coils are wound on separately, the wire being carried over the outside of the ring, then through the center opening and again around the outside, this operation being repeated until the winding for that individual section is completed. The adjacent coil is then wound in the same way, the ends of each being brought out to the commutator side of the armature, the arrangement of the coils on the ring and connections with the commutator being shown in fig. 247, examples of actual construction being shown in figs. 248 and 249.

Ques. For what conditions of operation is the ring armature specially adapted, and why?

Ans. It is well suited to the generation of small currents at high voltage, as for series arc lighting, because the numerous coils can be very well insulated.

Fig. 250.—Distribution of magnetic lines of force through a Gramme ring. Since the metal of the ring furnishes a path of least reluctance, most of the magnetic lines will follow the metal of the ring and very few will penetrate into the aperture of the interior. This condition causes a serious defect in the action of ring armatures rendering the winding around the interior useless for the production of electromotive force. Hence, in ring armatures only about half of the winding is effective, the rest or "dead wire," adding its resistance to the circuit, thus decreasing the efficiency of the machine.

Ques. Why does a ring armature require more copper in the winding than a drum armature?

Ans. For the reason that those inductors which lie on the inner side of the iron ring, being screened from practically all the lines of force, as shown in fig. 250, do not generate any current.

Numerous attempts have been made to utilize this part of the winding by making the pole pieces extend around the ring in such a manner that lines of force will pass to the inside of the ring, also by arranging an additional pole piece on the inside of the armature, but mechanical considerations have shown these methods to be impractical.

Ques. Is any portion of the winding of a drum armature inactive?

Ans. Yes; the end connectors do not generate any current.

Fig. 251.—Illustrating the principle of Siemens' drum winding. In order to make the winding and connections clear, one coil and the commutator is shown assembled, although the latter is not put in place until after all the sections have been wound, the ends of the wires being temporarily twisted together until all can be soldered to the risers. The cores of these early machines were of wood overspun circumferentially with iron wire before receiving the longitudinal copper windings.

Ques. What is the chief advantage of the drum armature?

Ans. It reduces considerably the large amount of dead wire necessary with the ring type.

Ques. How is this accomplished?

Ans. By winding the wire entirely on the outer surface of a cylinder or drum, as it is called, as shown in fig. 251, thus none of the wire is screened by the metal of the core.

Fig. 252.—Elementary four coil drum winding, showing the connections with the commutator segments, and directions of currents in the several coils. The action of this type of armature is fully explained in the text.

Fig. 252 shows an elementary four coil drum armature. Starting from the point a and following the winding around without reference at first to the commutator, it will be found that the rectangular turns of the wire form a closed circuit, and are electrically in series with one another in the order of the numbers marked on them.

With respect to the connections to the four segments w, x, y, z, of the commutator it will be found that at two of these, x and y, the pressures in the windings are both directed from, or both directed toward the junction with the connecting wire. At the other two segments, z and w, one pressure is toward the junction and the other directed from it. If, therefore, the brushes be placed on x and y they will supply current to an external circuit, z and w, for the moment being idle segments.

Disc Armatures.—The inductors of a disc armature move in a plane, perpendicular to the direction of the lines of force, about an axis parallel to them as shown in fig. 253. The main difficulty with this type has been in constructing it so that it will be strong and capable of resisting wear and tear. It was introduced in an effort to avoid the losses due to eddy currents and hysteresis present in the other types of armature.

Fig. 253.—Disc armature of Niaudet. It is equivalent to a ring armature, having the coils turned through an angle of 90°, so that all the coils lie in a plane perpendicular to the axis of rotation. The connections of the coils with each other and with the commutator remain the same, the beginning and the end of adjacent coils leading to a common commutator bar as shown. The magnetic field is arranged by the use of two magnets, so arranged as to present the north pole of one to the south pole of the other, and vice versa. In the figure one of these magnets is considered as above the paper, and the other below. If this armature be rotated through the magnetic field as shown, a reversal of current takes place in each coil, when it is in such a position that one of its diameters coincides with the pole line, NS. If the brushes be set so as to short circuit the coils that are in this position, the armature will be divided into two branchings, the current flowing in an opposite direction in each, and a direct current will flow in the exterior circuit.

On account of the nature of the construction of a disc armature, it is necessary that the coils subject to