Circular Beam of Electrons

This is a pretty standard first or second year physics undergraduate experiment, where the charge-to-mass ratio of an electron can be measured. See, for example:
http://www.ic.sunysb.edu/Class/phy122ps/labs/dokuwiki/doku.php?id=phy124summer:lab_8
http://www.clemson.edu/ces/phoenix/labs/cupol/eoverm/index.html
http://physics.nyu.edu/~physlab/Eng_PhysIII/eng_physIII.html
http://www.physics.upenn.edu/undergraduate/undergraduate-physics-labs/experiments/electron-motion-magnetic-field
It's actually quite elegant, and was quite similar to the work that J.J. Thomson did to show that cathode rays were in fact charged particles.

I didn't know those things had an actual name. I had seen examples of them in science catalogs for years, although they are expensive enough to typically be found only in university demo collections. And at that level, someone in the department might end up making their own custom one.

In particular, I had come across this page which shows a range of motions possible if you had a giant version of one of those tubes. I've used one of the images near the end in talks before as an example of a magnetic mirror motion, a topic important to some fusion plasmas and to behavior of plasmas near the Earth too. Unfortunately that seems to be another topic that could use a better intro level explanation somewhere on the web (assuming there isn't one I haven't found yet), although one could easily spend a whole hour long lecture trying to explain all of the motions seen in just the second to last image on that one page.