The Weston armature has the drum surface cut up into longitudinal poles; there is a similar armature by Jablochkoff, in which the poles are oblique.

Ring armatures are found in many machines, but the ingenuity of inventors has been exercised chiefly in 3 directions:- The securing of practical con tinuity; the avoidance of Foucault currents in the cores; and the reduction of useless resistance. In the greater part of these machines, the coils that form the sections of the ring are connected in series, the end of one to the beginning of the next, so that there is a continuous circuit all round, an attachment being made between each pair to a bar or segment of the collector. Most inventors have been content to secure approximate continuity by making the number of sections numerous. Prof. Perry has built up a ring with coils wound obliquely, so that the one coil reaches the neutral point before the preceding one has passed it; this arrangement presents mechanical difficulties in construction. Pacinotti's early dynamo had the coils wound between projecting teeth upon an iron ring. Gramme rejected these cogs, preferring that the coils should be wound round the entire surface of the endless core. To prevent wasteful currents in the cores, Gramme employed for that portion a coil of varnished iron wire of many turns.

In Gulcher's latest dynamo, the ring - core is made up of thin flat rings cut out of sheet iron, furnished with projecting cogs, and laid upon one another. The parts of the coils which pass through the interior of the ring are comparatively idle. They cut very few lines of force as they rotate, and therefore offer a wasteful resistance. Inventors have essayed to reduce this source of loss, by either fitting projecting flanges to the pole - pieces (as in Fein's dynamo), or by using internal magnets (as in Jurgensen's), or by flattening the ring into a disk - form, so as to reduce the interior parts of the ring - coils to an insignificant amount. This is done in the dynamos of Schuckert and Gulcher. In the latest form of Gulcher's dynamo, the field - magnets, at front and back of the ring, are united on the right and left sides in a pair of hollow pole - pieces, which form cases over the ring, covering a considerable part of it. The collector is identical with that of Gramme, but very substantial.

Drum - armatures may all be regarded as modifications of Siemens's longitudinal shuttle - form armature of 1856, the multiplicity of sections of the coils affording practical continuity in the currents. In some of Siemens's machines, the cores are of wood, overspun with iron wire circumferentially, before receiving the longitudinal windings; in another, is a stationary iron core, outside which the hollow drum revolves; in others is no iron in the armature beyond the driving - spindle. In all the Siemens armatures, the individual coils occupy a diametral position with respect to the cylindrical core, but the mode of connecting up the separate diametral sections is not the same in all. In the older of the Alteneck - Siemens windings, the sections were not connected together symmetrically, but in more recent machines, a symmetrical plan has been adhered to, as shown in Fig. 72.

Fig. 72.

In this system, as in the Gramme ring, the successive sections of coils ranged round the armature are connected together continuously, the end of one section and the beginning of the next being united to one segment or bar of the collector. A symmetrical arrangement is of course preferable, not only for ease of construction, but because it is important that there should never be any great difference of potential between one segment of the collector and its next neighbour; otherwise there will be increased liability to spark, and form arcs across the intervening gap. In Edison's •modification of the drum - armature, the winding, though symmetrical in one sense, is singular, inasmuch as the number of sections is an odd number. In the first machines were 7 paths, as shown in Fig. 73; in his latest giant machines, the number of sections is 49. One consequence of this peculiarity of structure is that, if the brushes are set diametrically opposite to one another, they will not pass at the same instant from section to section of the collector; one of them will be short - circuiting one of the sections, whilst the other is at the middle of the opposite collector.

The armature of the latest of Edison's dynamos is not wound up with wire, but, like some of Siemens's electroplated dynamos, is constructed of solid bars of copper, arranged around the periphery of a drum. The ends are connected across by washers or disks of copper, insulated from each other, and having projecting lugs, to which the copper bars are attached. Such disks present much less resistance than mere strips would do. The connections are in the following order: - Each of the 49 bars of the collector is connected to a corresponding one of the 49 disks at the anterior end of the drum, which is connected, by a lug - piece on one side, to one of the 98 copper bars. The cur - rent generated in this bar runs to the farther end of the machine, enters a disk at that end, crosses the disk, and returns along a bar diametrically opposite that along which it started. The anterior end of this bar is attached to a lug - piece of the next disk to that from which we began to trace the connections; it crosses this disk to the bar next but one to that first considered, and so round again. The 2 lug - pieces of the individual disks at the anterior end are therefore not exactly opposite each other, diametrically, as the connections advance through 1/40 of the circumference at each of the 49 paths.

The collector is very substantially built, and a screen is fixed between the collector and the rest of the armature, to prevent any copper - dust from flying back or clogging the insulation between the bars or disks. There are 5 pairs of brushes, the tendency to sparking being thereby greatly reduced. The core of the armature is made of very thin iron disks, separated by mica or asbestos paper from each other, and clamped together. Some exception may be taken to the use of such stout copper bars, as being more likely to heat from local currents than would be the case if bundles of straps or laminae of copper were substituted. And, indeed, the presence of a 4 h. - p. fan to. cool the armature is suggestive that continuous running is liable to heat it.