POLYPHASE CURRENTS.

77

same. Fig. (>4 is a central cross-section of the same, the arma-ture being shown mainly in elevation.

Let a a designate two plates built up of thin sections orlaminae of soft iron insulated more or less from one another andheld together by bolts a and secured to a base b. The innerfaces of these plates contain recesses or grooves in which a coilor coils n are secured obliquely to the direction of the lamina-tions. Within the coils i> is a disc e, preferably composed ofa spirally-wound iron wire or ribbon or a series of concentricrings and mounted on a shaft f, having bearings in the platesa a. Such a device when acted upon by an alternating currentis capable of rotation and constitutes a motor, the operation ofwhich may be explained in the following manner: A current orcurrent-impulse traversing the coils n tends to magnetize the

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cores a a and e, all of which are within the inflnence of thefield of the coils. The poles thus established would naturallylie in the same line at right angles to the coils i>, but in theplates a they are deflected by reason of the direction of thelaminations, and appear at or near the extremities of these plates.In the disc, however, where these conditions are not present, thepoles or points of greatest attraction are on a line at rightangles to the plane of the coils; hence there will be a torque es-tablished by this angular displacement of the poles or magneticlines, which starts the disc in rotation, the magnetic lines of thearmature and held tending toward a position of parallelism.This rotation is continued and maintained by the reversals ofthe current in coils n n, which change alternately the polarity ofthe field-cores A a. This rotary tendency or effect will be greatly