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INVENTIONS OF NIKOLA TESLA.

revolving between the two opposite poles of a magnet, and thepolar surfaces completely covering both sides of the disc, andassume the current to be taken off or conveyed to the same bycontacts uniformly from all points of the periphery of the disc.Take first the case of a motor. In all ordinary motors the opera-tion is dependent upon some shifting or change of the resultantof the magnetic attraction exerted upon the armature, this pro-cess being effected either by some mechanical contrivance oil themotor or by the action of currents of the proper character. Wemay explain the operation of such a motor just as we can that ofa water-wheel. But in the above example of the disc surroundedcompletely by the polar surfaces, there is no shifting of the mag-netic action, no change whatever, as far as we know, and yetrotation ensues. Here, then, ordinary considerations do notapply ; we cannot even give a superficial explanation, as in ordi-nary motors, and the operation will be clear to us only when weshall have recognized the very nature of the forces concerned,and fathomed the mystery of the invisible connecting mechan-ism.

Considered as a dynamo machine, the disc is an equally inter-esting object of study. In addition to its peculiarity of givingcurrents of one direction without the employment of commutat-ing devices, such a machine differs from ordinary dynamos inthat there is no reaction between armature and field. The arma-ture current tends to set up a magnetization at right angles tothat of the field current, but since the current is taken off uni-formly from all points of the periphery, and since, to be exact,the external circuit may also be arranged perfectly symmetricalto the field magnet, no reaction can occur. This, however, istrue only as long as the magnets are weakly energized, for whenthe magnets are more or less saturated, both magnetizations atright angles seemingly interfere with each other.

For the above reason alone it would appear that the output ofsuch a machine should, for the same weight, be much greaterthan that of any other machine in which the armature currenttends to demagnetize the field. The extraordinary output of theForbes unipolar dynamo and the experience of the writer con-firm this view.

Again, the facility with which such a machine may be made toexcite itself is striking, but this may be due—besides to the ab-sence of armature reaction—to the perfect smoothness of the cur-rent and non-existence of self-induction.