*Question T6C10:
What is component 3 in figure T-3?*  

*Correct Answer is  D: Variable inductor*  

### AI Response: (may contain errors)  

In question T6C10, we need to identify component 3 in figure T-3, which is a variable inductor.

### Explanation of the Correct Answer:
- **Variable Inductor**: A variable inductor is a component that allows the inductance to be adjusted. It typically consists of a coil of wire with a movable core, which changes the inductance as the core is moved in or out of the coil. This is commonly used in tuning circuits, such as in radio transmitters and receivers.

### Why the Other Choices are Incorrect:
- **A: Connector**: A connector is a device used to join electrical circuits together. It does not have variable properties and is not typically represented as a component that can be adjusted like an inductor.
  
- **B: Meter**: A meter is an instrument used to measure electrical quantities (like voltage, current, or resistance). It does not have variable inductance and is not used for tuning circuits.
  
- **C: Variable Capacitor**: While a variable capacitor is also an adjustable component, it changes capacitance rather than inductance. The question specifically asks for an inductor, making this choice incorrect.

### Useful Formulas or Rules of Thumb:
- **Inductance Formula**: The inductance $L$ of a coil can be calculated using the formula:
  
  $$L = \frac{N^2 \cdot \mu \cdot A}{l}$$
  
  where:
  - $N$ = number of turns in the coil
  - $\mu$ = permeability of the core material
  - $A$ = cross-sectional area of the coil
  - $l$ = length of the coil

- **Tuning Circuits**: In radio applications, variable inductors and capacitors are often used together in LC circuits to tune to specific frequencies. The resonant frequency $f_0$ of an LC circuit is given by:

  $$f_0 = \frac{1}{2\pi\sqrt{LC}}$$

  where $L$ is the inductance and $C$ is the capacitance.

Understanding these components and their functions is crucial for the Extra Class exam, especially in the context of radio frequency applications.