An ammter is basically a meter movement shunted with a low resistance, hence its total resistance Ra will be low.
* The low shunt resistance is used to bypass the excess large current and hence keep the meter movement safe.
* An ammeter is connected in series in a circuit to measure current, its resistance has to me low so that the original current in the circuit does change after introducing the ammeter.
* Also the voltage drop across the ammter when introduced in a circuit should be zero or very low, hence its resistance should be low
* Also the power loss across the ammeter I2Ra should be low, which is possible if Ra is low
An voltmeter is basically a meter movement in series with a high resistance, hence its total resistance Rv will be high.
* The high sseries resistance is used to drop the excess large voltage and hence keep the meter movement safe.
* A voltmeter is connected in parallel in a circuit to measure voltage, its resistance has to high so that the original current/voltage in the circuit does change after introducing the ammeter.
* Also the current through the voltmeter when introduced in a circuit should be zero or very low, hence its resistance should be high
* Also the power loss across the voltmeter V2 /Rv should be low, which is possible if Rv is high.