14
INVENTIONS OF NIKOLA TESLA.
call)' the same. Instead of using four wires, with like result?three wires may be used, one forming a common return for bothcircuits.
This rotation or whirling of the poles manifests itself in a seriesof curious phenomena. If a delicately pivoted disc of steel orother magnetic uietal is approached to the ring it is set in rapidrotation, the direction of rotation varying with the position of
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the disc. For instance, noting the direction outside of the ringit will be found that inside the ring it turns in an opposite direc-tion, while it is unaffected if placed in a position symmetrical tothe ring. This is easily explained. Each time that a pole ap-proaches, it induces an opposite pole in the nearest point on thedisc, and an attraction is produced upon that point; owing to this,as the pole is shifted further away from the disc a tangential pullis exerted upon the same, and the action being constantly repeat-ed, a more or less rapid rotation of the disc is the result. As thepull is exerted mainly upon that part which is nearest to thering, the rotation outside and inside, or light and left, respectively,is in opposite directions, Fig. 9 . When placed symmetricallyto the ring, the pull on the opposite sides of the disc being equal,no rotation results. The action is based on the magnetic inertiaof iron; for this reason a disc of hard steel is much more af-fected than a disc of soft iron, the latter being capable of veryrapid variations of magnetism. Such a disc has proved to be avery useful instrument in all these investigations, as it has en-abled me to detect any irregularity in the action. A curious ef-fect is also produced upon iron tilings. By placing some upon apaper and holding them externally quite close to the ring, theyare set in a vibrating motion, remaining in the same place, althoughthe paper may be moved back and forth ; but in lifting the paperto a certain height which seems to be dependent on the intensityof the poles and the speed of rotation, they are thrown away in