nd a fairly large flow
of current obtained thereby.

One thing must be observed: A current will be generated only so long as
there exists a difference in temperature between the inner and the outer
ends of the bars (A, B). This may be accomplished by water, or any other
cooling means which may suggest itself.

CHAPTER XV

ALTERNATING CURRENTS, CHOKING COILS, TRANSFORMERS, CONVERTERS AND
RECTIFIERS

DIRECT CURRENT.--When a current of electricity is generated by a cell,
it is assumed to move along the wire in one direction, in a steady,
continuous flow, and is called a _direct_ current. This direct current
is a natural one if generated by a cell.

ALTERNATING CURRENT.--On the other hand, the natural current generated
by a dynamo is alternating in its character--that is, it is not a
direct, steady flow in one direction, but, instead, it flows for an
instant in one direction, then in the other direction, and so on.

A direct-current dynamo such as we have shown in Chapter IV, is much
easier to explain, hence it is illustrated to show the third method used
in generating an electric current.

It is a difficult matter to explain the principle and operation of
alternating current machines, without becoming, in a measure, too
technical for the purposes of this book, but it is important to know the
fundamentals involved, so that the operation and uses of certain
apparatus, like the choking coil, transformers, rectifiers and
converters, may be explained.

THE MAGNETIC FIELD.--It has been stated that when a wire passes through
the magnetic field of a magnet, so as to cut the lines of force flowing
out from the end of a magnet, the wire will receive a charge of
electricity.

[Illustration: _Fig. 102._ CUTTING A MAGNETIC FIELD]

To explain this, study Fig. 102, in which is a bar magnet (A). If we
take a metal wire (B) and bend it in the form of a loop, as shown, and
mount the ends on journal-bearing blocks, the wire may be rotated so
that the loop will pass through the magnetic field. When this takes
place, the wire receives a charge of electricity, which moves, say, in
the direction of the darts, and will make a complete circuit if the ends
of the looped wire are joined, as shown by the conductor (D).

ACTION OF THE MAGNETIZED WIRE.--You will remember, also that we have
pointed out how, when a current passes over a wire, it has a magnetic
field extending out around it at all points, so that while it is passing