Hi all, I am currently reading a book that tries to compare Hertz transmission and reception of radio waves with the photoelectric effect. (I am aware that Hertz discovered the photoelectric effect and what it is although not why it occurs). The book states that; the magnitude of sparks produced at the spark gap during transmission was greater when electromagnetic radiation (Ultraviolet) was being projected towards the spark gap. So now I hope you can see my need to understand what causes a spark. I have searched the web but too many suggestions left me confused. So I was wondering whether someone might be able to help. Thank you.