Final answer:
A summing circuit with adjustable resistors can control timing in devices such as windshield wipers by changing the RC time constant. Adjustments to the voltage are not needed for this purpose but may be necessary to maintain a certain current level. Resistance in series affects the total resistance and circuit characteristics.
Step-by-step explanation:
A possible use of the summing circuit with manually adjustable resistances, such as those changed using a potentiometer, is for controlling the timing in applications like intermittent windshield wipers. By adjusting the resistances, you alter the RC time constant, which determines the time delay in these applications. Increasing resistance will increase the RC time constant, leading to longer intervals between operations. Variations in resistance directly change the total resistance of the circuit, impacting the charging time of the capacitor within the RC circuit.
No adjustment to the applied voltage is necessary to alter the RC time constant since it is dependent on the product of resistance and capacitance. However, if the goal is to keep the current at a specific value while changing the resistance, then alterations to the voltage may be required to maintain Ohm's law (V = IR).
It is important to note that the resistance in series adds up in the circuit, which means the total resistance is the sum of all individual resistances. Therefore, if resistances R1 and R2 are changed, this will affect the overall resistance in the circuit, potentially changing the circuit's characteristics, which may be crucial for the precise operation of devices represented by these resistors.