The principal difference between the continuous-and alternating-current
dynamo, is in the number of magnets used. Most of the former have only
four magnets, while the latter have frequently as many as thirty-two. In
reality, as I have shown, these are all alternating-current dynamos,
only that in the so-called continuous-current ones, the current is
commutated, whereas in the others it is not, but is used as it is
produced. In the principal alternating-current dynamos, a number of
small magnets, usually sixteen, are attached to a framework directly
opposite a similar number of others of the same size, the space between
the ends being only about an inch or two. These are all electro-magnets,
and are wound in such manner that when excited by a current, every
alternate one shall have the same magnetism, as in Fig. 10, and every
opposite one a contrary magnetism.
This produces an intense magnetic field between the ends of the magnets,
and in this space revolves the armature. This armature, in the Siemens
dynamo, is composed of a disc having as many bobbins on the periphery as
there are magnets on each side of the dynamo. As each bobbin approaches
each magnet a current is induced in one direction, which is reversed
when the bobbin recedes; thus an alternating current is produced, which
is collected by connecting the ends to insulated rings or collars on the
spindle, and having small copper brushes or rubbers in contact with
them. In the Ferranti dynamo, the armature is quite different, and much
more simple, as comparison of Figs. 11 and 12 will show.
[Illustration: FIG. 10.]
[Illustration: FIG. 11. Siemens Armature.]
[Illustration: FIG. 12. Ferranti Armature.]
It consists of a copper tape bent in and out so as to form a sort of
star with eight arms, the number of layers of insulated copper tape
being from ten to thirty, according to requirements. The centre is made
in a similar shape with bolts or rivets holding each convolution in
place. The two ends of the tape are attached respectively to two
collector-rings on the spindle, against which press two solid metal
rubbers which carry off the current for use in the circuit. It can be
shown that as each arm approaches a magnet a current will be induced in
one direction, which will be reversed as each arm recedes; and therefore
an alternating current will be produced. As there are sixteen magnets
for the armature to pass at each revolution, there must be sixteen
alternations of the current during the same time, so that if the speed
of the armature is 500 revolutions per minute, there will be 500 x 16 =
8000 alternations in one minute. These alternations being so extremely
rapid, when this current is used for electric lighting, the steadiness
of the light will be in no way affected, but will remain as constant as
with a continuous current.
[Illustration: FIG. 13. Siemens Alternating Dynamo.]
The alternating current produced by these dynamos cannot be used for
exciting an electro-magnet, as the magnetism would be reversed at every
alternation; a separate small dynamo of the continuous type is therefore
used as an exciter to magnetise all the electro-magnets in the field,
and it is usually coupled on to the same spindle, and therefore goes at
the same speed as the alternating-current dynamo. The exciter is usually
of a size to be able to do alone about one-tenth to one-twentieth of
the work that the larger machines does in the way of lighting; so that
if from any cause the latter is disabled while the ship lighted by it is
at sea, the exciter may be used alone to do a portion of the lighting,
in the first-class saloon for instance. This can only be done if the
exciter is so constructed as to give the proper E.M.F. that the lamps
require.
[Illustration: FIG. 14. Ferranti Alternating Dynamo.]
Figs. 13 and 14 are illustrations of two of the alternating current
dynamos in use on board ship and elsewhere.