Bells are used to cleanse the surrounding air, by purifying and cleansing evil thoughts.  Can be thought of as auditory smoke.

 

Some Wiccans use a bell in their rituals, though certainly not all.  Sound is vibration, and vibrations are important, not only in everyday life but especially in magick.  To help achieve the right vibrations, the right 'mood' or frequency, it helps to ring a bell.  It can actually be just as effective  - and therefore as important as the burning of incense, which also affects vibrations.  A very occasional single / double / triple ring, scattered throughout the ritual, can be far more effective that a constant jangling.

 

The bell you use should be chosen carefully.  Don't just take any bell - each has its own particular timbre.  Find one which seems 'right' to you, one which soothes and mellows rather than one which is harsh and strident.  If possible, remove the handle and replace it with a leather loop.  In this way you can then carry the bell from your belt.

 

The bell is a feminine symbol and so is often used to invoke the Goddess in ritual.  It is also rung to ward off evil spells and spirits, to halt stomrs, or to evoke good energies.  Placed in cupboards or hung on the door, it guards the home.