Final answer:
A manually adjustable resistor is known as a rheostat. A rheostat allows the user to change the resistance in a circuit by manually adjusting a knob or lever. This can be done to adjust parameters like the period of wiper blades in a system.
Step-by-step explanation:
A manually adjustable resistor is known as a rheostat. A rheostat is a type of resistor that allows the user to change the resistance in a circuit by manually adjusting a knob or lever. This is done by varying the length of the resistor's conductive pathway. By increasing or decreasing the resistance, the current in the circuit can be adjusted accordingly.
For example, in the intermittent windshield wipers system mentioned in Example 10.9, a rheostat is used to control the period of the wiper blades. By adjusting the resistance of the rheostat, the time it takes for the voltage-controlled switch to close and activate the wipers can be modified.
In some cases, adjusting the applied voltage may also be necessary to ensure that the current delivered through the resistor has an appropriate value. This can be done by connecting the circuit to a variable voltage source or using a potentiometer, which is another type of manually adjustable resistor that can alter the input voltage.