| Rate of Alternations

—Usually machines used for electric lighting work at from fifty to hundred alternations per second. To obtain the latter with a single coil in a single field would require six you sand revolutions per minute. A rapidly alternating current can be easily obtained with a large number of pole pieces; however a disadvantage results from the fact that these pole pieces alternate in polarity and a large percentage of the lines of force leak acro between adjacent limbs, instead of pa ing through the armature core. Sometimes nearly half the lines of force developed by the field magnets are rendered inoperative by leakage in machines with corele armatures, but the absence of iron in which repeated changes in the direction of the number Ot Yee of force is taking place obviates the lo by eddy currents and hysteresis, and may often more than compensate for the lo by leakage