How Everything Works
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Page 31 of 160 (1595 Questions and Answers)

301. How do the magnets that redirect the electron beam in the picture tube move it to the exact point that it's supposed to?
The electromagnets that steer the electron beam are very carefully designed and constructed so that they steer the beam very accurately. They are coils of wire that are built on a form and then glued together so that they cannot move. There are some adjustments made electronically inside the television set to make sure that the beam follows a very start path as it sweeps across the screen. When you adjust the horizontal and vertical sizes of the picture, you are adjusting the currents flowing through these electromagnets.

302. How does a magnet change the picture on a television—does this hurt the TV?
When you hold a magnet up to the front of a television, you are introducing an additional magnetic field in the system. This field exerts forces on the moving electrons inside the tube and they are deflected. The picture is distorted. With a black and white television, no harm is done because there is nothing to magnetize inside the picture tube. But color television picture tubes contain metal shadow masks that can become permanently magnetic. The picture remains distorted, even after you remove the magnet. To clear up the "damage", you would have to demagnetize the picture tube. Although this is not a particularly difficult task, it requires a demagnetizing coil and is best done by a professional repairperson. The bottom line is, don't play with magnets near a color television set.

303. How does the horizontal sync signal work?
The brightness information comes to the television as a steady stream. While the television knows that this information should control the brightness of adjacent spots on the screen, from left to right, it needs to be told when each horizontal line begins and when each vertical sweep begins. It knows that a new line is coming when the brightness information contains a "blacker-than-black" level. This level seems to say that the electron gun should not only stop sending electrons at the screen, it should send less than no electrons at the screen! Actually, this level is an instruction to the television's electronics, telling the television to bring its electron beam back to the left side of the screen to begin a new horizontal line. A long "blacker-than-black" level is an instruction to the television to begin a new vertical scan down the screen.

304. How does the picture get to the TV itself? How does a radio wave make a picture?
The television can reconstruct an image from a series of brightness measurements. It takes these brightness measurement and uses them to control the electron beam as it sweeps across the screen of the picture tube. It paints the picture one dot at a time and then starts over when it has finished. Thus all that the radio wave has to send to the television is a series of brightness measurements and some synchronization information (when to start a horizontal scan and when to start a vertical scan). It uses an AM technique to send the brightness measurements on a radio wave. The transmitter's power varies up and down to indicate brightness just as an AM radio transmitter's power varies up and down to indicate which way to push the speaker cone.

305. How does the picture tube know where to push the electrons onto the right areas/dots?
The television and picture tube simply scans the electron beam across the screen, one horizontal row after the next as it moves "slowly" down the screen. When it gets to the bottom of the screen, the picture tube brings the beam back to the top of the screen and starts over again. While the TV is scanning the beam across the set, it uses the signal from the television station to control the intensity of the electron beam and those the brightness of the spots on the screen. It also watches for sync information to know when to begin new horizontal lines and vertical sweeps.

306. How does the television camera record the picture?
Like the television picture tube, the camera generates a signal that indicates the brightnesses of individual spots one at a time. It first measures the brightness of light reaching it from the upper left hand spot, then the spot to its immediate right and so on horizontally across the field of view. It then moves down to a low horizontal line and repeats this sweep. It eventually records the light levels from the entire scene in front of it and begins again. It detects this light using an optical system that forms an image of the scene on a light sensitive surface. This surface may be part of an imaging vacuum tube (sort of a reverse picture tube), or it may be a semiconductor device that resembles a vast array of tiny photocells.

307. If black is a high current from the television's radio receiver and white is a low current, why do you get a bright spot when you increase the flow of electrons at that instant. Isn't white a bright spot?
Yes, white is created by a strong flow of electrons. There are two separate circuits here. The current from the receiver section of the television isn't what is sent through the electron gun. Instead, that current controls the electron gun. When a large current arrives at the electron gun (actually the grid) from the receiver, the flow of electrons toward the screen is pinched off and a dark spot is created. When a small current arrives from the receiver, the electron beam remains intense and a bright spot is created.

308. If you stand between the two satellites, would you have light on you?
When two satellites beam their radio waves at you, you are exposed to both of those waves. A normal antenna would not be able to distinguish between them and it would be hard to receive the transmissions of one and not the other. But with a satellite dish, you can easily select the transmissions of one and exclude those of the other. The satellite dish is directional, meaning that it focuses and collects radio waves from a particular direction while ignoring those from other directions. With a satellite dish aimed at a particular satellite, you can receive only transmissions from that satellite.

309. What is one doing when changing the brightness, contrast, and color adjustments on a television?
The brightness control determines the maximum strength of the electron beam and thus the peak brightness of the phosphors on the screen. The contrast control determines the extent to which the electron beam current changes between bright regions and dim regions on the screen. If the contrast is high, then even a less-than-white spot in the image may produce full beam current and full brightness in the phosphors and a more-than-black spot in the image may be cast as full black (no beam at all). If the contrast is low, then almost the entire screen will be illuminated by a medium electron beam and the image have no full black or full white. The color adjustments control the relative intensities of the red, green, and blue guns. Because of the way color is encoded in the television signal, the traditional controls are hue and tint, which involve mixtures of red, green, and blue. All these controls involve adjustments to the voltages and currents in the electron guns (cathodes), grids, and anodes of the picture tube.

310. Are microwaves distributed unevenly in the oven? Why do manufacturers claim that microwaves with turntables are more effective than microwaves without turntables?
As the microwaves bounce around the inside of the cooking chamber, they tend to interfere with one another. There are usually regions in which the waves that follow various paths almost cancel one another and regions in which the waves reinforce one another. These regions don't cook food equally well. If the microwaves are canceled in one region, cooking will be slow there. If the microwaves reinforce one another in another region, cooking will be fast there. If you simply leave food in one place and try to cook it in the microwaves, the cooking will be uneven. However, if the food is rotated continuously, these good and bad cooking regions will be blurred away so that the food will all cook at about the same speed.
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