What is component 3 in figure T-3?

In schematic diagrams, an inductor is represented by a series of coiled loops. When an arrow is drawn diagonally through or across the coil, it indicates that the component's value can be adjusted by the user. Therefore, this specific symbol in figure T-3 represents a variable inductor. Distractors like a "variable capacitor" would be drawn as two parallel lines with an arrow through them, while a "meter" would simply be a circle with a letter inside.