Bells used in ritual to open or close magic circles, bells for opening and closing Rites, various bells used by Witches for religious purposes.
The Bell is used by many in the process of casting the protective circle. It has been used for many hundreds of years in various religious rituals for different thing:
"Ring the bell, close the book, snuff out the candle..."
The above was from the ritual of exorcism from the Catholic church.
Bells are lovely way to open a circle as their clear, sweet sounds can act as the signal for all participating that the time has arrived to focus seriously on the ceremony at hand.