The correct option is Ammeter.
Explanation
In electrical circuits, specific instruments are employed to measure fundamental quantities such as current, potential difference, and resistance. The design and connection of these instruments depend on the specific property being measured to ensure accuracy and minimal interference with the circuit's operation.
Analysis of Options:
- Ammeter Ammeter: Correct. An ammeter is an instrument designed to measure the electric current flowing through a circuit. It is always connected in series with the component through which the current is to be measured. To minimize its effect on the circuit, an ideal ammeter has zero internal resistance, while a practical ammeter has very low resistance.
- Voltmeter Voltmeter: Incorrect. A voltmeter is used to measure the electric potential difference (voltage) between two points in a circuit. It is connected in parallel across the points of interest and has very high resistance.
- Galvanometer Galvanometer: Incorrect. A galvanometer is primarily used to detect the presence and direction of small electric currents. While it can be converted into an ammeter or voltmeter using shunts or multipliers, its primary function in its raw form is detection rather than the measurement of substantial current magnitudes.
- Rheostat Rheostat: Incorrect. A rheostat is a variable resistor used to control or adjust the current flowing in a circuit by changing the resistance. It is not a measuring instrument.
Key Takeaway:
An Ammeter measures electric current and is connected in series (low resistance), whereas a Voltmeter measures potential difference and is connected in parallel (high resistance).