An ammeter is used to measure the current flowing through a component/circuit. Since, current remains same in series connection and also the resistance of an ammeter is very small due to which it doesn’t affect the current to be measured, hence ammeter is connected in series with the circuit. Figure below shows the connection of the Ammeter in the circuit.

A voltmeter is used to measure the potential difference across two point in a circuit. Since, in parallel connection the voltage in the branches remains same, and also the resistance of the voltmeter is very high due to which a very small current flow through the voltmeter, therefore voltmeter is connected in parallel to the circuit.. Figure below shows the connection of the Voltmeter in the circuit.