Alternating current can not be measured by D.C. ammeter because
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A.C. cannot pass through D.C.
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A.C. changes direction
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average value of current for complete cycle is zero
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D.C. ammeter will get damaged.
C.
average value of current for complete cycle is zero
The full cycle of alternating current consists of two half cycles. For one half, currently is positive and for second half, current is negative. Therefore, for an a.c. cycle, the net value of current average out to zero. While for the half cycle, the value of current is different at different points. Hence, the alternating current cannot be measured by D.C. ammeter.