Final answer:
A potentiometer is the most commonly used variable resistor in low power circuits, used for controlling current flow and measuring voltages. For accurate measurements, the standard emfs should be of similar magnitude to those being measured and the resistance should be a few ohms. Moreover, a higher resistance leads to lower current flow in a circuit.
Step-by-step explanation:
The variable resistor most commonly used in low power circuits is known as a potentiometer. This device allows for variable resistance in the circuit and can be used to control the current flow, measure voltage levels, or adjust signal levels. In a circuit, the current flow is inversely proportional to the resistance, as per Ohm's law. Therefore, for a given voltage, a higher resistance results in a lower current, and vice versa.
When measuring cell emfs on the order of a few volts, a potentiometer provides accurate results when the standard emfs are of the same order of magnitude and the resistance is in the range of a few ohms. This is because the potential drop across the potentiometer's wire needs to match the measured emf for accurate null measurements without drawing significant current from the source.
Resistors are common components in electronic circuits, and resistance values are commonly measured in ohms (Ω). The smaller the resistor value, the higher the potential for current flow in the circuit; however, care should be taken as too small a resistance can lead to overheating or damage to components.