Why should the resistance of the ammeter be small and the resistance of the voltmeter large?

The resistance of the ammeter should be low, because it is included in the circuit in series: 1 = U / (R + Ra) The lower the resistance Ra, the less it affects the measured value l.
The resistance of the voltmeter must be large, because it is connected in parallel in the circuit: U = IR = IvRv. The greater the resistance Rv, the lower the current I will be. the less it affects the measured value U.

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