Electric current (B): cells, resistance and symbols

Electric resistance is how much a conductor opposes the flow of charge. It is measured in ohms (Ω) with an ohmmeter. A fixed resistor has a set value. A variable resistor, or rheostat, can be changed by sliding a contact along a wire: more wire in the circuit means more resistance and so less current, and less wire means less resistance and more current. This is how the volume knob of a radio works: turning it changes the resistance, so it changes the current. A larger resistance for the same current also gives more heat. Careful: resistance does not depend on the current or on the voltage, at a given temperature. It belongs to the conductor itself. A rheostat does not “store” current: it only changes how hard it is for the current to flow.