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Short clip to see his experiment (CRT).

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Cathode Ray Tubes in Your Home

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.

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.

The Three Experiments

J.J. Thomson's discovery of electrons didn't happen all at once. Instead, it was a result that was slowly built towards over the course of three different experiments. Before Thomson's experiments, it had already been discovered that the cathode rays deposit an electric charge. This was done by putting an electrometer at the opposite end from the cathode and anode. When the cathode rays hit the electrometer, it measured an electric charge.

The first experiment was done in order to see if the charge was separate from the cathode rays. Thomson used a magnet to bend the cathode rays away from the electrometers. When he did this, he discovered that the electrometers stopped measuring electric charge. From this he deduced that the electric charge and the cathode rays must be combined together and not separate entities.

Next, J.J. Thomson set out to determine if the charge carried by the cathode rays was negative or positive. To do this he would use an electric field. He put a negatively charged metal plate on one side of the cathode rays let out past the anode, and a positively charged metal plate on the other side. Instead of an electrometer at one end of the CRT he now had a fluorescent coating that would glow where the cathode ray hit it. When the charged metal plates were introduced he found that the cathode rays bent toward the positive plate and away from the negative plate. This showed that the charge carried by the cathode rays was negative.

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j.j. thomson's cathode ray tube (crt): definition

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experiment & diagram - video & lesson transcript | study.com

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J.J. Thomson's Cathode Ray Tube (CRT): Definition, Experiment & Diagram - Video & Lesson Transcript | Study.com

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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.

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J.J Thomson was a scientist from 1889 AD. He was the first to discover the negatively charged electron. He also created the Plum Pudding model of an atom. http://hi.fi.tripod.com/timeline/

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Dalton was a scientist in 1903 AD. Dalton was the one who created the atomic theory of matter. Which included
"1. Elements are composted of small particles called atoms.
2. All atoms of a given element are identical.
3. Atoms are neither created nor destroyed in chemical reactions.
4. Compounds always contain the same proportion of elements"
http://hi.fi.tripod.com/timeline/.

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Around 400 B.C Democritus said that everything is made up of atoms/ small particles that can not be split or divided. To bring that all together he basically was the start to everything and the study of the atom. Aristotle was the one who brought the science into Democritus thought.
http://www.ph.surrey.ac.uk/partphys/chapter1/EarlyHistory.html

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In 1778 Lavoisier heated mercury oxide. He noticed that its weight decreased, but when he measured the weight of oxygen released, the two weights equaled the original mass of mercury oxide. He discovered the Conservation of Mass.
http://bit.ly/2eJX2ev

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Rutherford's Gold Foil Experiment - 1909

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Rutherford's Gold Foil Experiment - 1909

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Rutherford shot Alpha particles at a sheet of gold foil. Most when right through the foil yet some were deflected. He concluded that atoms form a net, most atoms can get through but some are deflected.

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This model is based off of wave quantum mechanics. It was discovered that electrons do not move in a definite path around the atom.   http://bit.ly/2fIcTuZ

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In 1913 Niels Bohr created the Bohr model to show how electrons orbit around a nucleus indefinitely.