Replica of Bell's 1876 'Centennial' telephone transmitter, made by Science Museum Workshops, South Kensington, London, England, 1959

Replica of Bell's 1876 'Centennial' telephone transmitter, made by Science Museum Workshops, South Kensington, London, England, 1959

The 'Centennial' telephone was so called because Alexander Graham Bell (1847-1922) first demonstrated it at the Centennial Exhibition in Philadelphia on 25 June 1876. It was a more refined version of Bell's original 'Gallows' telephone: the hinged armature was replaced with an iron disk glued directly to a parchment diaphragm. Although this was an improvement, it was still problematic, as the parchment was hygroscopic, meaning that just a few minutes of talking allowed the parchment to absorb enough moisture from the breath to cause it to lose its taughtness, resulting in a reduced quality of sound, and eventually in the iron disk becoming stuck to the electromagnet, so ending the telephone call.