Current distribution in ring and drum armatures—connection of brushes—variation of voltage around the commutator—cross magnetization; field distortion—remedies for field distortion—angle of lead—demagnetizing effect of armature reaction—effect of lead—eddy currents; lamination—remedy for eddy currents—magnetic drag on the armature—smooth and slotted armatures—comparison of smooth and slotted armatures—magnetic hysteresis in armature cores—core loss or iron loss—dead turns—friction.

Period of commutation—commutating plane—normal neutral plane—neutral plane—plane of maximum induction—commutation—position of the brushes—sparking—effect of self-induction—construction of commutators—points relating to commutators—types of commutator.

Classification—gauze brushes—wire brushes—strip brushes—carbon brushes—adjustment—comparison of copper and carbon brushes—size of brushes—number—contact angle of brush— brush contact—drop in voltage at brushes—brush holders—brush rigging—multipolar brush gear.

Parts—shaft—core—slotted core—core laminations—core bolts—attachment to shaft—insulation of core discs—teeth—advantages and defects of slotted armatures—slotted cores; built up construction—ventilation—insulation of core—armature windings—construction of inductors—objection to copper bars—various windings: hand winding—evolute or butterfly winding—connectors—barrel winding—bastard winding—former winding—former coils—peculiarity of evolute coil—"straight out" coil—coil retaining devices—driving horns.

Definition—principles—propelling drag—essential requirements of construction—the reverse electromotive force—hydraulic analogy—action of current supplied to motor—armature reaction in motors—method of starting a motor— classes of motor; series, shunt, and compound—power of a motor—brake horse power—mutual relations of motor torque and speed—speed regulation—series parallel controller—interpole motors.

General conditions governing selection—construction—efficiency—adaptation of series and shunt motors—location—foundations—erection of dynamos and motors—connecting up dynamos—marine generating set—belt clamp—belt lacing—belt speed—points relating to belts—gear drive—friction drive—electrical connections.

Switches—switch classification and construction—difficulty encountered in opening the circuit—various switches: knife, snap, and quick break types—fuses—circuit breakers: maximum, minimum, reverse current, maximum and reverse current, no voltage breaker—discriminating cut out—time limit attachments—rheostats—starting boxes—switchboards.

CHAPTER XVII
THE ARMATURE

The armature of a dynamo consists of coils of insulated wire wound around an iron core, and so arranged that electric currents are induced in the wire when the armature is rotated in a magnetic field or the field magnets rotated and armature held stationary.