Knob on the popper is a dimmer in series with the heating element to lower the heat. Without it, this popper roasted WAY too fast.
So that's a modified hot air popcorn popper?
How long does it take to roast the beans in that?
20+ years ago I used to roast in this popper all the time. When I first tried it, it would be over done in less than 5 minutes. I put a dimmer switch in-line with the heating coil, assuming it would just burn up, since the dimmer said "600W" and I think the popper said 1200W. I just tied it onto the outside of the popper since I thought it was a temporary experiment. Lo and behold it worked, and eventually I put the dimmer inside the case. I can adjust the heat from too hot (as it was), to too cold (never roast). I have it set to a point where 2nd crack occurs around 9 minutes. Back when I did this as my regular way of doing coffee, I would buy green beans from a local roaster for I think $3 a pound less than he charged for his roasted coffee. I was the only coffee drinker in my house (kids all young). My primary motivation for roasting was that if I bought a whole pound of roasted coffee, it wouldn't be fresh enough for my tastes when I got near the end of the bag. I generally drink a morning and an afternoon cup. That's all. I grind enough for a few days, which for my tastes is fine.
Later I changed my coffee habits and started drinking espresso. I couldn't figure out how to roast anything that I liked as well as Starbuck's, so I gave up roasting. With medium roast, I could roast such that it tasted the same to me as the local roaster's coffee. I'm thinking now that for espresso I roasted too fast and too dark. Starbuck's is really dark, and when I roast to that color, it just tasted burnt.