Bell patents the telephone and makes the first successful call

Background. Bell's research on hearing and speech led to experiments with hearing devices, culminating in the first U.S. patent for the telephone. In 1874, telegraph traffic expanded rapidly, becoming the 'nervous system of commerce.' Bell developed an acoustic telegraph and drew up a patent application. Gray developed a telephone prototype in 1876, working in Highland Park, Illinois — a development site, not one of the filing or call locations. On February 14, 1876, Gray signed a caveat describing a liquid transmitter.

Competing Filings. On February 14, 1876, Gray signed and notarized a caveat for a telephone using a liquid transmitter. Bell's lawyer learned of Gray's idea before filing. Bell had previously filed a mercury application in February 1875. Gray abandoned his caveat and did not contest Bell's priority, leading the examiner to approve Bell's patent on March 3, 1876.

First Successful Call. On March 10, 1876, Bell used the instrument in Boston to call Thomas Watson, who was in another room but out of earshot. Watson, an assistant to Bell, had worked in a Boston machine shop since 1872. After March 1876, Bell focused on improving the electromagnetic telephone and never used Gray's liquid transmitter in public demonstrations.