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.
Oscilloscope An oscilloscope is used to display the shape of electrical signals - showing how they vary with time. It can be used to measure voltage and time periods.
Electricity can flow only if there is a complete circuit from the battery through wires to components and back to the battery again.