Fourthly, of collecting mechanism, the commutator in direct current
dynamos, attached to the armature shaft and rotating with it. This
consists of insulated rings, or segments of rings to which the wire
coils of the armature are connected, and on which two springs of copper
or plates of carbon or some other conductor presses. The electro-motive
force developed by the cutting of lines of force, by the wires of the
armature, shows itself as potential difference between the two springs.
If the ends of a conductor are attached, one to each of these brushes,
the potential difference will establish a current through the wire. By
using properly divided and connected segments on the commutator the
potential difference and consequent direction of the current may be kept
always in the same sense or direction. It is now clear that the external
wire may be connected with the windings of the field-magnet. In such
case the excitement of the field-magnets is derived from the armature
and the machine is self-excited and entirely self-contained.
The above is a general description of a dynamo. Sometimes the coils of
the field-magnets are not connected with the armature, but derive their
current from an outside source. Such are termed separately excited
dynamos.
Some general features of dynamo generators may be seen in the
definitions under this head and elsewhere. The general conception is to
cut lines of force with a conductor and thus generate electromotive
force, or in some way to change the number of lines of force within a
loop or circuit with the same effect.
Dynamo, Electroplating.
A dynamo designed for low potential and high current intensity. They are
wound for low resistance, frequently several wires being used in
parallel, or ribbon, bar or rectangular conductors being employed. They
are of the direct current type. They should be shunt wound or they are
liable to reverse. They are sometimes provided with resistance in the
shunt, which is changed as desired to alter the electro-motive force.
Dynamo, Equalizing.
A combination for three and five-wire systems. A number of armatures or
of windings on the same shaft are connected across the leads. If the
potential drops at any pair of mains, the armature will begin to be
driven by the other mains, acting to an extent as an element of a motor,
and will raise the potential in the first pair.
Dynamo, Far Leading.
A motor dynamo, used to compensate the drop of potential in long mains.
Into the mains at a distant point a series motor is connected, driving a
dynamo placed in shunt across the mains. The dynamo thus driven raises
the potential difference between the two mains.