Some time it is required to test the circuit element values. e.g. voltage, current, resistance values becomes important to know before attaching or replacing some of the elements. For the purpose to investigate, the required meter must be inserted in a correct way in the circuit. The insertion of any meter in a circuit will affect the circuit.
You must use meters that minimize the impact on the response of the circuit. we will assume that the meters are ideal and do not affect the networks to which they are applied so that we can concentrate on their proper usage.
Fig.no.1: Voltmeter connected across the resisters.
Fig.no.2: Ammeter connected through the series circuit.
The voltages of a circuit can be measured without disturbing (breaking the connections in) the circuit. (Fig.no.1)
The resistance of a circuit can be measured without disturbing (breaking the connections in) the circuit. (Fig.no.1)(replace voltmeter with ohmmeter)
Using an ammeter to measure the current of a circuit requires that the circuit be broken at some point and the meter inserted in series with the branch in which the current is to be determined. (Fig.no.2)