Alexanderson alternator
Alexanderson would receive a patent in 1911 for his device. The Alexanderson alternator followed Fessenden's rotary spark-gap transmitter as the second radio transmitter to be modulated to carry the human voice. Until the invention of vacuum-tube (valve) oscillators in 1913 such as the Armstrong oscillator, the Alexanderson alternator was an important high-power radio transmitter, and allowed amplitude modulation radio transmission of the human voice. The last remaining operable Alexanderson alternator is at the VLF transmitter Grimeton in Sweden and was in regular service until 1996. It continues to be operated for a few minutes on Alexanderson Day, which is either the last Sunday in June or first Sunday in July every year.