Higher tier - variable resistor
Moving the position of the slider on this resistor, changes the length of wire in the circuit which changes the resistance.
A variable resistor is used in some dimmer switches and volume controls.
As the slider moves towards B the length of wire in the circuit increases.
This increases the resistance in the circuit and decreases the current.
It would dim a lamp.
As the slider moves towards A the length of wire in the circuit decreases.
This decreases the resistance in the circuit and increases the current.
It would make a lamp shine more brightly.