CHAPTER IV.

Motors with Circuits or Different Resistance.

As lias been pointed out elsewhere, the lag or retardation ofthe phases of an alternating current is directly proportional tothe self-induction and inversely proportional to the resistance ofthe circuit through which the current flows. Hence, in orderto secure the proper differences of phase between the two motor-circuits, it is desirable to make the self-induction in one muchhigher and the resistance much lower than the self-induction andresistance, respectively, in the other. At the same time themagnetic quantities of the two poles or sets of poles which thetwo circuits produce should be approximately equal. Theserequirements have led Mr. Tesla to the invention of a motorhaving the following general characteristics: The coils whichare included in that energizing circuit which is to have thehigher self-induction are made of coarse wire, or a conductor ofrelatively low resistance, and with the greatest possible lengthor number of turns. In the other set of coils a comparativelyfew turns of finer wire are used, or a wire of higher resistance.Furthermore, in order to approximate the magnetic quantities ofthe poles excited by these coils, Mr. Tesla employs in the self-induction circuit cores much longer than those in the other orresistance circuit.

Fig. 65 is a part sectional view of the motor at right angles tothe shaft. Fig. 66 is a diagram of the field circuits.

In Fig. 66, let a represent the coils in one motor circuit, and nthose in the other. The circuit a is to have the higher self-induction. There are, therefore, used a long length or a largenumber of turns of coarse wire in forming the coils of this cir-cuit. For the circuit is, a smaller conductor is employed, or aconductor of a higher resistance than copper, such as Germansilver or iron, and the coils are wound with fewer turns. In apply-ing these coils to a motor, Mr. Tesla builds up a field-magnet ofplates c, of iron and steel, secured together in the usual manner