Invention of the telephone
The carbon microphone was independently developed around 1878 by David Edward Hughes in England and Emile Berliner and Thomas Edison in the US. Although Edison was awarded the first patent in mid-1877, Hughes had demonstrated his working device in front of many witnesses some years earlier, and most historians credit him with its invention.
Public address system
Their first experiment used a carbon microphone. When the 12 V battery was connected to the system, they experienced one of the first examples of acoustic feedback, a typically unwanted effect often characterized by high-pitched sounds. They then placed the loudspeaker on the laboratory's roof, and claims say that the amplified human voice could be heard away. Jensen and Pridham refined the system and connected a phonograph to the loudspeaker so it could broadcast recorded music. They did this on a number of occasions, including once at the Napa laboratory, at the Panama–Pacific International Exposition, and on December 24, 1915 at San Francisco City Hall alongside Mayor James Rolph. This demonstration was official presentation of the working system, and approximately 100,000 people gathered to hear Christmas music and speeches "with absolute distinctness".