Retrospective: The $100 idea: How Russell and Sigurd Varian, with the help of William Hansen and a $100 appropriation, invented the klystron

I first met Russell Varian in 1939 on a stairway landing in the Stanford University Physics Building. I had just been hired as a part-time research associate on the new Klystron Project at a then munificent $90 per month, and was going upstairs to meet the Varians and their small staff. I had heard of the invention of this new electron tube that could generate centimeter waves, but I did not know how it worked, and indeed I had never even seen one. Thus, when I stopped and introduced myself to Russ, he thought the first order of business ought to be an explanation of the principle of the klystron. I remember his saying something like this: