Loudspeaker A transducer which converts electrical energy to sound.
Voltmeter Measures voltage. The proper name for voltage is potential difference but voltage is more widely used.
A variable capacitor is used in a radio tuner.
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.