It was rather a fine bell: heavy, metal, it rested on the table of whoever was on duty. Some of the more sympathetic of the masters would permit a pupil sitting next to him to take over the duty of ringing it. To do so required delicacy of touch. There was, in effect, a button on the top of the bell, attached to a short narrow shaft. If you pressed down for too long, no sound would emerge. A short, sharp strike was the preferred method and a surprisingly loud "Ting" would sound.
The bell was used throughout the meal for the purposes of silencing everyone so an announcement of some kind or another - "There are seconds" - could be made. Towards the end of the meal, the bell would sound twice: "The double bell means silence!" Presumably the logic behind this was to ensure speed in finishing our meals and clearing the tables.
The final bell was the signal for chairs and benches to scrape back and everyone to rise to their feet: "Benedictus benedicata per iesum christum dominum nostrum Amen." Curiously, the Latin master, John Herbert, never said the grace in Latin but went instead, briskly, for this considerably more austere form: "For what we have just received, we return thanks. Amen."