12. Approach being a finite process the approaching and receeding must give alternating directions to the current.
13. By the use of a commutator all the currents can be turned in the same direction.
14. In a steady circuit it makes no difference what kind of magnets are used to procure the requisite magnetic field whether permanent or electromagnets.
15. Hence the current of the generator may be used to excite the magnetism of field magnets.
Now the Thomson-Houston dynamo comes under that class of dynamos in which there is a rotation of coils in a uniform field of force, such rotation (Fig. 6.) being affected round an axis in the plane of the coil. Of course this dynamo is made like all others of its class to have first, as powerful field-magnets as possible, second, the armature or rotating coil has as great a lenght of wire in it as possible the wire being thick to offer little resistance and third, built to stand high rotative speed.
The simple theoretical dynamo is shown at Fig. 8, consisting of a single rectangular loop of wire rotating in the magnetic field formed by large magnets, and in order to take the current so generated from the loop so as to give a continuous current, we use a two part commutator (Fig. 9) consisting of a metal tube split in two and mounted on wood, each half connected to one end of the loop. The current is taken off by brushes which lead to the main circuit. But manifestly this dynamo would give no appreciable current becase it has a very small length of wire on the armature, so a great number of loops were used which at present constitute the so-called drum armature.
We may rotate the loops of wire in Fig. 8, on one of its sides as an axis or even push it farther from the center of revolution than that. To do this, wrap the wire around a ring and connect both ends to a two part commutator (Fig. 10). If instead of the ring in Fig. 10, being solid it be a number of coils of wire and if instead of there being one coil around the ring there be thirty we will have Pacinnotti’s ring before spoken of. If we used four to ten coils or “bobbins” of large size which is shown diagramatically at Fig. 11, we would have the Brush dynamo.
So with exceptions we may say that there are practically two types of dynamos as regards armatures, the ring type as Brush, Pacinnotti’s Gramme, and the drum armature (page 20).
The Thomson-Houston dynamo is like the rest of that dynamo, unique. To quote S. P. Thompson; “The Thomson-Houston spherical armature is unique among armatures, its cup shaped field magnets are unique among field magnets, its three part commutator is unique commutators.”
