POLYPHASE CURRENTS.

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screened by the shields n. As the increasing electromotiveforce then urges more current through the coils e, the iron wireh becomes magnetically saturated and incapable of carrying allthe lines of force, and hence ceases to protect the cores c, whichbecomes magnetized, developing their maximum effect after aninterval of time subsequent to the similar manifestation of strengthin the other set of magnets, the extent of which is arbitrarilydetermined by the thickness of the .shield h, and other well-un-derstood conditions.

From the above it will be seen that the apparatus or deviceacts in two ways. First, by retarding the current, and, second,by retarding the magnetization of one set of the cores, fromwhich its effectiveness will readily appear.

Many modifications of the principle of this invention are pos-sible. One useful and efficient application of the invention isshown in Fig. 59. In this figure a motor is shown similar in allrespects to that above described, except that the iron wire h, whichis wrapped around the coils e, is in this case connected in serieswith the coils d. The iron-wire coils n, are connected and wound,so as to have little or no self-induction, and being added to theresistance of the circuit r, the action of the current in that cir-cuit will be accelerated, while in the other circuit o it will beretarded. The shield h may be made in many forms, as will beunderstood, and used in different ways, as appears from theforegoing description.

As a modification of his type of motor with “ shielded ” fields^Mr. Tesla has constructed a motor with a field-magnet havingtwo sets of poles or inwardly-projecting cores and placed sideby side, so as practically to form two fields of force and alter-nately disposed—that is to say, with the poles of one set or fieldopposite the spaces between the other. He then connects the freeends of one set of poles by means of laminated iron bands orbridge-pieces of considerably smaller cross-section than the coresthemselves, whereby the cores will all form parts of completemagnetic circuits. When the coils on each set of magnets areconnected in multiple circuits or branches from a source of al-ternating currents, electromotive forces are set up in or im-pressed upon each circuit simultaneously; but the coils on themagnetically bridged or shunted cores will have, by reason ofthe closed magnetic circuits, a high self-induction, which retardsthe current, permitting at the beginning of each impulse but lit-