Step 1: Understanding the Concept:
To measure the total current flowing through a branch, the measuring device must allow all that current to pass through it.
Step 2: Detailed Explanation:
1. The device is called an Ammeter.
2. It is always connected in series with the circuit components because it has very low resistance. Connecting it in series ensures that the same current flows through the ammeter as through the circuit, without significantly altering the circuit's total resistance.
Step 3: Final Answer:
Ammeter, connected in series.