Question

What should be the resistances of an ideal ammeter and an ideal voltmeter ?

Answer 1

An ideal voltmeter requires no current to flow through it. In that case, all the current passes through resister and for the that to happen, one needs infinite resistance.

For an ideal ammeter, no voltage should pass through it.

In that case, the voltage should pass trough the resister in order to measure the current accurately and for that to take place, zero resistance is required.