The Cathode Ray Tube

J. J. Thomson 1897

J. J. Thomson was a British physicist studying the properties of Cathode Rays. For his studies he used a Crookes tube which is also known as a Cathode Ray Tube or CRT. A CRT is a small vacuum sealed tube that contains a trace amount of gas, a phosphorescent screen, and electrodes embedded on either side. Below you will conduct a simulation similar to Thomson's experiment.

This is a picture of a CRT similar to the one used in Thomson's experiments.

CRT's were commonly used in "tube" televisions and older computer monitors

These are pictures of an updated, yet nearly outdated, version of a cathode ray tube.

Instructions

1. At the bottom of the diagram to the left, slide the white circle to "None". This removes the charges from the metal plates.

2. Turn on the beam by clicking the white box next to "TURN ON"

3. Observe the beam

4. Move the white circle on the charge slider all the way to the left. Where are the positive and negative changes? What happens to the beam?

5. Move the white circle on the charge slider all the way to the right. Where are the positive and negative changes? What happens to the beam?

An additional part of the experiment involved placing a small paddle wheel on 2 slightly upward sloped runners in a modified cathode ray tube. Watch the video below to see what happens.

In order to move an object a force must be applied. If you remember from Physics the formula to calculate force is F=ma, or Force equals mass times acceleration. The fact that the paddle wheel moved along the runners when the beam was turned on suggests that the beam must have mass. Thomson concluded the the beam of light must be made of negatively charged particles.

Thomson called these negatively charged particles electrons.

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