CATHODE RAY TUBE
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
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.Even without consciously realizing it, most of us are already aware of what a cathode ray tube is.
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.
WORKING OF CRT
How the Cathode Ray Tube Works:
- A CRT is composed of several parts, all working together to for a coherent picture. A CRT contains a cathode, or a negative electronic terminal. It consists of a thick, heated wire that is contained within a glass tube. This tube is vacuum-sealed to eliminate resistance. The cathode emits a stream of electrons into the tube, which travel down its length and are attracted and accelerated by an anode, a positive terminal. After the electrons are sped up to extremely high speed, they strike a phosphorescent screen at the end of the tube, causing it to glow. However, it's important to note that, in addition to these parts, CRTs also require steering coils. These coils consist mainly of copper wire wrapped around the picture tube itself, and create magnetic fields that steer the electron beam to the desired pixel on the screen. These magnetic fields can be manipulated with extreme precision by changing the voltage of the wiring to focus the electron beam to any point on the screen.Color television screens use three electron beams at once. They use one each for red, blue and green. These three colors are used in the phosphor coating on of the picture tube. When each color is struck by its respective electron beam, it glows, creating a color picture. In black and white picture sets, there is a single phospor, which glows white when struck by the beam of electron radiation emitted from the picture tube. The human eye has been trained into assembling these small dots into a cohesive picture. Obviously, the more "dots per inch" the higher the "resolution", and the clearer the picture. The highest quality pictures may consist of hundreds of thousands of "dots per inch" creating an extremely clear picture.
The Future of CRT Televisions
- While CRTs certainly dominate the market now, what may happen in the future is a completely different story. Plasma televisions, once prohibitively expensive, have dropped considerably in price since their introduction. The newest possible CRT replacement are LCD televisions. They are providing a serious challenge to older CRT televisions. The advent of High Definition Television, or HDTV, has given the LCD and Plasma screens a growing sector of the market. CRT screens are limited as to the precision of the picture quality they offer, and as broadcast picture quality improves over time, CRTs may not be able to translate that picture quality onto the screen.
PICTURES:
The Braun Tube
This is one of five CRT oscilloscopes developed by Ferdinand Braun in 1897. Using a bellows, it took a strong man to evacuate the air. The successor to Sir William Crookes' vacuum tubes some 20 years earlier, these tubes used "cold" cathodes, which means they were unheated, but required a huge voltage. (Image courtesy of O'Neill's Electronic Museum)
Flat Panel Vs. CRT
The L66 was Eizo's first 18" desktop LCD display. Sitting next to its CRT counterpart, the flat panel not only took up less space, but used less energy and emitted less radiation. It was also glare proof. Formerly selling in the U.S. under the Nanao brand, Eizo is known for its high-quality monitors. (Image courtesy of EIZO Nanao Technologies Inc.)
CRT Front Projection
The first data and TV projectors used CRTs, and although mostly abandoned, they continue to provide the highest quality. In 2000, this home theater was built by a serious video enthusiast. (Images courtesy of Kal of CurtPalm.com)
CRT Rear Projection
Although big and bulky, the Pioneer Elite Pro-107 was perhaps the best CRT-based rear-projection TV ever made. Still working fine after 17 years, this unit was sold for a pittance in 2010. See rear-projection TV .
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