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INVENTIONS OB' NIKOLA TESLA.

Another interesting exhibit, shown at a, is an early model of atwo held motor energized by currents of different phase. Thereare two independent fields of laminated iron joined by brassbolts; in each field is mounted an armature, both armatures be-ing on the same shaft. The armatures were originally so ar-ranged as to be placed in any position relatively to each other,and the fields also were arranged to be connected in a numberof ways. The motor has served for the exhibition of a numberof features; among other things, it has been used as a dynamofor the production of currents of any fretpiency between widelimits. In this case the field, instead of being energized by di-rect current, was energized by currents differing in phase, which

Fig. 298.

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produced a rotation of the field; the armature was then rotatedin the same or in opposite direction to the movement of the field;and so any number of alternations of the currents induced in thearmature, from a small to a high number, determined by thefrequency of the energizing field coils and the speed of the arma-ture, was obtained.

The models h, i, j, represent a variety of rotating field, synchron-ous motors which are of special value in long distance transmis-sion work. The principle embodied in these motors was enunci-ated by Mr. Tesla in his lecture before the American Institute ofElectrical Engineers, in May, 1888 1 . It involves the production

1. See Part I, Chap. Ill, page 9.