(a) The direction of rotation is directly indicated by the polarity of the output voltage.
(b) The output voltage is typically 10mV/r.p.m. and can be measured with conventional type d.c. voltmeter. Disadvantages (a) Brushes on small tachometer generators often produce maintenance problems, as their contact resistance may vary and produce appreciable error.
(b) The input resistance of meter should be very high as compared with output resistance of generator. This is required to limit the armature current to small value. If the armature current is large, the field of permanent magnet is distorted giving rise to non-linearity.
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