Final answer:
In a series circuit, the electrical current is the same throughout the circuit. Resistors in series carry the same current, but the voltage drop across each can differ. An ammeter must be connected in series to measure the current accurately.
Step-by-step explanation:
The concept you're referring to is a fundamental property of series circuits in electricity. In a series circuit, electrical current is constant throughout; it has the same value at the supply, return path, and across any loads, such as resistors that are present. This occurs because there's only one pathway for the current to flow, and this pathway has to accommodate the entire current from the source. If the current were to change at any point in the circuit, it would imply a buildup or deficiency of charge, which is not possible in a continuous and closed electrical circuit.
When multiple resistors are connected in a series circuit, each one experiences the same current flow. However, the voltage drop across each resistor is proportional to its resistance, following Ohm's Law. The total voltage drops across all resistors add up to the total voltage provided by the source. When measuring current, an ammeter is always connected in series to ensure it measures the actual current flowing through the component of interest.