Why should ammeter have low resistance and voltmeter high resistance??
Ammeter is used to measure the strength of current in an electric circuit, it is always connected in series with a circuit, hence its introduction should not change the current, so it has low resistance.
Voltmeter is an instrument to measure the potential difference between two points to which terminals of the instrument are connected.It is always connected in parallel to the circuit and its introduction does not change the pot. diff. to be measured, hence it may have high resistance.