An ammeter should have a low internal resistance. This is because an ammeter is connected in series with the circuit to measure the current. If the ammeter has a high resistance, it will significantly alter the current in the circuit, leading to inaccurate measurements. A low internal resistance ensures that the ammeter has minimal impact on the circuit, allowing for a more accurate reading of the current.
On the other hand, a voltmeter should have a high resistance. A voltmeter is connected in parallel with the component or circuit whose voltage is to be measured. If the voltmeter has a low resistance, it will draw a significant amount of current from the circuit, which can alter the voltage across the component and lead to inaccurate measurements. A high resistance in the voltmeter ensures that it draws very little current from the circuit, thus providing a more accurate measurement of the voltage.