Sometimes an armature is made of steel and is permanently magnetized.

Such an armature, termed a polarized armature, is repelled when its like poles are opposed to like poles of the magnet and otherwise is attracted with force due to the sums of the magnetism. If the magnet is sufficiently powerful depolarization of the armature may ensue when like poles are opposed to like poles. Polarized armatures are used in various appliances, magneto generators, telegraphic instruments and others.

(b) In a dynamo or Motor the ma.s.s of laminated iron or of wire which carries the coils of insulated wires which are caused to rotate in the field of force of the field magnets in order to establish and maintain potential difference with its accompanying current, or which rotates under the effects of a current in a motor. (See Dynamo Electric Generator.)

The work of the armature core is twofold. It acts as a portion of the magnetic circuit, conducting the lines of force, and by virtue of its high permeability or multiplying power concentrating a number of the lines of force through its own substance. To enable it to act with efficiency in this direction it should be made of iron of the highest permeability, and should approach as closely as possible to the armature cores consistent with leaving s.p.a.ce for the wire winding. It next acts as a support for the wires which are to be swept through the field of force. Thus it acts both to establish a strong field and then acts as a carrier for the wires which are to be cut by the wires in question. In connection with this subject the different definitions under Armature, Dynamo, Commutator, Induction and similar topics may be consulted.

(c) See Armature of Influence Machine.

(d) See Armature of Leyden Jar or Static Condenser.

42 STANDARD ELECTRICAL DICTIONARY.

Armature, Bar.

An armature in a dynamo or motor whose winding is made up of conductors in the form of bars, round, rectangular and of other sections. This type of armature conductor is objectionable as Foucault currents are produced in it. It is found best to laminate or subdivide low resistance armature windings.

[Transcriber's Note: Foucault currents are also called eddy currents.]

Armature, Bipolar.

An armature in which two poles are induced by the field. A bipolar field magnet produces a bipolar armature.

Armature Bore.

The cylindrical s.p.a.ce defined by the pole pieces of a dynamo or motor within which the armature rotates.

Synonym--Armature Chamber.

43 STANDARD ELECTRICAL DICTIONARY.

Armature, Closed Coil.

An armature for a motor or dynamo, the ends of all of whose coils are united, so as to be in one closed circuit all the way around.

Fig. 23. CLOSED COIL GRAMME RING ARMATURE.

Armature Coil, or Coils.

The insulated wire wound around the core of the armature of an electric current generator or motor.

Armature Core.

The central ma.s.s of iron on which the insulated wire, to be rotated in the field of an electric current generator or motor, is wound. (See Dynamo-electric Machine and Motor, Electric.)

Armature, Cylinder.

An armature of the Gramme ring type, but longer in the axial direction, so that its core resembles a long hollow cylinder, the wire being wound inside and outside as in the Gramme ring. (See Gramme Ring.)

Armature, Disc.

(a) An armature of a dynamo electric machine or motor in which the coils are wound so as to be flat and are carried on the face of a disc forming the core or part of the core of the armature. S. P. Thompson treats it as a modified drum armature extended radially, the outer periphery corresponding to the back end of the drum. The poles of the field are generally placed to face the side or sides of the disc.

(b) Another type of disc armature has its wire wound on bobbins arranged around the periphery of a disc.

In disc armatures there is often no iron core, their thinness enabling this to be dispensed with.

44 STANDARD ELECTRICAL DICTIONARY.

Fig. 24. DISC ARMATURE OF FRITSCHE MACHINE.

Fig. 25. PLAN OF WINDING PACINOTTI'S DISC ARMATURE.

Armature, Discoidal Ring.

In a dynamo an armature of the shape of a ring of considerable radial depth of section as compared to its axial depth. It is generally made of iron ribbon or thin band wound to the proper size.

Synonym--Flat Ring Armature.

45 STANDARD ELECTRICAL DICTIONARY.

Armature, Drum.

An armature for a dynamo or motor, consisting of a cylinder of iron preferably made up of discs insulated from each other by thin sh.e.l.lacked paper, or simply by their oxidized surfaces, and wound with wire parallel to the axis where it lies on the cylindrical periphery and crossing the heads approximately parallel to the diameter. It operates practically on the same principle as a Gramme Ring Armature. (See Gramme Ring.)

Synonym--Cylindrical Armature.

Armature Factor.

The number of conductors on an armature, counted or enumerated all around its external periphery.

Armature, Hinged.

An armature pivoted to the end of one of the legs of an electro-magnet so as to be free to swing and bring its other end down upon the other pole.

Fig. 26. HINGED ARMATURES OF CLUB-FOOT ELECTRO MAGNETS.

Armature, Hole.

An armature whose core is perforated to secure cooling.

Synonym--perforated Armature.