BELL IN VACUUM (0072) Demonstrates that sound waves need material medium for propagation and cannot travel through vacuum. For use on pump plates with diameter at least 15cm. It consists of an electric Bell operating on 4-6 volts AC / DC, suspended inside bell jar through a pair of thick metal wires from a rubber bung that seals the jar from the top. The wires terminating in 4mm socket terminals on the upper side of rubber bung for electrical connection to the bell Qty :