Question
Wed February 27, 2013 By: Lekha
 

why cannot current produced by an ac generator be measured with a moving coil ammeter?

Expert Reply
Wed February 27, 2013
Because the magnetic field is polarised, the meter needle acts in opposite directions for each direction of current.
A DC ammeter is thus sensitive to which way round it is connected; most are marked with a positive terminal, but some have centre-zero mechanisms and can display currents in either direction.
 
A moving coil meter indicates the average (mean) of a varying current through it,which is zero for AC. For this reason moving-coil meters are only usable directly for DC, not AC.
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