Although Lee DeForest invented the triode vacuum  in 1906, it was not employed in hearing aids until 1921. The first hearing aid using a vacuum tube was purported to have been made by Earl C. Hanson. He called his hearing aid the Vactuphone. It used one small peanut tube (below right) to amplify the output of a carbon hearing aid. The Globe Hearing Aid Company and Western Electric collaborated to manufacture the Vactuphone. By the early 1940s, Raytheon had developed miniature vacuum tubes. It was these miniature vacuum tubes that made body-worn hearing aids a reality. Unfortunately, the power requirements of these aids made it necessary to use two relatively large batteries—an "A" battery to power the filaments of the vacuum tubes and a "B" battery to supply the high-voltage for the plate current. The batteries were worn external to the hearing aid itself and the name "Two-piece" hearing aid was used to describe them. Until about 1945 "Two-piece" hearing aids were the norm. However, improved vacuum tube design reduced current requirements. As a result, battery manufacturers were able to reduce the size of the batteries used in hearing aids. At the same time, electronic components were also being miniaturized. This allowed the hearing aid manufacturers to design hearing aids with internal batteries. After 1945, these "One-piece" hearing aids quickly replaced the more cumbersome "Two-piece" hearing aids. With continuing improvements in both batteries and components, hearing aids continued to reduce in size until 1953 when the vacuum tube was suddenly supplanted by the transistor.Fawn plastic hearing aid with metal clip on back and wires leading from the bottom. Made in the USA, sold by Angus & Coote as the Sole Australian Agents. The Model is the Sonotone 700. Two dials one on each of the top corners."ANGUS & COOTE/Sole Australian/Agents/C D C" Printed in black on the back, top left hand corner. "SONOTONE/700/SERIAL NO/843218/MAD IN U.S.A./SEE PATENT NOTICE/IN INSTRUCTION BOOK" imprinted on back, centre middle.deaf children australia, hearing aid, sonotone hearing aid, sonotone 700, angus & coote