March 3, 1995
One Minute Papers - Questions and Answers
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.
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.
How does a magnet manufacturer make the magnet -- how does it vary in strength?
The manufacturer assembles the magnet from hard magnetic materials. These materials are intrinsically magnetic (ferromagnetic) so that they have tiny magnetic domains inside. They are hard, meaning that these domains have great difficulty changing their magnetic orientations. As the final processing step, the finished magnets are exposed to an extremely strong magnetic field; so strong that it flips all of the domains into the desired direction. The domains become trapped in this new orientation and the magnet becomes permanently magnetized. Unless it is exposed to other very strong fields or excessive heat or shock, it will remain permanently magnetized indefinitely.
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.
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.
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.
Was the repair man able to fix the TV you permanently magnetized? It seems to me that the TV would be ruined.
Yes, the repair man was able to demagnetize it with surprising ease. It was fun to watch (very colorful; better than I could ever do). Any magnet can be demagnetized with the right tools. It just requires a strong fluctuating magnetic field that can scramble all of the magnetic domains and leave them randomly arranged.
How do notebook computer monitors work?
These displays use liquid crystals, liquids that contain long chain or disk-shaped molecules. These molecules can be aligned by external electric fields or by their own interactions with one another to form very orderly arrays; hence the name "liquid crystals". The extent to which these molecules are oriented determines their optical properties. A notebook computer uses electric fields to orient or disorient the liquid crystals and control their optical properties. With some help from other optical devices, the notebook computer can make these liquid crystals block or unblock light to appear dark or light. Adding color filters allows them to produce colored images on their screens.