Relay An electrically operated switch, for example a 9V battery circuit connected to the coil can switch an AC mains circuit. The rectangle represents the coil. NO = Normally Open, COM = Common, NC = Normally Closed.
A variable capacitor is used in a radio tuner.
Galvanometer A very sensitive meter used to measure tiny currents, usually 1mA or less.
Aerial (Antenna) A device to receive or transmit radio signals. It is also known as an antenna.