Cathode Rays & Canal Rays

When the Crookes dark space fills the tube, something invisible streams from the cathode and makes the far wall glow. Crookes, Thomson and others pinned down what it was.

Properties of cathode rays#

1. They leave the cathode at right angles to its surface, wherever the anode is. A concave cathode focuses them at its centre of curvature, where a fluorescent material glows brightest.
2. They travel in straight lines: a metal cross in their path casts a sharp shadow on the far wall.
3. They push on what they hit: a light mica paddle wheel in their path turns.
4. They heat what they hit: a blackened platinum strip at the focus of a concave cathode becomes red-hot.
5. They make many crystals, minerals and salts fluoresce.
6. They produce X-rays when they strike a solid, especially a metal.
7. Electric and magnetic fields deflect them exactly as they would a stream of negative particles, whatever gas, cathode metal or anode position is used. Thomson showed this with a tube that makes a small glowing dot, which moves when a magnet or charged rod is brought near.
8. They ionize gas they pass through.
9. They pass through thin metal foils.
10. They blacken photographic plates.

All of this is explained at once if cathode rays are simply a stream of fast electrons.

Canal rays (positive rays)#

Drill holes in the cathode and run the tube at about 1 mm of Hg: faint luminous streams emerge from the holes on the far side of the cathode. These canal rays are the slow positive ions, made near the cathode, that pass through the holes. Electric and magnetic fields deflect them like positive particles, and they too cause fluorescence.