CHAPTER XXIV.

A Constant Current Transforms with .Magnetic ShieldBetween Coils of Primary and Secondary.

Mr. Tesla has applied his principle of magnetic shielding ofparts to the construction also of transformers, the shield beinginterposed between the primary and secondary coils. In trans-formers of the ordinary type it will be found that the wave ofelectromotive force of the secondary very nearly coincides withthat of the primary, being, however, in opposite sign. At the sametime the currents, both primary and secondary, lag behind theirrespective electromotive forces; but as this lag is practically ornearly the same in the case of each it follows that the maximumand minimum of the primary and secondary currents will nearlycoincide, but differ in sign or direction, provided the secondaryhe not loaded or if it contain devices having the property ofself-induction. On the other hand, the lag of the primarybehind the impressed electromotive force may be diminished byloading the secondary with a non-inductive or dead resistance—such as incandescent lamps—whereby the time interval betweenthe maximum or minimum periods of the primary and secondarycurrents is increased. This time interval, however, is limited,and the results obtained by phase difference in the operation ofsuch devices as the Tesla alternating current motors can only beapproximately realized by such means of producing or securingthis difference, as above indicated, for it is desirable in such casesthat there should exist between the primary and secondary cur-rents, or those which, however produced, pass through the twocircuits of the motor, a difference of phase of ninety degrees;or, in other words, the current in one circuit should be a maxi-mum when that in the other circuit is a minimum. To attainto this condition more perfectly, an increased retardation of thesecondary current is secured in the following manner: Insteadof bringing the primary and secondary coils or circuits of atransformer into the closest possible relations, as has hitherto

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