the amateur to receive them. As it was next to impossible for a boy to
get a current having a high enough voltage for operating an
oscillation arc lamp, wireless telephony was out of the question for
him, so he had to stick to the spark coil transmitter which needed
only a battery current to energize it, and this, of course, limited
him to sending Morse signals. As the electrolytic detector was
cumbersome and required a liquid, the crystal detector which came into
being shortly after was just as sensitive and soon displaced the
former, even as this had displaced the coherer.

A few years ahead of these amateurs, that is to say in 1905, J. A.
Fleming, of England, invented the vacuum tube detector, but ten more
years elapsed before it was perfected to a point where it could
compete with the crystal detector. Then its use became general and
workers everywhere sought to, and did improve it. Further, they found
that the vacuum tube would not only act as a detector, but that if
energized by a direct current of high voltage it would set up
sustained oscillations like the arc lamp, and the value of sustained
oscillations for wireless telegraphy as well as wireless telephony had
already been discovered.

The fact that the vacuum tube oscillator requires no adjustment of its
elements, that its initial cost is much less than the oscillation arc,
besides other considerations, is the reason that it popularized
wireless telephony; and because continuous waves have many advantages
over periodic oscillations is the reason the vacuum tube oscillator is
replacing the spark coil as a wireless telegraph transmitter.
Moreover, by using a number of large tubes in parallel, powerful
oscillations can be set up and, hence, the waves sent out are radiated
to enormous distances.