Danish physicist Valdemar Poulsen took Duddell's audio oscillator and, by placing the arc in a transverse magnetic field, and in a hydrogen atmosphere (and somehow not getting blown up in the process), moved the frequency of oscillation up into the low radio range, around 500 kHz or so. This was the arc radio transmitter. It differed from the more common spark transmitter in that the arc's output oscillation was continuous, while that of the spark transmitter was a damped (decaying) oscillation.
I learned something on Slashdot, my day is done.
(I'm a software geek, so my electronics only had to go as far as a wheatstone bridge. Which is kind of embarrassing when you consider that my grandfather was an electrical engineer, I'll bet he could have whipped up an arc transmitter for fun.)