Suppose, for instance, that there are twenty-four coils—that is, twelve in each part—and consequently twenty-four commutator plates. There will be in each part three groups, each containing four coils, and the coils will be connected as follows:
| Groups. | Commutator Bars. | |
| First | 1—5 | |
| First part of wire | Second | 17—21 |
| Third | 9—13 | |
| First | 13—17 | |
| Second part of wire | Second | 5—9 |
| Third | 21—1 | |
In constructing the armature core and winding and connecting the coils in the manner indicated, the passive or electrically inactive wire is reduced to a minimum, and the coils at each side of the plates that are in contact with the brushes are practically equal. In this way the electrical efficiency of the machine is increased.
| Fig. 275. | Fig. 276. |
The commutator plates t are shown as outside the bearing b of the armature shaft. The shaft H is tubular and split at the end portion, and the wires are carried through the same in the usual manner and connected to the respective commutator plates. The commutator plates are upon a cylinder, u, and insulated, and this cylinder is properly placed and then secured by expanding the split end of the shaft by a tapering screw plug, v.
Fig. 277.
The arc lamps invented by Mr. Tesla for use on the circuits from the above described dynamo are those in which the separation and feed of the carbon electrodes or their equivalents is accomplished by means of electro-magnets or solenoids in connection with suitable clutch mechanism, and were designed for the purpose of remedying certain faults common to arc lamps.
He proposed to prevent the frequent vibrations of the movable carbon "point" and flickering of the light arising therefrom; to prevent the falling into contact of the carbons; to dispense with the dash pot, clock work, or gearing and similar devices; to render the lamp extremely sensitive, and to feed the carbon almost imperceptibly, and thereby obtain a very steady and uniform light.