Alternators produce an electrical field that can interfere with radio reception. An alternator condenser corrects this problem. Automobile alternators are necessary to recharge the car battery and to run the car electrical systems without using the battery.
An alternator converts the mechanical energy of the engine into electrical energy in the form of alternating current. It makes this conversion by spinning wire brushes at high speed near copper coils to create an electrical field. The electrical current the alternator produces is the discharge of this field.
The word condenser has been replaced largely by the word capacitor when referring to the type of device used as an alternator condenser. A capacitor or condenser has no moving parts. It consists of an insulator connected to two conductors.
Alternator condensers are seen mainly in older automobiles. Modern car radios are designed largely with a built-in shield against alternator-generated static. When a condenser is used, one of the leads connects to the alternator and one connects to ground. When a difference in voltage exists between the leads, a field is generated. Excess energy stores in the insulator, shielding the radio from the interference that would otherwise be discharged.