Cathode Ray Tubes

   Cathode ray tubes are the product of two 19th century technologies: high vacuum and high voltage. The Geissler pump, developed in 1855, allowed glass tubes, equipped with metallic electrodes, to be evacuated and sealed off. The induction coil  was fully developed in the early part of the second half of the century, and provided the high voltage necessary to produce a discharge between the electrodes.

   A number of specialized tubes were used to illustrate the properties of the rays emanating from the cathode. 

1. Cathode Rays Travel in Straight Lines. This is usually called the Maltese Cross Tube, from the shape of the aluminum cross mounted inside this tube. The shadow of the cross may be seen on the convex end of the tube. The cross is designed to flip down so that students may see the diffuse green spot formed by the cathode rays as the glass fluoresces. Of course, x rays are also being given off from the end of the tube.

2. Cathode Rays Carry Energy. The cathode rays from the hemispherical cathode are focussed on a thin piece of platinum held above it by a glass arm. The platinum eventually heats up and glows red, showing that the cathode rays carry kinetic energy.

3. Cathode Rays Carry Linear Momentum. There are two similar electrodes in this tube, either of which may be used as a cathode. The beam of cathode rays collides with a paddle wheel whose axle runs on parallel glass rails inside the tube, and causes it to spin. Fluorescent paint strips on the mica vanes glows in various colors as the wheel spins.

   The device is often called a Railway Tube. This example is in the Greenslade Collection.

4. Cathode Rays Are Emitted at Right Angles to the Cathode. This tube has a concave cathode that brings the cathode rays emitted from it to a focus in the middle of the tube. The rays then diverge and produce a green fluorescent patch on the side wall of the tube. The path of the cathode rays can be followed by the blue glow due to the residual gas in the tube as in the example at the right, below.

5. Cathode Rays Travel to All Anodes. This rather minor tube shows cathode rays being emitted from the cathode and traveling to the three anodes. The vacuum is intentionally slightly soft, and so the path of the cathode rays is traced out by the visible radiation from the excited air molecules.

   This tube is in the Greenslade Collection.

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