You can use an ammeter in a series circuit because in a series circuit, the current is the same throughout the circuit. An ammeter measures the current flowing through a specific point in the circuit, and in a series circuit, there's only one path for the current to flow, so it's easy to measure.
In a parallel circuit, the current splits and flows through multiple paths, making it challenging to measure with a single ammeter. To measure the total current in a parallel circuit, you would need to use a clamp ammeter or multiple ammeters in each branch.
On the other hand, you can use a voltmeter in a parallel circuit because in a parallel circuit, the voltage across each component is the same. A voltmeter measures the voltage difference between two points, and in a parallel circuit, you can place it across any component to measure the voltage across that specific component.
In a series circuit, you can't easily use a voltmeter because the entire voltage of the source is dropped across the components in sequence, making it challenging to connect a voltmeter without disrupting the circuit.