_Magneto-electric Machines._
I have shown how the electric current is produced by the action of
chemical or primary batteries, and how this current will flow through
suitable conductors. I shall now explain how mechanical power may be
converted into electricity. It has been found that if a wire, preferably
of copper, of which the ends are joined together, is moved past a magnet
a current is induced in the wire, flowing in one direction while the
wire is approaching the magnet, and in the opposite direction while it
is receding from it. This is then not a continuous current like we
obtained from the chemical battery, but an alternating one, and you will
see later on how it can be made to produce similar effects. The oftener
the wire passes the magnet the more electricity is generated, so that if
we make a coil of the wire and move a large number of parts of wire past
at one time, the effects on each part are accumulated; and if instead of
having one magnet to pass before, we have several, the effects will be
doubled or trebled, &c., in proportion to the number. If, again, the
coil is moved at an increased speed past the magnets, the effects will
be still further increased.
[Illustration: FIG. 2.]
[Illustration: FIG. 3.]
The knowledge of these facts led to the construction of the various
magneto-electric machines, of which a familiar type is seen in those
small ones used for medical purposes. They contain a large horse-shoe
magnet, close to the end of which two bobbins of copper wire are made to
revolve at a high speed, and all who have used these machines know that
the more quickly they turn the handle the greater shock the person
receives who is being operated upon. The current generated is really
very feeble, the shock being produced by interrupting it at every half
revolution by means of a small spring or other suitable mechanism. If
the current is not so interrupted, it cannot be felt at all, which may
be proved by lifting up the spring on the spindle of the ordinary kind.
The current is an alternating one, and changes its direction throughout
the circuit, however extended it may be, at every half revolution. If it
is required to have a continuous current, use must be made of what is
termed a commutator, and I shall endeavour to explain the manner in
which it acts as simply as possible. Without going into any further
details as to the construction of the bobbins, and their action at any
particular moment, I shall content myself with saying that if the wire
on the two bobbins is continuous, and the ends are connected, the
current will flow one way during half a revolution, and the other way
during the other half. Now, in Fig. 2, on the spindle A on which the
bobbins are fixed, is fitted a split collar formed of two halves B and
C, to which are joined respectively the ends of the wires + and -. This
collar is insulated from the spindle by a suitable insulating material,
that is to say, a material which does not conduct electricity, such as
wood, ivory, &c., and is represented in Fig. 2 by the dark parts D. So
far the circuit is not complete, so that however quickly you turn the
machine no current is produced. If, however, some means is employed for
joining B and C by a conductor, the alternating current is produced as
before. In Fig. 3, I show a section through B A C. On a base E made of
wood, are fixed two metal springs F and G, which are made to press
against B and C respectively; wires are connected at H and K, which,
joined together, complete the circuit. A continuous current is said to
be + or positive where it leaves a battery, and - or negative where it
returns; it will be convenient to use these signs and terms in the
following explanation. At one portion of the revolution the spindle will
be in the position shown in Fig. 3, and the + current is flowing into B,
through F, to the terminal H, thence through the circuit to the terminal
K, through G to C, and so back through the - wire to the bobbins of the
machine. In Fig. 4 the spindle has made a half revolution, bringing B in
contact with G, and C with F. But by this half turn the current is
reversed in the bobbins, and the + current flows into C, through F, to
terminal H as before, and through the circuit to K, through G and B,
back to the bobbins. Thus you see that in the circuit the current will
be always in the same direction, or continuous, although in the bobbins
it is alternating, and may be used for any purpose for which a
continuous current is required, such as electro-plating, &c.
[Illustration: FIG. 4.]
There are various forms of the magneto-electric machines, as well as of
commutators, but the foregoing shows the general principle of them all.
_Dynamo-electric Machines._
It will now be necessary to explain the nature of a dynamo-electric
machine, called, for shortness, a dynamo, and to show in what it
differs from a magneto-electric machine.
I have explained how an electric current is produced by a wire passing
in front of a magnet; now, this magnet may either be of the ordinary
kind, or it may be what is termed an electro-magnet. One of the effects
which electricity can be made to produce is the magnetising of steel
bars to form the ordinary and well-known permanent magnets which are
used in ships' compasses, &c. To produce this effect, part of the wire
in a circuit is made into a spiral as in Fig. 5.
[Illustration: FIG. 5.]
The steel rod to be magnetised is placed within the spiral, and a
continuous current of electricity is then sent through the wire, which
causes the rod to become magnetised with a North pole at one end, and a
South pole at the other. The more current is passed through the circuit,
and the more turns are in the spiral, the more quickly and strongly is
the rod magnetised; and it will retain its magnetism for an indefinite
time if made of suitable steel. There is a point at which the metal is
said to be saturated with magnetism, and the strength it has then
acquired will be that which it will retain afterwards, although while
under the influence of the current that strength may be considerably
exceeded. If instead of a steel rod one of iron is placed in the
spiral, and the current is passed through as before, it will be
magnetised in the same manner; but as soon as the current is stopped,
the rod loses almost all its magnetism, and if the current is then
passed in the opposite direction the rod will be magnetised in the
opposite way. The softer and more homogeneous is the iron, the more
instantaneously will it acquire and lose its magnetism, and the greater
strength of magnetism it is able to acquire. An iron bar, round which
are wound a large number of turns of insulated or covered wire,
constitutes an electro-magnet. The difference then between a
magneto-electric and a dynamo-electric machine is, that in the former
permanent magnets are used, and in the latter electro-magnets take their
place. I do not intend to go into particulars as to the construction of
the various dynamos in present use, as there are many books to be had in
which these machines are fully described. I need merely say that in the
so-called continuous-current dynamos, the whole or part of the current
produced is made to pass through the coils of the electro-magnets, thus
inducing in them the required magnetism. I showed how, in the
magneto-electric machine, the currents are collected by means of a
commutator, and it is evident that in Figs. 2, 3, and 4 there might be
separate wires coming from each bobbin to B and C; and if there were
more than two bobbins, there might still be two wires from each to B
and C. On the other hand the collecting collar might be split into more
sections; in fact there might be as many sections as bobbins. To show
how the current is collected in continuous-current dynamos, I must give
a short explanation of the revolving part or armature of a standard type
of machine.
In Fig. 6 is shown a horse-shoe magnet, with its North and South poles,
N and S. Between these poles is made to revolve the armature, composed
of a number of coils of wire made to form a ring like a life-buoy. The
ends of the wires are made to lie along a collar on the spindle, made of
some insulating material, each wire being parallel to its neighbour, and
kept separate from it, as shown in Fig. 7.
[Illustration: FIG. 6.]
[Illustration: FIG. 7.]
These wires are so arranged that if one end of a sectional coil is on
top of the spindle at a given moment, the other will be on the under
side. If then, as shown in Fig. 7, a rubber of copper, made in the form
of a brush of copper wire for convenience, is placed in contact with the
upper part of the commutator collar, and another similar one with the
lower, it is evident the circuit will be completed in the same manner as
before explained.
[Illustration: FIG. 8. Edison Dynamo.]
A wire which is + when above the spindle, will be - when below it,
and as the spindle revolves the current changes in the various wires
from - to + as they reach the top, so that it will always therefore
be + in the upper brush and - in the lower one, and will accordingly be
continuous through the circuit. It will be seen in the illustrations
of various continuous-current dynamos, that though their shape and
arrangement differ, the mode of collecting the current is much about the
same as I have described above. Figs. 8 and 9 show some of the
continuous-current dynamos at present in use.
[Illustration: FIG. 9. Brush Dynamo.]