From the Guidebook:
This is the original site
of the laboratory and factory of Federal Telegraph Company,
founded in 1909 by Cyril F. Elwell. Here, Dr. Lee de Forest,
inventor of the three-element radio vacuum tube, devised
the first vacuum tube amplifier and oscillator in 1911-13.
Worldwide developments based on this research led to modern
radio communication, television, and the electronics age.