From Alfred N. Goldsmith, "Radio Telephony" (Wireless Press, 1918). The Alexanderson Alternator was an alternative to the spark gap for transmitters. It had a rotating disk with many individual windings such that the edge of the disk could cut magnetic flux lines at a relatively high rate ... perhaps even 100 KHz! These were popular for a while, but eventually transmitting tubes replaced them for continuous signal generation at high power.
