Galvanometer A very sensitive meter used to measure tiny currents, usually 1mA or less.
Electricity is the flow of charge around a circuit carrying energy from the battery (or power supply) to components such as lamps and motors.
A capacitor stores electric charge. It can be used with a resistor in a timing circuit, for smoothing a supply (it provides a reservoir of charge) and can be used as a filter (blocking DC signals but passing AC signals). Unpolarised capacitors usually have small values, less than 1µF.
Diode A device which allows current to flow in only one direction.