Lessons in practical electricity; principles, experiments, and arithmetical problems, an elementary text-book . gnetic field of three permanent stationary-steel horseshoe N -poles of thesemagnets are above andthe S-poles below thecopper disc, and it cutsthe magnetic lines offorce as it revolves. Eddycurrents, ^[ 292, are in-duced in the copper disc,and the reaction of theirmagnetic field tends toretard the rotation. Theamount of this retardingeffect is directly propor-tional to the speed ofrotation. Since theangular force causing thearmature to rotate is di-rectly proportional to t
Lessons in practical electricity; principles, experiments, and arithmetical problems, an elementary text-book . gnetic field of three permanent stationary-steel horseshoe N -poles of thesemagnets are above andthe S-poles below thecopper disc, and it cutsthe magnetic lines offorce as it revolves. Eddycurrents, ^[ 292, are in-duced in the copper disc,and the reaction of theirmagnetic field tends toretard the rotation. Theamount of this retardingeffect is directly propor-tional to the speed ofrotation. Since theangular force causing thearmature to rotate is di-rectly proportional to themagnetic field of the cur-rents in the two coils,and the retarding angularforce also proportional tothe magnetic field set up,the armature must rotateat such a speed that theelectromagnetic drivingtorque is exactly equalto the electromagneticretarding torque. Then with a constant pressure maintainedin the wattmeter coils for any length of time, the number ofrevolutions of the armature, and therefore the travel of the dialhands will also be constant during the time, and proportionalto the energy 258.—Drum-Wound Armature ofhomson Recording Wattmeter. ELECTR OD YNA MICS. 291
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