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.
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.