Hi,
To add to that; the number on the bit is the temperature in degrees Fahrenheit divided by 100. I tend to use a number 8 (800 degrees F) in fine pointy conical style for most stuff but have a big chunky screwdriver no. 9 for soldering car battery terminals and bits of brass and steel. I've some random 5, 6 and 7 bits in a tin but these don't see much use. I think 7 is considered "normal" but my soldering style is fast and very well refined so I prefer an 8
How it works: It works by using the Curie Point of the metal. Above a certain temperature it decides not to be magnetic. It cools down a bit and becomes magnetic again. This operates a switch; the switch turns the element (24V 45W usually) on and off. Hence "Magnastat" - Magnetic Thermostat. Simple, not much to go wrong, in my opinion these irons rock - been using one since 1986
Regards, Kat