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