Memory: Crooke’s tube

Towards the end of high school, my physics teacher gathered the class in the laboratory in the back part of the room. We had covered mechanics in the first half of the year, but slipped into increasingly esoteric concepts since: electric current, magnetic fields, relativity, photon momentum, and finally a lecture on quarks thrown in at the end of the year. He had, probably years ago, decided to end his class with a demonstration to show that these concepts had some grounding in fact. He produced two objects he said he had found in the supply closet long ago: a high voltage DC source and, from a wooden case, a Crooke’s tube.

A Crooke’s tube is a glass container, roughly cylindrical, with a near vacuum inside. Two pieces of metal at either end – anodes – were attached to wires that ran out of the tube. Between them was a small flywheel on a track that ran the length of the tube. The paddles of the flywheel had been coated in a special fluorescing paint. Someone turned off the lights, we huddled close in the semidarkness, and the teacher turned on the power.

The flywheel began to spin. Slowly at first, but gradually accelerating until it hit the end of the track. Whatever part of the wheel was in the line between anodes lit up, a pale green in color. He reversed the polarity and the wheel moved in the other direction.

“I don’t know about quarks,” said the teacher, “whether they are real or relevant. But you can see the flow of electrons and photons push the wheel back and forth. This stuff – this stuff is real.”


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