During all these events the boys had been busy at developing the receiving
set, and at last it was finished to their satisfaction. In the course of
their work they gathered a large amount of familiarity with radio which
proved of immense value later on, as will be seen in the next volume of
this series, entitled: "The Radio Boys at Mountain Pass; Or, The Midnight
Call For Assistance."

The special set that represented the advance they had made in radio
reception included the regenerative principle. This feature added
immensely to the sensitiveness of the set. It consisted of a coil,
variously known as the tickler, the intensity coil, and the regeneration
coil. It involved three controls, the wave-length tuning, the regenerative
coil, and the filament rheostat. The result of the combination was not
only that the radio frequency waves could be carried over into the plate
circuit, but that they could be amplified there by the energy derived from
the local battery in the plate circuit without change of frequency or wave
form, and that they could be fed into the grid circuit, where they
increased the potential variations on the grid so that the operation
constantly repeated itself.

This "feed-back" regeneration enormously increased the loudness of the
receiving signals, and its value to the boys was demonstrated one night
when the air was unusually free of static and they clearly heard the
signals from Nauen, Germany, and the Eiffel Tower, Paris. They looked at
each other incredulously at first, and then as they heard the signals
again too certainly to admit of doubt, they jumped to their feet, clapped
each other on the shoulder, and fairly went wild with delight.

"The first boys in this old town to pick up a message from Europe!" cried
Joe. "What next?"

"Asia perhaps," suggested Jimmy.

"Then Australia," ventured Herb.

"Or Mars," predicted Bob. "Who knows?" he added, as he saw the smile of
doubt on his comrades' faces. "Marconi thought he might, and he's no
dreamer. What is impossible to radio?"

THE END

This Isn't All

Would you like to know what became of the good friends you have made in
this book?

Would you like to read other stories continuing their adventures and
experiences, or other books quite as entertaining by the same author?

On the _reverse side_ of the wrapper which comes with this book, you will
find a wonderful list of stories which you can buy at the same store where
you got this