[4] Note.—Walmsley attributes the first production of rotating fields to Walter Bailey in 1879, who exhibited a model at a meeting of the Physical Society of London, but very little was done, it is stated, until Ferraris took up the subject.
Fig. 1,658.—Experiment made by Faraday being the reverse of Arago's first observation. Faraday assumed that since the presence of a metal at rest stops the oscillations of a magnetic needle, the neighborhood of a magnet at rest ought to stop the motion of a rotating mass of metal. He suspended a cube of copper by a twisted thread, which was placed between the poles of a powerful electromagnet. When the thread was left to itself, it began to spin round with great velocity, but stopped the moment a powerful current was passed through the electromagnet.
[5]This discovery was commercially applied a few years later by Tesla, Brown, and Dobrowolsky.
[5] Note.—The Tesla patents were acquired in the U.S. by the Westinghouse Co. in 1888, and polyphase induction motors, as they were called, were soon on the market. Brown of the Oerlikon Machine Works developed the single phase system and operated a transmission plant over five miles in length at Kassel, Germany, which operated at 2,000 volts.
The principles of polyphase motors can be best understood by means of elementary diagrams illustrating the action of polyphase currents in producing a rotating magnetic field, as explained in the paragraphs following.
Production of a Rotating Magnetic Field by Two Phase Currents.—Fig. 1,659 represents an iron ring wound with coils of insulated wire, which are supplied with a two phase current at the four points A, B, C, D, the points A and B, and C and D, being electrically connected.
Fig. 1,659.—Production of a rotating magnetic field by two phase currents. The figure represents an iron ring, wound with coils of insulated wire, and supplied with two phase currents at the four points A, B, C, and D. The action of the two phase current on the ring in producing a rotating magnetic field is explained in the accompanying text.