Preset variable resistor A preset is operated with a small screwdriver or similar tool. It is designed to be set when the circuit is made and then left without further adjustment. Presets are cheaper than standard variable resistors so they are sometimes used in projects to reduce the cost.
A variable capacitor is used in a radio tuner.
LDR A transducer which converts brightness (light) to resistance (an electrical property). LDR = Light Dependent Resistor
Electricity can flow only if there is a complete circuit from the battery through wires to components and back to the battery again.