Areometer, Bead.
A tube of glass containing beads of different specific gravities. It has
apertures at top and bottom. When immersed in a liquid, the same fills
it, and the specific gravity within certain limits, depending on the
factors of the beads, is shown by the beads which float and those which
sink. It is used for storage batteries and other purposes where acids
and solutions have to be tested.
Argyrometry.
The method of ascertaining the weight and inferentially the thickness of
an electroplater's deposit of silver. It is done by weighing the article
before and after plating.
Arm.
The four members of a Wheatstone bridge, q. v., are termed its arms.
Referring to the diagram of a bridge, P, Q, R, S, are the arms.
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.