What is component 9 in figure T-2?

Component 9 in figure T-2 is identified as a variable resistor, often referred to as a rheostat. A variable resistor allows for the adjustment of resistance in a circuit, enabling the user to control the amount of current flowing through it. This capability is essential in various applications, such as tone adjustments in audio circuits or controlling the brightness of lights.

Variable resistors are crucial components because they provide flexibility in circuit design. By changing the resistance, they can effectively modify voltage and current levels, which is particularly valuable for testing and tuning circuits.

In contrast, a variable capacitor adjusts capacitance and typically serves different functions, such as tuning radio frequencies. A variable inductor modifies inductance, which can affect circuit response at different frequencies, while a variable transformer adjusts voltage levels.

Understanding the function and application of a variable resistor helps in comprehending how adjustments can be made in an electronic circuit to achieve desired performance characteristics.