The Black-and-White TV Signal

In a black-and-white TV, the screen is coated with white phosphor and the electron beam "paints" an image onto the screen by moving the electron beam across the phosphor a line at a time. To "paint" the entire screen, electronic circuits inside the TV use the magnetic coils to move the electron beam in a "raster scan" pattern across and down the screen. The beam paints one line across the screen from left to right. It then quickly flies back to the left side, moves down slightly and paints another horizontal line, and so on down the screen, like this:

In this figure, the blue lines represent lines that the electron beam is "painting" on the screen from left to right, while the red dashed lines represent the beam flying back to the left. When the beam reaches the right side of the bottom line, it has to move back to the upper left corner of the screen, as represented by the green line in the figure. When the beam is "painting," it is on, and when it is flying back, it is off so that it does not leave a trail on the screen. The term horizontal retrace is used to refer to the beam moving back to the left at the end of each line, while the term vertical retrace refers to its movement from bottom to top.

As the beam paints each line from left to right, the intensity of the beam is changed to create different shades of black, gray and white across the screen. Because the lines are spaced very closely together, your brain integrates them into a single image. A TV screen normally has about 480 lines visible from top to bottom. In the next section, you'll find out how the TV "paints" these lines on the screen.

TV Phosphors

A phosphor is any material that, when exposed to radiation, emits visible light. The radiation might be ultraviolet light or a beam of electrons. Any fluorescent color is really a phosphor -- fluorescent colors absorb invisible ultraviolet light and emit visible light at a characteristic color.

In a CRT, phosphor coats the inside of the screen. When the electron beam strikes the phosphor, it makes the screen glow. In a black-and-white screen, there is one phosphor that glows white when struck. In a color screen, there are three phosphors arranged as dots or stripes that emit red, green and blue light. There are also three electron beams to illuminate the three different colors together.

There are thousands of different phosphors that have been formulated. They are characterized by their emission color and the length of time emission lasts after they are excited.

TV Steering Coils

The following pictures give you three different views of a typical set of steering coils:

(Note the large black electrode hooked to the tube near the screen -- it is connected internally to the conductive coating.)

The steering coils are simply copper windings (see How Electromagnets Work for details on coils). These coils are able to create magnetic fields inside the tube, and the electron beam responds to the fields. One set of coils creates a magnetic field that moves the electron beam vertically, while another set moves the beam horizontally. By controlling the voltages in the coils, you can position the electron beam at any point on the screen.

Inside a CRT

As you can see in the below drawing, there's not a whole lot to a basic cathode ray tube.

There is a cathode and a pair (or more) of anodes. There is the phosphor-coated screen. There is a conductive coating inside the tube to soak up the electrons that pile up at the screen-end of the tube. However, in this diagram you can see no way to "steer" the beam -- the beam will always land in a tiny dot right in the center of the screen.

That's why, if you look inside any TV set, you will find that the tube is wrapped in coils of wires. On the next page, you'll get a good view of steering coils.

How Television Works

Almost all TVs in use today rely on a device known as the cathode ray tube, or CRT, to display their images. LCDs and plasma displays are sometimes seen, but they are still rare when compared to CRTs. It is even possible to make a television screen out of thousands of ordinary 60-watt light bulbs! You may have seen something like this at an outdoor event like a football game. Let's start with the CRT, however, because CRTs are the most common way of displaying images today.

The terms anode and cathode are used in electronics as synonyms for positive and negative terminals. For example, you could refer to the positive terminal of a battery as the anode and the negative terminal as the cathode.

In a cathode ray tube, the "cathode" is a heated filament (not unlike the filament in a normal light bulb). The heated filament is in a vacuum created inside a glass "tube." The "ray" is a stream of electrons that naturally pour off a heated cathode into the vacuum.

Electrons are negative. The anode is positive, so it attracts the electrons pouring off the cathode. In a TV's cathode ray tube, the stream of electrons is focused by a focusing anode into a tight beam and then accelerated by an accelerating anode. This tight, high-speed beam of electrons flies through the vacuum in the tube and hits the flat screen at the other end of the tube. This screen is coated with phosphor, which glows when struck by the beam.

The Cathode Ray Tube 1855 - 1896

1855

German inventor Heinrich Geissler develops mercury pump produces first good vacuum tubes. These tubes, as modified by Sir William Crookes, become the first to produce cathode rays, leading eventually to the discovery of the electron (and a bit farther down the road to television).

1858

Julius Plücker shows that cathode rays bend under the influence of a magnet suggesting that they are connected in some way; this leads in 1897 to discovery that cathode rays are composed of electrons.

1865

H. Sprengel improves the Geissler vacuum pump. Plücker uses Geissler tubes to show that at lower pressure, the Faraday dark space grows larger. He also finds that there is an extended glow on the walls of the tube and that this glow is affected by an external magnetic field.

1869

J.W. Hittorf finds that a solid body put in front of the cathode cuts off the glow from the walls of the tube. Establishes that "rays" from the cathode travel in straight lines.

1871

C.F. Varley is first to publish suggestion that cathode rays are composed of particles. Crookes proposes that they are molecules that have picked up a negative charge from the cathode and are repelled by it.

1874

George Johnstone Stoney estimates the charge of the then unknown electron to be about 10-20 coulomb, close to the modern value of 1.6021892 x 10-19 coulomb. (He used the Faraday constant (total electric charge per mole of univalent atoms) divided by Avogadro's Number.

James Clerk Maxwell had recognized this method soon after Faraday had published, but he did not accept the idea that electricity is composed of particles.) Stoney also proposes the name "electrine" for the unit of charge on a hydrogen ion. In 1891, he changes the name to "electron."

1876

Eugen Goldstein shows that the radiation in a vacuum tube produced when an electric current is forced through the tube starts at the cathode; Goldstein introduces the term cathode ray to describe the light emitted.

1881

Herman Ludwig von Helmholtz shows that the electrical charges in atoms are divided into definite integral portions, suggesting the idea that there is a smallest unit of electricity.

1883

Heinrich Hertz shows that cathode rays are not deflected by electrically charged metal plates, which would seem to indicate (incorrectly) that cathode rays cannot be charged particles.

1886

Eugen Goldstein observes that a cathode-ray tube produces, in addition to the cathode ray, radiation that travels in the opposite direction - away from the anode. These rays are called canal rays because of holes (canals) bored in the cathode; later these will be found to be ions that have had electrons stripped in producing the cathode ray.

1890

Arthur Schuster calculates the ratio of charge to mass of the particles making up cathode rays (today known as electrons) by measuring the magnetic deflection of cathode rays.

Joseph John (J.J.) Thomson first becomes interested in the discharge of electricity through a gas a low pressure, that is to say, cathode rays.

1892

Heinrich Hertz who has concluded (incorrectly) that cathode rays must be some form of wave, shows that the rays can penetrate thin foils of metal, which he takes to support the wave hypothesis.

Philipp von Lenard develops a cathode-ray tube with a thin aluminum window that permits the rays to escape, allowing the rays to be studied in the open air.

1894

J.J. Thomson announces that he has found that the velocity of cathode rays is much lower than that of light. He obtained the value of 1.9 x 107 cm/sec, as compared to the value 3.0 x 1010 cm/sec for light. This was in response to the prediction by Lenard that cathode rays would move with the velocity of light. However, by 1897, he distrusts this measurement.

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Special Note:

At this time there was great rivalry between German and British researchers. As concerning the nature of the cathode ray, the Germans tended to the explanation that cathode rays were a wave (like light). The British tended to believe that the cathode ray was particle.

As events unfold over the next few decades, both will be proven correct. In fact, J.J. Thomson will be awarded the Nobel Prize in Physics in 1906 for proving the electron is a particle. His son, George Paget Thomson, will be awarded the Nobel Prize in Physics in 1937 for showing that the electron is a wave.

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1895

Jean-Baptiste Perrin shows that cathode rays deposit a negative electric charge where they impact, refuting Hertz's concept of cathode rays as waves and showing they are particles.

1896

Pieter P. Zeeman discovers that spectral lines of gases placed in a magnetic field are split, a phenomenon call the Zeeman effect; Hendrik Antoon Lorentz explains this effect by assuming that light is produced by the motion of charged particles in the atom.

Lorentz uses Zeeman's observations of the behavior of light in magnetic field to calculate the charge to mass ratio of the electron in an atom, a year before electrons are discovered and 15 years before it is known that electron are constituents of atoms.

Source: http://dbhs.wvusd.k12.ca.us/Disc-of-Electron-History.html

The development of electronic television systems was based on the development of the cathode ray tube (CRT). A cathode ray tube aka picture tube, was found in all electronic television sets up until the invention of the less bulky LCD screens.

Definitions

• A cathode is a terminal or electrode at which electrons enter a system, such as an electrolytic cell or an electron tube.
• A cathode ray is a stream of electrons leaving the negative electrode, or cathode, in a discharge tube (an electron tube that contains gas or vapor at low pressure), or emitted by a heated filament in certain electron tubes.
• A vacuum tube is an electron tube consisting of a sealed glass or metal enclosure from which the air has been withdrawn.
• A cathode ray tube or CRT is a specialized vacuum tube in which images are produced when an electron beam strikes a phosphorescent surface.

Besides television sets, cathode ray tubes are used in computer monitors, automated teller machines, video game machines, video cameras, oscilloscopes and radar displays.

The first cathode ray tube scanning device was invented by the German scientist Karl Ferdinand Braun in 1897. Braun introduced a CRT with a fluorescent screen, known as the cathode ray oscilloscope. The screen would emit a visible light when struck by a beam of electrons.

In 1907, the Russian scientist Boris Rosing (who worked with Vladimir Zworykin) used a CRT in the receiver of a television system that at the camera end made use of mirror-drum scanning. Rosing transmitted crude geometrical patterns onto the television screen and was the first inventor to do so using a CRT.

Modern phosphor screens using multiple beams of electrons have allowed CRTs to display millions of colors.