What is component 4 in figure T-2?

In the context of figure T-2, component 4 is identified as a transformer. This conclusion can be drawn based on the fundamental characteristics and functions of a transformer, which typically consists of two or more wire coils that transfer energy between them through electromagnetic induction. Transformers are essential in AC circuits for stepping up or stepping down voltage, making them integral in various applications in amateur radio and electronic projects.

In the identification of components in schematic representations, transformers are usually depicted with specific symbols that represent their function—a pair of coils connected through a magnetic core. Recognizing these symbols is crucial for interpreting circuit diagrams accurately.

The other options represent distinct components with their unique symbols and applications. A variable inductor would typically have a different symbol associated with inductance, and a potentiometer has a symbol indicating a resistive element with an adjustable tap. A double-pole switch would also have a symbol indicating its switching mechanism. These differences in symbols help in easily identifying and distinguishing among various components in electronic diagrams.