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.