114

INVENTIONS OF NIKOLA. TESLA.

been done, Mr. Tesla protects in a measure the secondary fromtlie inductive action or effect of the primary by surroundingeither the primary or the secondary with a comparatively thinmagnetic shield or screen. Under these modified conditions,as long as the primary current has a small value, the shieldprotects the secondary; but as soon as the primary currenthas reached a certain strength, which is arbitrarily determined,the protecting magnetic shield becomes saturated and the induc-tive action upon the secondary begins. It results, therefore, thatthe secondary current begins to How at a certain fraction of aperiod later than it would without the interposed shield, andsince this retardation may he obtained without necessarily retard-hig the primary current also, an additional lag is secured, andthe time interval between the maximum or minimum periods ofthe primary and secondary currents is increased. Such a trans-

Fro. 85.

former may, by properly proportioning its several elements anddetermining the proper relations between the primary andsecondary windings, the thickness of the magnetic shield, andother conditions, he constructed to yield a constant current at allloads.

Fig. 95 is a cross-section of a transformer embodying this im-provement. Fig. 96 is a similar view of a modified form oftransformer, showing diagrammatically the manner of using thesame.

a a is the main core of the transformer, composed of a ringof soft annealed and insulated or oxidized iron wire. Upon thiscore is wound the secondary circuit or coil b b. This latter isthen covered with a layer or layers of annealed and insulatediron wires c c, wound in a direction at right angles to the secondary