A puzzling phenomenon observed by Edison in one of his early light bulbs seemed to disclose a kind of electric current that did not fit the received ideas, for it was not confined to a conductor, and it would flow in one direction but not in the reverse direction.  From this “Edison effect” arose the modern vacuum tube, which enabled, first, the detection of electromagnetic waves by a process readily understood (rectification), and second, a means for effectively multiplying the strength of an electric current (amplification).  Both of these capabilities brought dramatic improvements in wireless telegraphy, and they also introduced the prospect of radio transmission of voice and sound.  

This is the second of three talks on early radio technology.