carbon lamps were tried, but some detected, or thought they detected, flickering when metallic filament lamps were tried. At slightly less than 25 frequency the majority of observers detected flickering. We may thus take 25 as a yet permissible lower limit for the frequency if the current is to be used in incandescent lighting. With arc lighting the lowest frequency permissible is 40. As a general rule lighting current is supplied at a frequency of 50. To get such a frequency with a machine built on the principles shown in Fig. 14 would require driving it at a speed of 3000 revolutions a minute. Such a speed is too high for ordinary steam engines, athough it is well within the range of steam turbines, Apart, however, from the question of driving, it is mechanically and electrically wrong to subject a coil, which must be highly insulated, to so high a speed. High speed means great centrifugal forces, and that means great mechanical stress on the insulating material. Such stresses should be avoided. as far as possible. For this reason the mechanical arrangement of field magnet and armature is reversed in modern machines. It is the magnetic field which is caused to rotate, and then it is possible to keep the armature
