secondary coil S C
is of long thin wire terminating in two discharging points D1 D2.
An interrupter or hammer "key" interrupts or "makes and breaks"
the circuit of the primary coil very rapidly, so as to excite a
great many induced currents in the secondary coil per second, and
produce energetic sparks between the terminals D1 D2. The
interrupter is actuated automatically by the magnetism of the iron
core I C, for the hammer H has a soft iron head which is attracted
by the core when the latter is magnetised, and being thus drawn
away from the contact screw C S the circuit of the primary is
broken, and the current is stopped. The iron core then ceases to
be a magnet, the hammer H springs back to the contact screw, and
the current again flows in the primary circuit only to be
interrupted again as before. In this way the current in the
primary coil is rapidly started and stopped many times a second,
and this, as we know, induces corresponding currents in the
secondary which appear as sparks at the discharging points. The
effect of the apparatus is enhanced by interpolating a "condenser"
C C in the primary circuit. A condenser is a form of Leyden jar,
suitable for current electricity, and consists of layers of
tinfoil separated from each other by sheets of paraffin paper,
mica, or some other convenient insulator, and alternate foils are
connected together. The wires joining each set of plates are the
poles of the condenser, and when these are connected in the
circuit of a current the condenser is charged. It can be
discharged by joining its two poles with a wire, and letting the
two opposite electricities on its plates rush together. Now, the
sudden discharge of the condenser C C through the primary coil P C
enhances the inductive effect of the current. The battery B, here
shown by the conventional symbol [Electrical Symbol] where the
thick dash is the negative and the thin dash the positive pole, is
connected between the terminals T1 T2, and a COMMUTATOR or pole-
changer R, turned with a handle, permits the direction of the
current to be reversed at will.
Figure 36 represents the exterior of an ordinary induction coil of
the Ruhmkorff pattern, with its two coils, one over the other C,
its commutator R, and its sparkling points D1D2, the whole being
mounted on a mahogany base, which holds the condenser.
The intermittent, or rather alternating, currents from the
secondary coil are often applied to the body in certain nervous
disorders. When sent through glass tubes filled with rarefied
gases, sometimes called "Geissler tubes," they elicit glows of
many colours, vieing in beauty with the fleeting tints of the
aurora polaris, which, indeed, is probably a similar effect of
electrical discharges in the atmosphere.
The action of the induction is reversible. We can not only send a
current of low "pressure" from a generator of weak electromotive
force through the primary coil, and thus excite a current of high
pressure in the secondary coil, but we can send a current of high
pressure through the secondary coil and provoke a current of low
pressure in the primary coil The transformer or converter, a
modified induction coil used in distributing electricity to
electric lamps and motors, can not only transform a low pressure
current into a high, but a high pressure current into a low. As
the high pressure currents are best able to overcome the
resistance of the wire convening them, it is customary to transmit
high pressure currents from the generator to the distant place
where they are wanted by means of small wires, and there transform
them into currents of the pressure required to light the lamps or
drive the motors.
We come now to another consequence of Oersted's great discovery,
which is doubtless the most important of all, namely, the
generation of electricity from magnetism, or, as it is usually
called, magneto-electric induction. In the year 1831 the
illustrious Michael Faraday further succeeded in demonstrating
that when a magnet M is thrust into a hollow coil of wire C, as
shown in figure 37, a current of electricity is set up in the coil
whilst the motion lasts. When the magnet is withdrawn again
another current is induced in the reverse direction to the first.
If the coil be closed through a small galvanometer G the movements
of the needle to one side or the other will indicate these
temporary currents. It follows from the principle of action and
reaction that if the magnet is kept still and the coil thrust over
it similar currents will be induced in the coil. All that is
necessary is for the wires to cut the lines of magnetic force
around the magnet, or, in other words, the lines of force in a
magnetic field We have seen already that a wire conveying a
current can move a magnetic pole, and we are therefore prepared to
find that a magnetic pole moved near a wire can excite a current
in it.
Figure 38 illustrates the conditions of this remarkable effect,
where N and S are two magnetic poles with lines of force between
them, and W is a wire crossing these lines at right angles, which
is the best position. If, now, this wire be moved so as to sink
bodily through the paper away from the reader, an electric current
flowing in the direction of the arrow will be induced in it. If,
on the contrary, the wire be moved across the lines of force
towards the reader, the induced current will flow oppositely to
the arrow. Moreover, if the poles of the magnet N and S exchange
places, the directions of the induced currents will also be
reversed. This is the fundamental principle of the well known
dynamo-electric machine, popularly called a dynamo.
Again, if we send a current from some external source through the
wire in the direction of the arrow, the wire will move OF ITSELF
across the lines of force away from the reader, that is to say, in
the direction it would need to be moved in order to excite such a
current; and if, on the other hand, the current be sent through it
in the reverse direction to the arrow, it will move towards the
reader. This is the principle of the equally well-known electric
motor. Figure 39 shows a simple method of remembering these
directions.
Let the right hand rest on the north pole of a magnet and the
forefinger be extended in the direction of the lines of force,
then the outstretched thumb will indicate the direction in which
the wire or conductor moves and the bent middle finger the
direction of the current. These three digits, as will be noticed,
are all at right angles to each other, and this relation is the
best for inducing the strongest current in a dynamo or the most
energetic movement of the conductor in an electric motor.
Of course in a dynamo-electric generator the stronger the magnetic
field, the less the resistance of the conductor, and the faster it
is moved across the lines of force, that is to say, the more lines
it cuts in a second the stronger is the current produced.
Similarly in an electric motor, the stronger the current and
magnetic field the faster will the conductor move.
The most convenient motion to give the conductor in practice is
one of rotation, and hence the dynamo usually consists of a coil
or series of coils of insulated wire termed the "armature," which
is mounted on a spindle and rapidly rotated in a strong magnetic
field between the poles of powerful magnets. Currents are
generated in the coils, now in one direction then in another, as
they revolve or cross different parts of the field; and, by means
of a device termed a commutator, these currents can be collected
or sifted at will, and led away by wires to an electric lamp, an
accumulator, or an electric motor, as desired. The character of
the electricity is precisely the same as that generated in the
voltaic battery.
The commutator may only collect the currents as they are
generated, and supply what is called an alternating current, that
is to say, a current which alternates or changes its direction
several hundred times a second, or it may sift the currents as
they are produced and supply what is termed a continuous current,
that is, a current always in the same direction, like that of a
voltaic battery. Some machines are made to supply alternating
currents, others continuous currents. Either class of current will
do for electric lamps, but only continuous currents are used for
electo-plating, or, in general, for electric motors.
In the "magneto-electric" machine the FIELD MAGNETS are simply
steel bars permanently magnetised, but in the ordinary dynamo the
field magnets are electro-magnets excited to a high pitch by means
of the current generated in the moving conductor or armature. In
the "series-wound" machine the whole of the current generated in
the armature also goes through the coils of the field magnets.
Such a machine is sketched in figure 40, where A is the armature,
consisting of an iron core surrounded by coils of wire and
rotating in the field of a powerful electro-magnet NS in the
direction of the arrows. For the sake of simplicity only twelve
coils are represented. They are all in circuit one with another,
and a wire connects the ends of each coil to corresponding metal
bars on the commutator C. These bars are insulated from each other
on the spindle X of the armature. Now, as each coil passes through
the magnetic field in turn, a current is excited in it. Each coil
therefore resembles an individual cell of a voltaic battery,
connected in series. The current is drawn off from the ring by two
copper "brushes" b, be which rub upon the bars of the commutator
at opposite ends of a diameter, as shown. One brush is the
positive pole of the dynamo, the other is the negative, and the
current will flow through any wire or external circuit which may
be connected with these, whether electric lamps, motors,
accumulators, electro-plating baths, or other device. The small
arrows show the movements of the current throughout the machine,
and the terminals are marked (+) positive and (-) negative.
It will be observed that the current excited in the armature also
flows through the coils of the electro-magnets, and thus keeps up
their strength. When the machine is first started the current is
feeble, because the field of the magnets in which the armature
revolves is merely that due to the dregs or "residual magnetism"
left in the soft iron cores of the magnet since the last time the
machine was used. But this feeble current exalts the strength of
the field-magnets, producing a stronger field, which in turn
excites a still stronger current in the armature, and this process
of give and take goes on until the full strength or "saturation"
of the magnets is attained.
Such is the "series" dynamo, of which the well-known Gramme
machine is a type. Figure 41 illustrates this machine as it is
actually made, A being the armature revolving between the poles NS
of the field-magnets M, M, M' M', on a spindle which is driven by
means of a belt on the pulley P from a separate engine The brushes
b b' of the commutator C collect the current, which in this case
is continuous, or constant in its direction.
The current of the series machine varies with the resistance of
the external or working circuit, because that is included in the
circuit of the field magnets and the armature. Thus, if we vary
the number of electric lamps fed by the machine, we shall vary the
current it is capable of yielding. With arc lamps in series, by
adding to the number in circuit we increase the resistance of the
outer circuit, and therefore diminish the strength of the current
yielded by the machine, because the current, weakened by the
increase of resistance, fails to excite the field magnets as
strongly as before. On the other hand, with glow lamps arranged in
parallel, the reverse is the case, and putting more lamps in
circuit increases the power of the machine, by diminishing the
resistance of the outer circuit in providing more cross-cuts for
the current. This, of course, is a drawback to the series machine
in places where the number of lamps to be lighted varies from time
to time. In the "shunt-wound" machine the field magnets are
excited by diverting a small portion of the main current from the
armature through them, by means of a "shunt" or loop circuit. Thus
in figure 42 where C is the commutator and b b' the brushes, M is
a shunt circuit through the magnets, and E is the external or
working circuit of the machine.
The small arrows indicate the directions of the currents. With
this arrangement the addition of more glow lamps to the external
circuit E DIMINISHES the current, because the portion of it which
flows through the by-path M, and excites the magnets, is less now
that the alternative route for the current through E is of lower
resistance than before. When fewer glow lamps are in the external
circuit E, and its resistance therefore higher, the current in the
shunt circuit M is greater than before, the magnets become
stronger, and the electromotive force of the armature is
increased. The Edison machine is of this type, and is illustrated
in figure 43, where M M' are the field magnets with their poles N
S, between which the armature A is revolved by means of the belt
B, and a pulley seen behind. The leading wires W W convey the
current from the brushes of the commutator to the external
circuit. In this machine the conductors of the armature are not
coils of wire, but separate bars of copper.
In shunt machines the variation of current due to a varying number
of lamps in use occasions a rise and fall in the brightness of the
lamps which is undesirable, and hence a third class of dynamo has
been devised, which combines the principles of both the series and
shunt machines. This is the "compound-wound" machine, in which the
magnets are wound partly in shunt and partly in series with the
armature, in such a manner that the strength of the field-magnets
and the electromotive force of the current do not vary much,
whatever be the number of lamps in circuit. In alternate current
machines the electromotive force keeps constant, as the field-
magnets are excited by a separate machine, giving a continuous
current.
We have already seen that the action of the dynamo is reversible,
and that just as a wire moved across a magnetic field supplies an
electric current, so a wire at rest, but conducting a current
across a magnetic field, will move. The electric motor is
therefore essentially a dynamo, which on being traversed by an
electric current from an external source puts itself in motion.
Thus, if a current be sent through the armature of the Gramme
machine, shown in figure 41, the armature will revolve, and the
spindle, by means of a belt on the pulley P, can communicate its
energy to another machine.
Hence the electric motor can be employed to work lathes, hoists,
lifts, drive the screws of boats or the wheels of carriages, and
for many other purposes. There are numerous types of electric
motor as of the dynamo in use, but they are all modifications of
the simple continuous or alternating current dynamo.
Obviously, since mechanical power can be converted into
electricity by the dynamo, and reconverted into mechanical power
by the motor, it is sufficient to connect a dynamo and motor
together by insulated wire in order to transmit mechanical power,
whether it be derived from wind, water, or fuel, to any reasonable
distance.