The principle of ammeter is that the resistance and inductive reactance must be very low. Ammeter has a very low impedance as the voltage drop across the ammeter should be low. The ammeter cannot be connected in parallel because of the above-mentioned reason. In a series connection, the current will be same. Also connecting an ammeter in parallel may result in short circuit and the current passing through ammeter might burn the instrument. For an ideal ammeter, the impedance must be zero so that the voltage drop across the instrument is zero.