What is component 9 in figure T-2?

In Figure T-2, component 9 is drawn as a zig-zag line with an arrow pointing to or passing through it. The zig-zag line is the standard symbol for a resistor, and the addition of an arrow indicates that its value can be adjusted, making it a variable resistor (often called a potentiometer). In electronic schematic diagrams, an arrow drawn through or pointing to a standard component symbol universally means "variable" or "adjustable." This allows the user to change the resistance to control current or voltage in the circuit. It cannot be a variable capacitor or variable inductor, as those would use the parallel plate or coiled wire symbols, respectively, rather than a zig-zag line.