452

INVENTIONS OF NIKOLA TESLA.

imum the loss of power due to Foucault currents and to thechange of polarity, and also to shorten as much as possible thelength of the inactive wire wound upon the armature core.

It is well knowm that when the armature is revolved betweenthe poles of the field-magnets, currents are generated in the ironbody of the armature which develop heat, and consequently cause

Fig. 271.

'Akkkkii I

a waste of power. Owing to the mutual action of the lines offorce, the magnetic properties of iron, and the speed of the dif-ferent portions of the armature core, these currents are generatedprincipally on and near the surface of the armature core, dimin-ishing in strength gradually toward the centre of the core.Their quantity is under some conditions proportional to thelength of the iron body in the direction in which these currentsare generated. By subdividing the iron core electrically in thisdirection, the generation of these currents can be reduced to agreat extent. For instance, if the length of the armature-core istwelve inches, and by a suitable construction it is subdividedelectrically, so that there are in the generating direction six inchesof iron and six inches of intervening air-spaces or insulating ma-terial, the waste currents will be reduced to fifty per cent.

As shown in the diagrams, the armature is constructed of thiniron discs n n n, of various diameters, fastened upon the arma-ture-shaft in a suitable manner and arranged according to theirsizes, so that a series of iron bodies, ii i, is formed, each of ■whichdiminishes in thickness from the centre toward the periphery.At both ends of the armature the inwardly curved discs d d, ofcast iron, are fastened to the armature shaft.

The armature core being constructed as shown, it will be easilyseen that on those portions of the armature that are the mostremote from the axis, and where the currents are principally de-veloped, the length of iron in the generating direction is only a