I wrote a new entry over at Nineteenth-Century Disability: A Digital Reader:
On September 1879, Richard Silas Rhodes (1842-1902), president of a publishing company in Chicago, received a patent for his “Audiphone for the Deaf” his various improvements to the device. (U.S. Patent No. 319,828). Rhodes had conductive hearing loss for twenty years following a bout of illness and was frustrated with his continuous failed encounters with ear trumpets. He observed he could hear the ticking of his watch when he held it in his mouth, and this inspired his construction of the Audiphone, which exploited the fact sound waves can be transmitted through the teeth or cranial bones.