What is component 3 in figure T-3?

In the context of electronic circuits, a variable inductor is a component that allows the inductance to be adjusted or varied. This capability is significant in tuning applications, such as in radio frequency circuits, where the inductance must be optimized to resonate with the capacitance at a specific frequency.

Variable inductors typically consist of a coil of wire that can have its physical shape or the number of windings altered to change its inductance. This feature makes them valuable in various electronic applications, including filters and oscillators.

While other components like connectors, meters, and variable capacitors serve important functions, they do not provide the same adjustable inductive properties that a variable inductor does. This is why recognizing component 3 in figure T-3 as a variable inductor emphasizes its role in tuning frequency responses and adjusting circuit characteristics effectively.