*Question T7D04:
Which instrument is used to measure electric current?*  

*Correct Answer is  D: An ammeter*  

### AI Response: (may contain errors)  

To measure electric current, the correct instrument is an ammeter. 

### Explanation of the Correct Answer:
- **Ammeter**: This device is specifically designed to measure the flow of electric current in a circuit, typically expressed in amperes (A). It is connected in series with the circuit to allow the current to pass through it.

### Why the Other Choices are Incorrect:
- **A: Ohmmeter**: 
  - An ohmmeter measures resistance, not current. It determines how much opposition a component offers to the flow of electric current.
  
- **B: Electrometer**: 
  - An electrometer is used to measure very small electric charges or voltages, often in the microvolt range. It is not designed for measuring current directly.
  
- **C: Voltmeter**: 
  - A voltmeter measures the voltage (potential difference) across two points in a circuit. It is connected in parallel with the component whose voltage is being measured, not in series like an ammeter.

### Useful Formulas:
- The relationship between voltage ($V$), current ($I$), and resistance ($R$) is given by Ohm's Law:
  
$$
V = I \times R
$$

This formula is useful for understanding how current, voltage, and resistance interact in a circuit, but remember that to measure current specifically, you need an ammeter.