INDUCTOR MACHINES 275 Its armature coils, A, are stationary. One stationary field coil, F, surrounds the magnetic circuit of the machine, which consists of two sections, the stationary external one, 5, which contains the armature, A, and a movable one, C, which contains the inductor, N. The inductor contains as many polar projec- tions, A7", as there are cycles or pairs of poles. The magnetic flux in the air gap and inductor does not reverse or alternate, as in the revolving-field type of alternator, Fig. 135, but is constant in direction, that is, all the inductor teeth are of the same polarity, but the flux density varies or pulsates, between a maxi- mum, Bi, in front of the inductor teeth, and a minimum, B2, though in the same direction, in front of the inductor slots. The magnetic flux, <£, which interlinks with the armature coils, does not alternate between two equal and opposite values, + $o and FIG. 136.—Inductor alternator. -~o, as in Fig. 135, but pulsates between a high value, $1, when an inductor tooth stands in front of the armature coil, and a low value in the same direction, $2, when the armature coil faces an inductor slot. 157. In the inductor alternator, the voltage induction thus is brought about by shifting the magnetic flux produced by a stationary field coil, or by what may be called magneto 'commu- tation, by means of the inductor. The flux variation, which induces the voltage in the armature turns of the inductor alternator, thus is $1 - $2, while that in the revolving-field or revolving-armature type of alternator-is 2 $0. The general formula of voltage induction in an alternator is: