The generator is known as the "Thomson spherical," on account of the
nearly spherical form of its armature, and differs radically from all
others in all essential portions, viz., its field magnets, armature,
and winding thereof, and in its commutator; both in principle and
construction, and, besides, it is provided with an automatic
regulator, an attachment not applied to other generators. The annexed
view of the complete machine will convey an idea of the general
appearance and disposition of its parts.
The revolving armature which generates the electrical current is made
internally of a hollow shell of soft iron secured to the central
portion of the shaft between the bearings, and is wound externally
with a copper conducting wire, constituting three coils or helices
surrounding the armature, which coils are, however, permanently
joined, and in reality act as a single three-branched wire.
This wire, being wound on the exterior of the armature, is fully
exposed to the powerful magnetic influence of the field poles, which
inclose the armature almost completely. The armature will thus be seen
to be thoroughly incased and protected, at the same time that all the
wire upon it is subject to a powerful action of the surrounding
magnets, resulting in an economy in the generation of current in its
coils. The form of the armature being spherical, very little power is
lost by air friction, and no injury can occur from increased speed
developing centrifugal force. The field magnets, which surround the
armature, are cast iron shells, wound outside with many convolutions
of insulated copper wire, and are joined externally by iron bars to
convey the magnetism. These outer bars serve also as a most efficient
protection to the wire and armature of the machine during
transportation or otherwise. Objects cannot fall upon or rest upon the
wire coils and injure them. The coils of wire upon the field magnets
surround not only the iron poles or shells, but are situated also so
as to surround likewise the revolving armature, and increase the
effect produced in it by direct induction and magnetism. This feature
is not used in any other generator, nor does any other make use of a
spherical armature. The shaft is mounted in babbitted bearings of
ample size, sustained by a handsome frame therefor, and is of steel,
finely turned and perfectly true. The shaft and armature together are
balanced with the utmost care, and run without buzz or rumble. The
armature wire is kept cool by an active circulation of air over its
whole surface during revolution. The commutator, or portion from which
the currents developed in the armature are carried out for use, is a
beautiful piece of mechanism. It is mounted upon the end of the shaft,
and has attached to it the wires, three only, coming from the armature
wire through the tubular shaft.
[Illustration: THE THOMSON SPHERICAL.]
The commutator is peculiar, consisting of only three segments of a
copper ring, while in the simplest of other continuous current
generators several times that number exist, and frequently 120!
segments are to be found. These three segments are made so as to be
removable in a moment for cleaning or replacement. They are mounted
upon a metal support, and are surrounded on all sides by a free air
space, and cannot, therefore, lose their insulated condition. This
feature of air insulation is peculiar to this system, and is very
important as a factor in the durability of the commutator. Besides
this, the commutator is sustained by supports carried in flanges upon
the shaft, which flanges, as an additional safeguard, are coated all
over with hard rubber, one of the finest known insulators. It may be
stated, without fear of contradiction, that no other commutator made
is so thoroughly insulated and protected. The three commutator
segments virtually constitute a single copper ring, mounted in free
air, and cut into three equal pieces by slots across its face. Four
slit copper springs, called commutator brushes or collectors, are
allowed to bear lightly upon the commutator when it revolves, and
serve to take up the current and convey it to the circuit. These
commutator brushes are carried by movable supports, and their position
is automatically regulated so as to control the strength of the
developed current--a feature not found in other systems. This feature,
as well as the fact that the commutator can be oiled to prevent wear,
saves attendance and greatly increases the durability of the wearing
surfaces, while the commutator brushes are maintained in the position
of best adjustment. The commutator and brushes, in consequence, after
weeks of running, show scarcely any wear.