DIRECT CURRENT ARC LIGHTING SYSTEM.

453

small fraction of tlie total length of the armature core, and be-sides this the iron body is subdivided in the generating direction,and therefore the.Foucault currents are greatly reduced. Anothercause of heating is the shifting of the poles of the armature core.In consequence of the subdivision of the iron in the armatureand the increased surface for radiation, the risk of heating islessened.

The iron discs n n n are insulated or coated with some insulat-ing-paint, a very careful insulation being unnecessary, as anelectrical contact between several discs can only occur at placeswhere the generated currents are comparatively weak. Anarmature core constructed in the manner described may be re-volved between the poles of the field magnets without showingthe slightest increase of temperature.

The end discs, <1 <7, which are of sufficient thickness and, forthe sake of cheapness, of cast-iron, are curved inwardly, as in-dicated in the drawings. The extent of the curve is dependenton the amount of wire to be wound upon the armatures. In thismachine the wire is wound upon the armature in two super-imposed parts, and the curve of the end discs, <ld, is so calculatedthat the first part—that is, practically half of the wire—just fills

Fig. 272.

Fig. 273.

up the hollow space to the line x x; or, if the wire is wound inany other manner, the curve is such that when the whole of thewire is wound, the outside mass of wires, w, and the inside massof wires, w', are equal at each side of the plane x x. In this casethe passive or electrically-inactive wires are of the smallestlength practicable. The arrangement has further the advantage