The thomson expirement

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Lesson Transcript

Instructor: Damien Howard

Damien has a master's degree in physics and has taught physics lab to college students.

J.J. Thomson's cathode ray tube experiments led to a very important scientific discovery, the electron. In this lesson learn what a cathode ray tube is and how J.J. Thomson made his discovery.

Cathode Ray Tubes in Your Home

Portrait of J. J. Thomson

You might have used a cathode ray tube even if you've never even heard of it until reading this lesson. Before LCD and Plasma TVs became commonplace, most people used bulkier cathode ray tube (CRT) televisions. The CRT in a television is used to display images on your screen. However, cathode ray tubes have been used for more than entertainment. It was cathode ray tubes that allowed the English physicist J.J. Thomson to discover the existence of electrons in 1897.

How Cathode Ray Tubes Work

Before we see how J.J. Thomson used the cathode ray tube to discover the electron, we need to know how a cathode ray tube works. We'll look at a basic CRT like what J.J. Thomson would've used, as seen in the diagram below.

Cathode Ray Tube Diagram

The CRT consists of several elements starting with a tube that's vacuum sealed to keep air out of it. On one side of the inside of the tube there's a cathode and an anode. The cathode is a negatively-charged conductor, and the anode is a positively-charged conductor. Electrons, which have a negative charge, flow off the cathode and are attracted towards the anode. A small hole in the anode allows some electrons to pass through it, creating a beam of electrons. On the opposite side of the tube is a coating that glows when struck by the electrons. This allowed J. J. Thomson to see where the electron beam was hitting.

Of course, before his experiment, we didn't know electrons existed. So, no one was calling it an electron beam. Instead, what flowed off the cathode toward the anode were called 'cathode rays.' Hence the name cathode ray tube.

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