Final answer:
An ammeter measures the electric current flowing through a circuit. It is designed to have a very low resistance to avoid affecting the circuit's behavior and typically includes a fuse for protection against high currents.
Step-by-step explanation:
An ammeter is an instrument used to measure the electric current in a circuit. It is placed in series with the path that the current is flowing so it can measure how much charge is passing through it in a given period of time. The correct answer to the question of what an ammeter can read is: D. Direct power across loads. However, this needs a correction, as the ammeter does NOT measure power directly but current. Hence, the accurate term should be current, not direct power.
Amperes (A) are the unit of current in the International System of Units (SI), and the ammeter is designed to provide a reading in these units. Ammeters can come in both analog and digital forms, with analog meters using a galvanometer mechanism, and digital meters often employing an analog-to-digital converter to display the reading. To measure current accurately without significantly altering the circuit behavior, ammeters must have a very low internal resistance and typically include a fuse to protect against currents that could damage the meter.