Poor charging, high draw - '82 CGT

[TWFAUST at aol.com]

My knowledge of electricity may be analogized to sitting Hannibal at a piano. 
Here are the symptoms. When I turn my heater on, the meter reading drops from 
13 to 12 volts. If, in addition, I turn on the headlights the meter drops to 
11 volts. Occasionally the alternator belt will screech (it seems tight). I am 
worried. Can anyone tell me what I am worried about?

Tom Faust