Radio
Picture of Tesla's radio patent and Marconi sending the first signal across the English Channel.
Tesla had originally made a demonstration and patented radio back in 1897 recieving his patent in November of 1900. Marconi recieved much more credit though when he sent a siganl across the English channel. Tesla was heard to say, let Marconi continue he's using seventeen of my patents.
An early radio transmitting and recieving unit used by the U.S. Navy