18 REPORT NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS (d) Rugged indicator.—It is common experience that B. Alternating Current Tachometer delicate electrical instruments will not withstand vi- This instrument consists of an alternating current bration of the severity found on instrument boards… … Figure 14— Horn d.c. electric tachometer. terial. In general, the ruggedness of an instrument of usually of cobalt steel. As the rotor of the generator a given design decreases with its sensitivity. It is revolves there is induced in the stator windings an therefore important that the generator…
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