esistance of
the external circuit is lowered, by turning on more and more lights,
more and more current flows from the armature, through the field
magnets. Each time the resistance is lowered, therefore, the current
passing through the field magnets becomes more dense in amperes, and
makes the field magnets correspondingly stronger.

We have seen that the voltage depends on the number of lines of
magnetic force cut by the armature conductors in a given time. If the
speed remains constant then, and the magnets grow stronger and
stronger, the voltage will rise in a straight line. When no current is
drawn, it is 0; at full load, it may be 100 volts, or 500, or 1,000
according to the machine. This type of machine is used only in street
lighting, in cities, with the lights connected in "series," or one
after another on the same wire, the last lamp finally returning the
wire to the machine to complete the circuit. This type of dynamo has
gained the name for itself of "mankiller," as its voltage becomes
enormous at full load. It is unsuitable, in every respect, for the
farm plant. Its field coils consist of a few turns of very heavy wire,
enough to carry all the current of the external circuit, without
heating.

[Illustration: Connections of a series dynamo]

_The Shunt Dynamo_

The shunt dynamo, on the other hand, has field coils connected
directly _across_ the circuit, from one wire to another, instead of in
"series." These coils consist of a great many turns of very fine wire,
thus introducing _resistance_ into the circuit, which limits the
amount of current (amperes) that can be forced through them at any
given voltage. As a shunt dynamo is brought up to its rated speed, its
voltage gradually rises until a condition of balance occurs between
the field coils and the armature. There it remains constant. When
resistance on the external circuit is lowered, by means of turning on
lamps or other devices, the current from the armature increases in
working power, by increasing its amperes. Its voltage remains
stationary; and, since the resistance of its field coils never
changes, the magnets do not vary in strength.

[Illustration: Connections of a shunt dynamo]

The objection to this type of machine for a farm plant is that, in
practice, the armature begins to exercise a de-magnetizing effect on
the field magnets after a certain point is reached--weakens them;
consequently the voltage begins to fall. The voltage of a sh