Exploring Invisible Particles by Visible Observation

Thompson's Cathode Ray Experiment

JJ Thompson did a famous experiment in which he passed an electric current through a glass tube that had a very low-pressure gas inside.  Any gas will do- hydrogen, argon, nitrogen, even mercury vapor.  The tube glows.  He passed the glowing beam through a slit to make a flat stream of glowing particles, which were known as "cathode rays."  Physicists called everything "rays" back then, and there were all kinds of exotic rays which we now know as belonging to the electromagnetic spectrum, or a small set of subatomic particles.  He noticed that when he passed a magnet or a charged set of plates near the cathode ray beam, the particles deflected toward the positive side. (By the way, magnets don't really have "positive" and "negative" ends, but they do influence moving charged particles, because a moving charged particle generates a magnetic field.)  He called the particles "corpuscles" and realized that they were both negative and were less massive than atoms, which led him to correctly hypothesize that the corpuscles were smaller pieces of atoms.  This was contra Dalton, but not completely accurate because Thompson suggested that the corpuscles were floating among a little blob of positive charge; this was endearingly termed the "plum pudding model" of the atom, which was showed by Rutherford to be bunk.

Now cathode ray tubes don't necessarily grow on trees, and neither do their power apparatus.  But I happened to get hold of a set of them, along with their power supply from DHS, courtesy of the chemistry department.

For a simple lab on atomic structure, I wanted students to make some of the puzzling observations that physicists made at the turn of the century, which- although confusing and sometimes self-contradictory when considered individually- can coalesce into a comprehensive understanding of subatomic particles.  Students observed uranium nitrate, uranium sulfate, and thorium nitrate with a small Geiger counter; they measured the radioactivity of alpha, beta, and gamma emitters with a "nuclear scaler" (a very precise Geiger counter); they looked at a simulation of Rutherford's gold foil experiment; and they tested the effect of an extremely powerful neodymium magnet on a powered cathode ray tube.

For a brief video on what they saw, see my YouTube video.

When the magnet is passed along the tube, the electrified gas swirls and quivers, showing the magnetic field lines in tight, alternating bands of brightness.

Students, with minimal help, reasoned thusly:

1.) The glowing substance in the tube cannot be neutrons; otherwise the magnet would not affect them.

2.) It could be protons, but this is inconsistent with what students know about them.

3.) It is probably electrons, because the gas is electrified.

Just one simple way I was lucky enough to stumble upon, to get kids working with subatomic particles on a personal, up-close level.