. Electrical news and engineering . ced—the resultant direction of the forces—will be MR,which is the diagonal of the parallelogram formed by drawingparallels to A and B. Now, it is plain that and ; angle B ^ sin angle BRM. side a = side A, therefore ngje RMBngle RMA side B sin RMAand that if we cause sides A, B, to vary harmonically we shallalso cause angles RMB, RM.\ to vary harmonically ; and conse-quently we can cause the diagonal MR lo swing between Now MR is the resultant of two forces acting-together ; consequently if we cause these forces to varj harmoni-cal
. Electrical news and engineering . ced—the resultant direction of the forces—will be MR,which is the diagonal of the parallelogram formed by drawingparallels to A and B. Now, it is plain that and ; angle B ^ sin angle BRM. side a = side A, therefore ngje RMBngle RMA side B sin RMAand that if we cause sides A, B, to vary harmonically we shallalso cause angles RMB, RM.\ to vary harmonically ; and conse-quently we can cause the diagonal MR lo swing between Now MR is the resultant of two forces acting-together ; consequently if we cause these forces to varj harmoni-cally in strength (remaining constant in direction) the resultantdirection will swing as above described. .Applying this principleto magnets, and an armature, we can cause the resultantmagnetic field to revolve. Two electromagnets, A, B, are placedradially, and are separately energized by currents which can bevaried in strength by any convenient means, say a the centre R of this arc, as a pivot. Is hung an armature M. \ / Diagram 2.—Harmonically \arving Magnet and Armatlre. more towards itself, so that when B has been made as strong as.•\, each of them attracts M with equal force, and M will be heldmidway between them ; RM2 being the new direction of magneticstrongest pull. If now .A is slowly decreased, it will attract Mless and less strongl)-, M will approach closer to B, the strongestdirection RM3 will be the resultant pull. Thus, by causing themagnets A, B to vary in strength, one up, the other down, wehave swung the resultant strongest field over the arc .\B, of thecircle. If we have other magnets, C, D, we could pull Mall round the circle, and that is what is done in polyphase two necessary conditions for the production of a revolvingfield are electromagnets whose strength can be varied up anddown, and some arrangement whereby adjacent magnets shallattain their maximum or minimum strength at different first condition is evidently me
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