A common requirement for both the Ammeter and Voltmeter is that the current flow through these should not disturb the electric circuit, i.e. should not change the current flowing in or voltages across various parts of circuits.
Voltmeter is a high resistance Galvanometer. It has the Multiplier resistor in series with the meter. A voltmeter is connected across the two points between which the potential difference is to be measured. An ideal voltmeter should have infinite resistance.
Ammeter is a low resistance Galvanometer. It has the shunt resistor to bypass most of the current around the coil. An ammeter, since it is to measure the current flowing through a circuit element, is connected in series with it. An ideal ammeter should have zero resistance.
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