UNIPOLAR GENERATORS.

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If the poles do not cover the disc completely on both sides,then, of course, unless the disc be properly subdivided, themachine will be very inefficient. Again, in this case there arepoints worthy of notice. If the disc be rotated and the heldcurrent interrupted, the current through the annature will con-tinue to flow and the field magnets will lose their strength com-paratively slowly. The reason for this will at once appear whenwe consider the direction of the currents set up in the disc.

Referring to the diagram Tig. 292, d represents the disc withthe sliding contacts b b' on the shaft and periphery, n and srepresent the two poles of a magnet. If the pole u be above, asindicated in the diagram, the disc being supposed to be in the

Fio. 292.

plane of the paper, and rotating in the direction of the arrow n,the current set up in the disc will flow from the centre to theperiphery, as indicated by the arrow a. Since the magnetic ac-tion is more or less confined to the space between the poles n s,the other portions of the disc may be considered inactive. Thecurrent set up will therefore not wholly pass through the externalcircuit f, but will close through the disc itself, and generally, ifthe disposition be in any way similar to the one illustrated, by farthe greater portion of the current generated will not appear ex-ternally, as the circuit f is practically short-circuited by the inac-tive portions of the disc. The direction of the resulting currentsin the latter may be assumed to be as indicated by the dotted