The Standard Electrical Dictionary / A Popular Dictionary of Words and Terms Used in the Practice of Electrical Engineering

by T. O'Conor Sloane

Fig. 22. DIAGRAM OF WHEATSTONE'S BRIDGE.
Armature.
(a.) A mass or piece of iron or steel, or a collection of pieces of iron
designed to be acted on by a magnet. While nickel or cobalt might be
used, they rarely or never are except in experimental apparatus. The
armature of a permanent horse shoe magnet is simply a little bar of soft
iron. When the magnet is not in use it is kept in contact with the poles
with the idea of retaining its magnetism. It is then said to be used as
a keeper. A bar magnet does not generally have an armature. The armature
is also used to exhibit the attraction of the magnet.
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 mass 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 space 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 space 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 mass 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 shellacked
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.
Armature, Intensity.
An armature wound for high electro-motive force. A term little used at
the present time.
Armature Interference.
A limit to the ampere turns permissible on a given armature is found in
the increase of cross magnetizing effect, q. v., the increased lead
necessitated, and the growth of the demagnetizing power. All such
perturbing effects are sometimes expressed as armature interference.
46 STANDARD ELECTRICAL DICTIONARY.
Armature, Load of.
The circumflux, q. v., of the armature, or the ampere turns of the same.
The maximum load which can be carried by an armature without sparking is
directly proportional to the radial depth of core and to the length of
the gap, and inversely proportional to the breadth of the polar span.
Armature, Multipolar.
An armature in which a number of poles greater than two is determined by
the field. A multipolar field is employed for its production.
Armature, Neutral.
An armature of a magnet or telegraph relay which is not polarized or
magnetized.
Synonym--Non-polarized Armature--Neutral Relay Armature.
Armature of Influence Machine.
Pieces of paper pasted on the stationary plate of an electric machine of
the Holtz type.
Armature of Leyden Jar or Static Condenser.
The inner and outer tin-foil coatings of a Leyden jar or other
condenser.
Armature, Open Coil.
An armature of a dynamo or motor on which the coils are not joined in
one closed circuit, but have their ends or some of them separated, and
connected each to its own commutator bar or each set to their own bar.