CHAPTER XLI.

Improvement in “Unipolar” Generators.

Another interesting class of apparatus to which Mr. Tesla liasdirected his attention, is that of “ unipolar ” generators, in which adisc or a cylindrical conductor is mounted between magneticpoles adapted to produce an approximately uniform field. Inthe disc armature machines the currents induced in the rotatingconductor flow from the centre to the periphery, or conversely,according to the direction of rotation or the lines of force as de-termined by the signs of the magnetic poles, and these currentsare taken off usually by connections or brushes applied to thedisc at points on its periphery and near its centre. In the caseof the cylindrical armature machine, the currents developed inthe cylinder are taken off by brushes applied to the sides of thecylinder at its ends.

In order to develop economically an electromotive force avail-able for practicable purposes, it is necessary either to rotate theconductor at a very high rate of speed or to use a disc of largediameter or a cylinder of great length; but in either case it be-comes difficult to secure and maintain a good electrical connectionbetween the collecting brushes and the conductor, owing to thehigh peripheral speed.

It has been proposed to couple two or more discs together inseries, with the object of obtaining a higher electro-motive force ;but with the connections heretofore used and using other condi-tions of speed and dimension of disc necessary to securing goodpracticable results, this difficulty is still felt to be a seriousobstacle to the use of this kind of generator. These objectionsMr. Tesla has sought to avoid by constructing a machine withtwo fields, each having a rotary conductor mounted between itspoles. The same principle is involved in the case of both formsof machine above described, but the description now given isconfined to the disc type, which Mr. Tesla is inclined to favor forthat machine. The discs are formed with flanges, after the

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