What is component 2 in figure T-1?

Component 2 is depicted as a circle with three leads connected to it, which is the standard symbol for a Bipolar Junction Transistor (BJT). The three leads represent the base, collector, and emitter of the device. You can always identify a BJT transistor symbol by looking for the central straight bar (the base) and the two angled lines coming off it, one of which will have a small arrow. This distinct three-terminal symbol is entirely unique among the basic components shown in standard beginner circuits.