cathode ray experiment showed that

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Cathode Ray Experiment

What is cathode ray tube.

A cathode-ray tube (CRT) is a vacuum tube in which an electron beam, deflected by applied electric or magnetic fields, produces a trace on a fluorescent screen.

The function of the cathode ray tube is to convert an electrical signal into a visual display. Cathode rays or streams of electron particles are quite easy to produce, electrons orbit every atom and move from atom to atom as an electric current.

Table of Contents

Cathode ray tube, recommended videos.

• J.J.Thomson Experiment

Apparatus Setup

Procedure of the experiment.

• Frequently Asked Questions – FAQs

In a cathode ray tube, electrons are accelerated from one end of the tube to the other using an electric field. When the electrons hit the far end of the tube they give up all the energy they carry due to their speed and this is changed to other forms such as heat. A small amount of energy is transformed into X-rays.

The cathode ray tube (CRT), invented in 1897 by the German physicist Karl Ferdinand Braun, is an evacuated glass envelope containing an electron gun a source of electrons and a fluorescent light, usually with internal or external means to accelerate and redirect the electrons. Light is produced when electrons hit a fluorescent tube.

The electron beam is deflected and modulated in a manner that allows an image to appear on the projector. The picture may reflect electrical wave forms (oscilloscope), photographs (television, computer monitor), echoes of radar-detected aircraft, and so on. The single electron beam can be processed to show movable images in natural colours.

J. J. Thomson Experiment – The Discovery of Electron

The Cathode ray experiment was a result of English physicists named J. J. Thomson experimenting with cathode ray tubes. During his experiment he discovered electrons and it is one of the most important discoveries in the history of physics. He was even awarded a Nobel Prize in physics for this discovery and his work on the conduction of electricity in gases.

However, talking about the experiment, J. J. Thomson took a tube made of glass containing two pieces of metal as an electrode. The air inside the chamber was subjected to high voltage and electricity flowing through the air from the negative electrode to the positive electrode.

J. J. Thomson designed a glass tube that was partly evacuated, i.e. all the air had been drained out of the building. He then applied a high electric voltage at either end of the tube between two electrodes. He observed a particle stream (ray) coming out of the negatively charged electrode (cathode) to the positively charged electrode (anode). This ray is called a cathode ray and is called a cathode ray tube for the entire construction.

The experiment Cathode Ray Tube (CRT) conducted by J. J. Thomson, is one of the most well-known physical experiments that led to electron discovery . In addition, the experiment could describe characteristic properties, in essence, its affinity to positive charge, and its charge to mass ratio. This paper describes how J is simulated. J. Thomson experimented with Cathode Ray Tube.

The major contribution of this work is the new approach to modelling this experiment, using the equations of physical laws to describe the electrons’ motion with a great deal of accuracy and precision. The user can manipulate and record the movement of the electrons by assigning various values to the experimental parameters.

A Diagram of JJ.Thomson Cathode Ray Tube Experiment showing Electron Beam – A cathode-ray tube (CRT) is a large, sealed glass tube.

The apparatus of the experiment incorporated a tube made of glass containing two pieces of metals at the opposite ends which acted as an electrode. The two metal pieces were connected with an external voltage. The pressure of the gas inside the tube was lowered by evacuating the air.

• Apparatus is set up by providing a high voltage source and evacuating the air to maintain the low pressure inside the tube.
• High voltage is passed to the two metal pieces to ionize the air and make it a conductor of electricity.
• The electricity starts flowing as the circuit was complete.
• To identify the constituents of the ray produced by applying a high voltage to the tube, the dipole was set up as an add-on in the experiment.
• The positive pole and negative pole were kept on either side of the discharge ray.
• When the dipoles were applied, the ray was repelled by the negative pole and it was deflected towards the positive pole.
• This was further confirmed by placing the phosphorescent substance at the end of the discharge ray. It glows when hit by a discharge ray. By carefully observing the places where fluorescence was observed, it was noted that the deflections were on the positive side. So the constituents of the discharge tube were negatively charged.

After completing the experiment J.J. Thomson concluded that rays were and are basically negatively charged particles present or moving around in a set of a positive charge. This theory further helped physicists in understanding the structure of an atom . And the significant observation that he made was that the characteristics of cathode rays or electrons did not depend on the material of electrodes or the nature of the gas present in the cathode ray tube. All in all, from all this we learn that the electrons are in fact the basic constituent of all the atoms.

Most of the mass of the atom and all of its positive charge are contained in a small nucleus, called a nucleus. The particle which is positively charged is called a proton. The greater part of an atom’s volume is empty space.

The number of electrons that are dispersed outside the nucleus is the same as the number of positively charged protons in the nucleus. This explains the electrical neutrality of an atom as a whole.

Uses of Cathode Ray Tube

• Used as a most popular television (TV) display.
• X-rays are produced when fast-moving cathode rays are stopped suddenly.
• The screen of a cathode ray oscilloscope, and the monitor of a computer, are coated with fluorescent substances. When the cathode rays fall off the screen pictures are visible on the screen.

Frequently Asked Questions – FAQs

What are cathode ray tubes made of.

The cathode, or the emitter of electrons, is made of a caesium alloy. For many electronic vacuum tube systems, Cesium is used as a cathode, as it releases electrons readily when heated or hit by light.

Where can you find a cathode ray tube?

Cathode rays are streams of electrons observed in vacuum tubes (also called an electron beam or an e-beam). If an evacuated glass tube is fitted with two electrodes and a voltage is applied, it is observed that the glass opposite the negative electrode glows from the electrons emitted from the cathode.

How did JJ Thomson find the electron?

In the year 1897 J.J. Thomson invented the electron by playing with a tube that was Crookes, or cathode ray. He had shown that the cathode rays were charged negatively. Thomson realized that the accepted model of an atom did not account for the particles charged negatively or positively.

What are the properties of cathode rays?

They are formed in an evacuated tube via the negative electrode, or cathode, and move toward the anode. They journey straight and cast sharp shadows. They’ve got strength, and they can do the job. Electric and magnetic fields block them, and they have a negative charge.

What do you mean by cathode?

A device’s anode is the terminal on which current flows in from outside. A device’s cathode is the terminal from which current flows out. By present, we mean the traditional positive moment. Because electrons are charged negatively, positive current flowing in is the same as outflowing electrons.

Who discovered the cathode rays?

Studies of cathode-ray began in 1854 when the vacuum tube was improved by Heinrich Geissler, a glassblower and technical assistant to the German physicist Julius Plücker. In 1858, Plücker discovered cathode rays by sealing two electrodes inside the tube, evacuating the air and forcing it between the electrode’s electric current.

Which gas is used in the cathode ray experiment?

For better results in a cathode tube experiment, an evacuated (low pressure) tube is filled with hydrogen gas that is the lightest gas (maybe the lightest element) on ionization, giving the maximum charge value to the mass ratio (e / m ratio = 1.76 x 10 ^ 11 coulombs per kg).

What is the Colour of the cathode ray?

Cathode-ray tube (CRT), a vacuum tube which produces images when electron beams strike its phosphorescent surface. CRTs can be monochrome (using one electron gun) or coloured (using usually three electron guns to produce red, green, and blue images that render a multicoloured image when combined).

How cathode rays are formed?

Cathode rays come from the cathode because the cathode is charged negatively. So those rays strike and ionize the gas sample inside the container. The electrons that were ejected from gas ionization travel to the anode. These rays are electrons that are actually produced from the gas ionization inside the tube.

What are cathode rays made of?

Thomson showed that cathode rays were composed of a negatively charged particle, previously unknown, which was later named electron. To render an image on a screen, Cathode ray tubes (CRTs) use a focused beam of electrons deflected by electrical or magnetic fields.

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Cathode Ray Experiment

Cathode Ray Experiment , also known as the Crookes tube experiment , is a historically significant experiment in the field of physics that helped scientists understand the nature of electrons. English scientist Sir J.J. Thomson performed an experiment using a Cathode Ray Tube, which led to the discovery of an electron.

In this article, we will discuss this significant experiment, including details of the Cathode Ray Tube, the procedure of the experiment, and J.J. Thomson's observations, which led to one of the greatest discoveries in the field of science.

Table of Content

• What is the Cathode Ray Experiment?

What is Cathode Ray Tube (CRT)?

• Experiment Setup

Applications of Cathode Ray Experiment

• Limitations of the Cathode Ray Experiment

What is Cathode Ray Experiment?

Cathode Ray Experiment, also known as the Cathode Ray Tube (CRT) Experiment, is a fundamental experiment in the history of physics that played a crucial role in understanding the nature of electrons and contributed to the development of modern electronics and television technology.

The experiment was first conducted by Sir William Crookes in the 1870s and later improved upon by scientists like J.J. Thomson in the late 19 th and early 20 th centuries.

Who is J.J. Thomson?

Joseph John Thomson, often called J.J. Thomson, was a British physicist celebrated for winning the Nobel Prize in Physics in 1906 for his research on how electricity moves through gases. His notable achievement was the discovery of the electron during the Cathode Ray Experiment.

A Cathode Ray Tube (CRT) is a special glass tube that played a big part in J.J. Thomson's important experiment. This clever device helped scientists understand tiny particles that make up atoms.

Structure of CRT

CRT has a simple structure. It's a sealed glass tube with two electrodes at each end – one is called the cathode (negative), and the other is the anode (positive). When these electrodes are connected to power, they create an electric field inside the tube. The tube is made empty, like a vacuum, so there's no air inside.

The vacuum is essential because it lets cathode rays move in a straight line from the cathode to the anode without any interference from air. This controlled setup helps scientists study the behavior of cathode rays in different situations. The CRT is a key tool that led to important discoveries about the tiniest building blocks of matter.

Cathode Ray Experiment Setup

Below is the detailed setup for the Cathode Ray Tube Experiment with the elements used along with the diagram:

• Cathode Ray Tube (CRT): A sealed glass tube with a cathode and anode at either end.
• Cathode: A negatively charged electrode inside the CRT.
• Anode: A positively charged electrode inside the CRT.
• High Voltage Generator: A power supply capable of providing a high voltage between the cathode and anode.
• Vacuum Pump: A pump to evacuate air from the CRT to create a low-pressure environment.
• Discharge Tube: The entire CRT assembly including the cathode, anode, and vacuum space.
• Perforated Anode Disk: Placed at the anode end to allow some cathode rays to pass through.

Procedure of Experiment

Below is the procedure steps for the experiment with the perspective of the JJ Thomson:

• JJ Thomson created a sealed cathode ray tube with minimal air inside.
• Connected the tube to a power source, causing electrons (cathode rays) to shoot out.
• Observed electrons moving in straight lines inside the vacuum of the tube.
• Introduced an electric field by adjusting the power, causing electrons to change their path.
• Experimented with magnets, observing electrons being affected and swerving in response.
• Adjusted power settings to observe changes in electron movement, establishing consistent patterns.
• Systematically recorded electron behavior in various situations.
• Determined the charge-to-size ratio of electrons, making a significant discovery.
• Concluded that cathode rays were composed of tiny particles known as electrons.
• Thomson's discovery revolutionized understanding of the microscopic world's building blocks.

Observation of Cathode Ray Experiment

In the Cathode Ray Experiment, J.J. Thomson made a ground breaking observation i.e., when cathode rays encountered electric and magnetic fields, they exhibited intriguing behavior. Thomson noticed their deflection, and the direction of this deflection pointed to a negative charge. This pivotal observation led Thomson to the groundbreaking conclusion that cathode rays were composed of negatively charged particles, now recognized as electrons.

Conclusion of Cathode Ray Experiment

Cathode Ray Experiment marked a revolutionary moment in the realm of science. J.J. Thomson's demonstration of cathode ray deflection and the identification of these rays as negatively charged particles conclusively affirmed the existence of subatomic particles. This groundbreaking experiment transformed our comprehension of atomic structure, shattering the notion that atoms were indivisible. Instead, Thomson's work revealed the presence of smaller components within atoms. This pivotal episode in the history of physics not only altered fundamental perspectives but also laid the foundation for subsequent advancements in the field.

The Cathode Ray Experiment, conducted by Sir J.J. Thomson in 1897, led to several significant applications and advancements in various fields:

• Discovery of the Electron: The most direct outcome of the Cathode Ray Experiment was the discovery of the electron, a fundamental component of atoms. This discovery was pivotal in the development of atomic theory and quantum physics.
• Television and Computer Monitors: The technology behind cathode ray tubes (CRTs) was essential in the development of early television and computer monitors. These devices used electron beams, controlled and focused by magnetic or electric fields, to illuminate phosphors on the screen, creating images.
• Medical Imaging: Cathode ray technology found applications in medical imaging, particularly in early forms of X-ray machines and later in more advanced imaging technologies.
• Electron Microscopy: The principles discovered in the Cathode Ray Experiment were integral to the development of electron microscopy, which uses a beam of electrons to create an image of a specimen. This technology allows for much higher resolution than traditional light microscopy.

Limitations of Cathode Ray Experiment

The Cathode Ray Experiment, while groundbreaking in its time, had several limitations:

• Lack of Precise Measurement Tools: At the time of Thomson's experiments, the precision and accuracy of measurement tools were limited. This meant that the measurements of the charge-to-mass ratio of electrons were not as accurate as what can be achieved with modern equipment.
• Incomplete Understanding of Subatomic Particles: Thomson's experiment was conducted at a time when the structure of the atom was not fully understood. This meant that while the experiment led to the discovery of the electron, it did not provide a complete picture of subatomic particles and their interactions.
• Limited Control over Experimental Conditions: The vacuum technology and methods to control the electric and magnetic fields in Thomson's time were rudimentary compared to today's standards. This limited the ability to control experimental conditions precisely.
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Cathode Ray Experiment - FAQs

J.J. Thomson, whose full name is Joseph John Thomson, was a British physicist born on December 18, 1856, in Cheetham Hill, Manchester, England, and he passed away on August 30, 1940. He is best known for his discovery of the electron, a fundamental subatomic particle.

What are Cathode Rays?

Cathode rays are streams of electrons observed in a vacuum when a high voltage is applied between electrodes in a cathode ray tube (CRT). These rays were first discovered and studied by J.J. Thomson in the late 19th century.

What was the Cathode Ray Experiment?

The cathode ray experiment, conducted by J.J. Thomson in the late 19th century, was a series of experiments that led to the discovery of electrons and provided crucial insights into the nature of subatomic particles.

What are Two Conclusions of the Cathode Ray Experiment?

Two conclusion of Cathode Ray Experiment are: Cathode rays are streams of negatively charged particles (electrons). These particles are fundamental components of all atoms.

Why did J.J. Thomson Experimented with Cathode?

J.J. Thomson experimented with cathode rays to investigate their nature and to understand the internal structure of atoms.

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Cathode Ray Experiment

The electric experiment by j.j. thomson.

J. J. Thomson was one of the great scientists of the 19th century; his inspired and innovative cathode ray experiment greatly contributed to our understanding of the modern world.

This article is a part of the guide:

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• 1 Physics Experiments
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• 4 Cathode Ray Experiment

Like most scientists of that era, he inspired generations of later physicists, from Einstein to Hawking .

His better-known research proved the existence of negatively charged particles, later called electrons, and earned him a deserved Nobel Prize for physics. This research led to further experiments by Bohr and Rutherford, leading to an understanding of the structure of the atom.

What is a Cathode Ray Tube?

Even without consciously realizing it, most of us are already aware of what a cathode ray tube is.

Look at any glowing neon sign or any ‘old-fashioned’ television set, and you are looking at the modern descendants of the cathode ray tube.

Physicists in the 19th century found out that if they constructed a glass tube with wires inserted in both ends, and pumped out as much of the air as they could, an electric charge passed across the tube from the wires would create a fluorescent glow. This cathode ray also became known as an ‘electron gun’.

Later and improved cathode ray experiments found that certain types of glass produced a fluorescent glow at the positive end of the tube. William Crookes discovered that a tube coated in a fluorescing material at the positive end, would produce a focused ‘dot’ when rays from the electron gun hit it.

With more experimentation, researchers found that the ‘cathode rays’ emitted from the cathode could not move around solid objects and so traveled in straight lines, a property of waves. However, other researchers, notably Crookes, argued that the focused nature of the beam meant that they had to be particles.

Physicists knew that the ray carried a negative charge but were not sure whether the charge could be separated from the ray. They debated whether the rays were waves or particles, as they seemed to exhibit some of the properties of both. In response, J. J. Thomson constructed some elegant experiments to find a definitive and comprehensive answer about the nature of cathode rays.

Thomson’s First Cathode Ray Experiment

Thomson had an inkling that the ‘rays’ emitted from the electron gun were inseparable from the latent charge, and decided to try and prove this by using a magnetic field.

His first experiment was to build a cathode ray tube with a metal cylinder on the end. This cylinder had two slits in it, leading to electrometers, which could measure small electric charges.

He found that by applying a magnetic field across the tube, there was no activity recorded by the electrometers and so the charge had been bent away by the magnet. This proved that the negative charge and the ray were inseparable and intertwined.

Thomson's Cathode Ray Second Experiment

Like all great scientists, he did not stop there, and developed the second stage of the experiment, to prove that the rays carried a negative charge. To prove this hypothesis, he attempted to deflect them with an electric field.

Earlier experiments had failed to back this up, but Thomson thought that the vacuum in the tube was not good enough, and found ways to improve greatly the quality.

For this, he constructed a slightly different cathode ray tube, with a fluorescent coating at one end and a near perfect vacuum. Halfway down the tube were two electric plates, producing a positive anode and a negative cathode, which he hoped would deflect the rays.

As he expected, the rays were deflected by the electric charge, proving beyond doubt that the rays were made up of charged particles carrying a negative charge. This result was a major discovery in itself, but Thomson resolved to understand more about the nature of these particles.

Thomson's Third Experiment

The third experiment was a brilliant piece of scientific deduction and shows how a series of experiments can gradually uncover truths.

Many great scientific discoveries involve performing a series of interconnected experiments, gradually accumulating data and proving a hypothesis .

He decided to try to work out the nature of the particles. They were too small to have their mass or charge calculated directly, but he attempted to deduce this from how much the particles were bent by electrical currents, of varying strengths.

Thomson found out that the charge to mass ratio was so large that the particles either carried a huge charge, or were a thousand times smaller than a hydrogen ion. He decided upon the latter and came up with the idea that the cathode rays were made of particles that emanated from within the atoms themselves, a very bold and innovative idea.

Later Developments

Thomson came up with the initial idea for the structure of the atom, postulating that it consisted of these negatively charged particles swimming in a sea of positive charge. His pupil, Rutherford, developed the idea and came up with the theory that the atom consisted of a positively charged nucleus surrounded by orbiting tiny negative particles, which he called electrons.

Quantum physics has shown things to be a little more complex than this but all quantum physicists owe their legacy to Thomson. Although atoms were known about, as apparently indivisible elementary particles, he was the first to postulate that they had a complicated internal structure.

Thomson's greatest gift to physics was not his experiments, but the next generation of great scientists who studied under him, including Rutherford, Oppenheimer and Aston. These great minds were inspired by him, marking him out as one of the grandfathers of modern physics.

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Martyn Shuttleworth (Sep 22, 2008). Cathode Ray Experiment. Retrieved Dec 08, 2024 from Explorable.com: https://explorable.com/cathode-ray-experiment

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J. J. Thomson

The Discovery of the Electron (J. J. Thomson)

In 1897, J. J. Thomson found that the cathode rays can be deflected by an electric field, as shown below. By balancing the effect of a magnetic field on a cathode-ray beam with an electric field, Thomson was able to show that cathode "rays" are actually composed of particles. This experiment also provided an estimate of the ratio of the charge to the mass of these particles.

In the SI system, charge is measured in units of coulombs. By definition, one coulomb is the charge carried by a current of one ampere that flows for one second: 1 C = 1 amp-s. When Thomson's data are converted to SI units, the charge-to-mass ratio of the particles in the cathode-ray beam is about 10 8 coulomb per gram.

Thomson found the same charge-to-mass ratio regardless of the metal used to make the cathode and the anode. He also found the same charge-to-mass ratio regardless of the gas used to fill the tube. He therefore concluded that the particles given off by the cathode in this experiment are a universal component of matter. Although Thomson called these particles corpuscles , the name electron, which had been proposed by George Stoney several years earlier for the fundamental unit of negative electricity, was soon accepted.

The Raisin Pudding Model of the Atom (J. J. Thomson)

Thomson recognized one of the consequences of the discovery of the electron. Because matter is electrically neutral, there must be a positively charged particle that balances the negative charge on the electrons in an atom. Furthermore, if electrons are very much lighter than atoms, these positively charged particles must carry the mass of the atom. Thomson therefore suggested that atoms are spheres of positive charge in which light, negatively charged electrons are embedded, much as raisins might be embedded in the surface of a pudding.  At the time Thomson proposed this model, evidence for the existence of positively charged particles was available from cathode-ray tube experiments.

JJ Thompson’s Discovery of Electron: Cathode Ray Tube Experiment Explained

JJ Thomson discovered the electron in 1897 and there are tons of videos about it.  However, most videos miss what JJ Thomson himself said was the motivating factor: a debate about how cathode rays move.  Want to know not only how but why electrons were discovered?

Table of Contents

The start of jj thomson, how thomson discovered electrons: trials and errors, thomson’s conclusion.

A short history of Thomson: Joseph John Thomson, JJ on papers, to friends, and even to his own son [1] , was born in Lancashire, England to a middle class bookseller.  When he was 14 years old, Thomson planned to get an apprenticeship to a locomotive engineer but it had a long waiting list, so, he applied to and was accepted at that very young age to Owen’s college. 

Thompson later recalled that, “the authorities at Owens College thought my admission was such a scandal – I expect they feared that students would soon be coming in perambulators  – that they passed regulations raising the minimum age for admission, so that such a catastrophe should not happen again.

[2] ”  While in school, his father died, and his family didn’t have enough money for the apprenticeship.  Instead, he relied on scholarships at universities – ironically leading him to much greater fame in academia. In 1884, at the tender age of 28, Thomson applied to be the head of the Cavendish Research Institute. 

He mostly applied as a lark and was as surprised as anyone to actually get the position!  “I felt like a fisherman who…had casually cast a line in an unlikely spot and hooked a fish much too heavy for him to land. [3] ”  Suddenly, he had incredible resources, stability and ability to research whatever he wished. 

He ended up having an unerring ability to pinpoint interesting phenomena for himself and for others. In fact, a full eight of his research assistants and his son eventually earned Nobel Prizes, but, of course, like Thomson’s own Nobel Prize, that was in the future.

Why did J. J. Thomson discover the electron in 1897?  Well, according to Thomson: “the discovery of the electron began with an attempt to explain the discrepancy between the behavior of cathode rays under magnetic and electric forces [4] .”  What did he mean by that? 

Well, a cathode ray, or a ray in a vacuum tube that emanates from the negative electrode, can be easily moved with a magnet.  This gave a charismatic English chemist named William Crookes the crazy idea that the cathode ray was made of charged particles in 1879! 

However, 5 years later, a young German scientist named Heinrich Hertz found that he could not get the beam to move with parallel plates, or with an electric field.  Hertz decided that Crookes was wrong, if the cathode ray was made of charged particles then it should be attracted to a positive plate and repulsed from a negative plate. 

Ergo, it couldn’t be particles, and Hertz decided it was probably some new kind of electromagnetic wave, like a new kind of ultraviolet light.  Further, in 1892, Hertz accidentally discovered that cathode rays could tunnel through thin pieces of metal, which seemed like further proof that Crookes was so very wrong.

Then, in December of 1895, a French physicist named Jean Perrin used a magnet to direct a cathode ray into and out of an electroscope (called a Faraday cylinder) and measured its charge.  Perrin wrote, “the Faraday cylinder became negatively charged when the cathode rays entered it, and only when they entered it; the cathode rays are thus charged with negative electricity .

[5] ”  This is why JJ Thomson was so confused, he felt that Perrin had, “conclusive evidence that the rays carried a charge of negative electricity” except that, “Hertz found that when they were exposed to an electric force they were not deflected at all.”  What was going on?

In 1896, Thomson wondered if there might have been something wrong with Hertz’s experiment with the two plates.  Thomson knew that the cathode ray tubes that they had only work if there is a little air in the tube and the amount of air needed depended on the shape of the terminals.

Thomson wondered if the air affected the results.  Through trial and error, Thomson found he could get a “stronger” beam by shooting it through a positive anode with a hole in it.  With this system he could evacuate the tube to a much higher degree and, if the vacuum was good enough, the cathode ray was moved by electrically charged plates, “just as negatively electrified particles would be.

[6] ” (If you are wondering why the air affected it, the air became ionized in the high electric field and became conductive.  The conductive air then acted like a Faraday cage shielding the beam from the electric field.)

As stated before, Heinrich Hertz also found that cathode rays could travel through thin solids.  How could a particle do that?  Thomson thought that maybe particles could go through a solid if they were moving really, really fast.  But how to determine how fast a ray was moving? 

Thomson made an electromagnetic gauntlet.  First, Thomson put a magnet near the ray to deflect the ray one-way and plates with electric charge to deflect the ray the other way.  He then added or reduced the charge on the plates so that the forces were balanced and the ray went in a straight line. 

He knew that the force from the magnet depended on the charge of the particle, its speed and the magnetic field (given the letter B).  He also knew that the electric force from the plates only depended on the charge of the particle and the Electric field.  Since these forces were balanced, Thomson could determine the speed of the particles from the ratio of the two fields. 

Thomson found speeds as big as 60,000 miles per second or almost one third of the speed of light.  Thomson recalled, “In all cases when the cathode rays are produced their velocity is much greater than the velocity of any other moving body with which we are acquainted. [7] ”  

Thomson then did something even more ingenious; he removed the magnetic field.  Now, he had a beam of particles moving at a known speed with a single force on them.  They would fall, as Thomson said, “like a bullet projected horizontally with a velocity v and falling under gravity [8] ”.  

Note that these “bullets” are falling because of the force between their charge and the charges on the electric plates as gravity is too small on such light objects to be influential.  By measuring the distance the bullets went he could determine the time they were in the tube and by the distance they “fell” Thomson could determine their acceleration. 

Using F=ma Thomson determine the ratio of the charge on the particle to the mass (or e/m).  He found some very interesting results.  First, no matter what variables he changed in the experiment, the value of e/m was constant.  “We may… use any kind of substance we please for the electrodes and fill the tube with gas of any kind and yet the value of e/m will remain the same.

[9] ”  This was a revolutionary result.  Thomson concluded that everything contained these tiny little things that he called corpuscles (and we call electrons).  He also deduced that the “corpuscles” in one item are exactly the same as the “corpuscles” in another.  So, for example, an oxygen molecule contains the same kind of electrons as a piece of gold!  Atoms are the building blocks of matter but inside the atoms (called subatomic) are these tiny electrons that are the same for everything .

The other result he found was that the value of e/m was gigantic, 1,700 times bigger than the value for a charged Hydrogen atom, the object with the largest value of e/m before this experiment.   So, either the “corpuscle” had a ridiculously large charge or it was, well, ridiculously small.   

A student of Thomson’s named C. T. R. Wilson had experimented with slowly falling water droplets that found that the charge on the corpuscles were, to the accuracy of the experiment, the same as the charge on a charged Hydrogen atom!   Thomson concluded that his corpuscles were just very, very, tiny, about 1,700 times smaller then the Hydrogen atom [1] .  These experiments lead Thomson to come to some interesting conclusions:

• Electrons are in everything and are well over a thousand times smaller then even the smallest atom. 
• Benjamin Franklin thought positive objects had too much “electrical fire” and negative had too little.  Really, positive objects have too few electrons and negative have too many.  Oops.
• Although since Franklin, people thought current flowed from the positive side to the negative, really, the electrons are flowing the other way.  When a person talks about “current” that flows from positive to negative they are talking about something that is not real!   True “electric current” flows from negative to positive and is the real way the electrons move. [although by the time that people believed J.J. Thomson, it was too late to change our electronics, so people just decided to stick with “current” going the wrong way!]
• Since electrons are tiny and in everything but most things have a neutral charge, and because solid objects are solid, the electrons must be swimming in a sea or soup of positive charges.  Like raisons in a raison cookie.

The first three are still considered correct over one hundred years later.  The forth theory, the “plum pudding model” named after a truly English “desert” with raisins in sweet bread that the English torture people with during Christmas, was proposed by Thomson in 1904. 

In 1908, a former student of Thomson’snamed Ernest Rutherford was experimenting with radiation, and inadvertently demolished the “plum pudding model” in the process.  However, before I can get into Rutherford’s gold foil experiment, I first want to talk about what was going on in France concurrent to Thomson’s experiments. 

This is a story of how a new mother working mostly in a converted shed discovered and named the radium that Rutherford was experimenting with.  That woman’s name was Marie Sklodowska Curie, and that story is next time on the Lightning Tamers.

[1] the current number is 1,836 but Thomson got pretty close

[1] p 14 “Flash of the Cathode Rays: A History of JJ Thomson’s Electron” Dahl

[2] Thompson, J.J. Recollections and Reflections p. 2 Referred to in Davis & Falconer JJ. Thompson and the Discovery of the Electron 2002 p. 3

[3] Thomson, Joseph John Recollections and Reflections p. 98 quoted in Davis, E.A & Falconer, Isabel JJ Thomson and the Discovery of the Electron 2002 p. 35

[4]   Thomson, JJ Recollections and Reflections p. 332-3

[5] “New Experiments on the Kathode Rays” Jean Perrin, December 30, 1985 translation appeared in Nature, Volume 53, p 298-9, January 30, 1896

[6] Nobel Prize speech?

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Discovering the electron: JJ Thomson and the Cathode Ray Tube

Concept Introduction: JJ Thomson and the Discovery of the Electron

The discovery of the electron was an important step for physics, chemistry, and all fields of science. JJ Thomson made the discovery using the cathode ray tube. Learn all about the discovery, the importance of the discovery, and JJ Thomson in this tutorial article.

Further Reading on the Electron

Electron Orbital and Electron Shapes Writing Electron Configurations Electron Shells What are valence electrons? Electron Affinity Aufbau Principle

Who was JJ Thomson?

JJ Thomson was an English physicist who is credited with discovery of the electron in 1897. Thompson was born in December 1856 in Manchester, England and was educated at the University of Manchester and then the University of Cambridge, graduating with a degree in mathematics. Thompson made the switch to physics a few years later and began studying the properties of cathode rays. In addition to this work, Thomson also performed the first-ever mass spectrometr y experiments, discovered the first isotope and made important contributions both to the understanding of positively charged particles and electrical conductivity in gases.

Thomson did most of this work while leading the famed Cavendish Laboratory at the University of Cambridge. Although he received the Nobel Prize in physics and not chemistry, Thomson’s contributions to the field of chemistry are numerous. For instance, the discovery of the electron was vital to the development of chemistry today, and it was the first subatomic particle to be discovered. The proton and the neutron would soon follow as the full structure of the atom was discovered.

What is a cathode ray tube and why was it important?

Prior to the discovery of the electron, several scientists suggested that atoms consisted of smaller pieces. Yet until Thomson, no one had determined what these might be. Cathode rays played a critical role in unlocking this mystery. Thomson determined that charged particles much lighter than atoms , particles that we now call electrons made up cathode rays. Cathode rays form when electrons emit from one electrode and travel to another. The transfer occurs due to the application of a voltage in vacuum. Thomson also determined the mass to charge ratio of the electron using a cathode ray tube, another significant discovery.

How did Thomson make these discoveries?

Thomson was able to deflect the cathode ray towards a positively charged plate deduce that the particles in the beam were negatively charged. Then Thomson measured how much various strengths of magnetic fields bent the particles. Using this information Thomson determined the mass to charge ratio of an electron. These were the two critical pieces of information that lead to the discovery of the electron. Thomson was now able to determine that the particles in question were much smaller than atoms, but still highly charged. He finally proved atoms consisted of smaller components, something scientists puzzled over for a long time. Thomson called the particle “corpuscles” , not an electron. George Francis Fitzgerald suggested the name electron.

Why was the discovery of the electron important?

The discovery of the electron was the first step in a long journey towards a better understanding of the atom and chemical bonding. Although Thomson didn’t know it, the electron would turn out to be one of the most important particles in chemistry. We now know the electron forms the basis of all chemical bonds. In turn chemical bonds are essential to the reactions taking place around us every day. Thomson’s work provided the foundation for the work done by many other important scientists such as Einstein, Schrodinger, and Feynman.

Interesting Facts about JJ Thomson

Not only did Thomson receive the Nobel Prize in physics in 1906 , but his son Sir George Paget Thomson won the prize in 1937. A year earlier, in 1936, Thomson wrote an autobiography called “Recollections and Reflections”. He died in 1940, buried near Isaac Newton and Charles Darwin. JJ stands for “Joseph John”. Strangely, another author with the name JJ Thomson wrote a book with the same name in 1975. Thomson had many famous students, including Ernest Rutherford.

Discovery of the Electron: Further Reading

Protons, Neutrons & Electrons Discovering the nucleus with gold foil Millikan oil drop experiment Phase Diagrams