This simple bell is intended for a crucial experiment: the demonstration that sound waves cannot travel in vacuum, contrary to light waves which are transmitted in vacuum. In this experiment the bell is placed under a bell-jar on the base of an air pump. The bell in the bell-jar can be activated by holding a magnet external to the bell-jar. The sound can be heard as long as there is air in the bell-jar. When the bell-jar has been evacuated, the bell cannot be heard anymore. Which shows that sound waves require molecules to be transmitted.