That's why what was later termed "thermionic emission":

https://en.wikipedia.org/wiki/Thermionic_emission

...is also known as the "Edison Effect" (though the effect was discovered by others much earlier, and re-discovered over time - Edison being just one in the chain).

His whole purpose was in trying to find out why his light bulbs filaments broke and burned out, and why there was darkening near the positive terminal of the bulb.

I won't go further with this, but I do often wonder if Tesla, while he worked with Edison, had any contact with this part of Edison's operations? In everything I've read on both inventors, it would seem like the answer is "no"; Tesla started working for Edison in June of 1884, and Edison took out a patent on the effect in 1883, and exhibited it in 1884.

The timeline is close, but for some reason Tesla either didn't have contact with the device, or dismissed it - as far as I can tell, though, it was likely the former, because I've never read anything from him where he discussed it.

Which is a pity. Had they not had the "breakup" of the century (which resulted in an insane rivalry ala "War of the Currents") - I am certain that Tesla would have both seen and understood the usefulness of the work, and would have been able to scientifically describe its workings most likely. Electronic technology would have been advanced by almost 30 years or so (whether or not that would have been a good thing we cannot know).

EDIT: years