Author: Lou Bloomfield

How long will the magnetic data last on a VCR tape before it becomes no longer u…

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How long will the magnetic data last on a VCR tape before it becomes no longer useable as read data? — KR, Urbana, IL

As long as the tape is kept cool and dry, its magnetization should remain stable for years. However, there is the problem of magnetic imprinting from one layer of tape to the adjacent layers on a spool. With time, one layer transfers some of its magnetization to those adjacent layers. In a videotape, this imprinting leads to a gradual appearance of noise in the video images. As long as you’re willing to tolerate a little video “snow,” this imprinting shouldn’t be too much of a problem. You can reduce its severity by occasionally winding and rewinding the tapes. But I don’t see any real reason why a tape won’t be reasonably useable for decades.

Can I soften small quantities of tap water by merely adding table salt to it? An…

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Can I soften small quantities of tap water by merely adding table salt to it? Any idea how much salt to add for tape water that is medium to very hard? I want enough to use in a steam iron regularly? — HD, Kintnersville, PA

There are two issues here. First, hard water is water that contains dissolved calcium, magnesium, and iron salts. The metal ions in these salts interfere with soaps and detergents, causing soaps to form soap scum and preventing detergents from effectively carrying away fats and oils. The standard way to soften water is to exchange sodium ions for the calcium, magnesium, and iron ions because sodium ions don’t have such bad effects on soaps and detergents. Adding salt to hard water, as you propose to do, won’t exchange sodium ions for the other ions. It will only add more metal ions to the water and the water will remain hard.

Second, a steam iron shouldn’t use hard water because when hard water boils away as steam, it leaves behind all the calcium, magnesium, and iron salts as unsightly scale. Again, adding salt to your hard water will simply leave more scale on the insides of your iron or on your clothes. You need demineralized water, not soft water, for your iron. The best way to demineralize water is to distill it.

If the condenser in a microwave is bad, what is the most likely reaction the mic…

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If the condenser in a microwave is bad, what is the most likely reaction the microwave generator will exhibit? — IF, Bakersfield, CA

According to a reader, most microwave oven capacitors have fuses in them so that when they fail, they usually become open (they lose all of their ability to store separated charge and behave as a simple open circuit). You’d need a capacitor checker to find this open circuit within the capacitor.

Why doesn’t a helium balloon pop when it reaches the ceiling?

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Why doesn’t a helium balloon pop when it reaches the ceiling?

The buoyant force lifts the helium balloon upward—the denser air flows downward to fill the space vacated as the balloon is squeezed upward. When the balloon finally reaches the ceiling, the ceiling exerts a downward force on the balloon and prevents it from rising further. But the force the ceiling exerts on the balloon’s skin is gentle enough and spread out enough that it doesn’t injure the rubber. The balloon simply comes to a stop and remains suspended until enough helium diffuses out of the balloon to cause it to descend.

If I want to create a radio controlled device, how do I make sure it does not cr…

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If I want to create a radio controlled device, how do I make sure it does not create interference with other devices or receive interference. How does digital RF work and does it stop interference problems? — KG, New York, NY

Radio interference occurs whenever two nearby radio transmitters are simultaneously emitting radio waves that overlap in space and frequency. The receivers for these two waves can’t tell them apart and end up receiving both at once. This interference is familiar with AM radio, where you can sometime hear two broadcasts at the same time. With FM radio, the receivers are clever enough to distinguish one radio wave from another, but they can’t determine which broadcast they’re supposed to follow. Instead, they lock onto whichever wave is strongest and will often flip back and forth from one station to the other as their signal strengths fluctuate.

The only way to avoid interference completely is to choose a radio frequency that no one else nearby is using. That way your transmission is certain to be stronger than any other at the same frequency and your receiver will follow only your broadcast. If you have no choice but to share a particular frequency, then you must use some encoding scheme such as digital transmission so that your receiver can tell when it’s receiving a broadcast from your transmitter and not from some other transmitter. Your receiver looks for your personal encoding scheme and won’t respond to that of some other transmitter. However, if that other transmitter is strong enough, it will probably prevent your receiver from detecting your transmission. That trick of overwhelming a receiver with a second transmission is the principle behind jamming of a radio transmission.

How can I check the magnetron in a home microwave oven? I have checked the HV (h…

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How can I check the magnetron in a home microwave oven? I have checked the HV (high voltage) transformer, the rectifier, and capacitor and all are OK. Does the magnetron output decrease with age? The oven has a hum that is much louder than normal. — AA, Ontario, CA

While I have only a little experience repairing microwave ovens, I can make reasonable guesses. The loud hum you hear is probably an indication that something is overloading the power transformer. That suggests that the diode, capacitor, or magnetron are bad. If you have checked the first two carefully, at full operating voltage, and found no problems, then I would suspect the magnetron. I have been told by a reader that magnetrons usually fail by shorting out, the result of electromigration of the filament material. The tube would then draw excessive currents from the high voltage transformer. That has probably happened in your case. Still, free advice like mine is only worth what you’ve paid for it. I’d suggest you consult a local repairperson, who has test equipment that can pinpoint the problem in seconds.

What is analog? I hear about digital audio being better than analog, but nobody …

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What is analog? I hear about digital audio being better than analog, but nobody defines what analog is. — DG, Houston, TX

In analog audio, the air pressure fluctuations of sound at the microphone are represented by a continuously variable physical quantity such as an electric current, a voltage, or a magnetization. Thus as the air pressure at a tape recorder’s microphone rises during one moment of a song, an electric current in the recorder will rise and a region of a magnetic tape surface will become particularly strongly magnetized in a particular direction. Overall, each value of air pressure is converted to a particular value of the physical quantity.

The problem with analog recording is that when the sound is recreated, any defect in the physical quantity representing air pressure will lead to an imperfection in the reproduced sound. For example, if the magnetization of the recording tape has changed slightly due to how it was stored, the sound that the tape recorder produces won’t be exactly the same as the sound that the microphone heard. Digital recording avoids this problem by recording the information as bits. The physical quantity such as magnetization is representing bits (which take only two possible values) rather than the air pressure itself (which can take a broad range of values). Minor changes in the physical quantity representing these bits won’t change the bits. Thus imperfections in the recording or playback process won’t affect the sound quality.

I have to do an experiment for school on the electromagnetic properties of iron,…

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I have to do an experiment for school on the electromagnetic properties of iron, steel, and aluminum. The only problem is that I am not too sure what I should be testing. Any ideas? — CP, Nassau, Bahamas

Iron and steel (not stainless) are ferromagnetic metals, meaning that they are intrinsically magnetic. While this magnetism is normally hidden by the formation of millions of tiny, randomly oriented magnetic domains, it becomes apparent when you hold a magnet near the iron or steel: they are attracted! Aluminum has no intrinsic magnetism and is not attracted to a magnet. There are far more non-magnetic metals than magnetic ones. Why don’t you try to see which metals will stick to a magnet. Only the ferromagnetic ones will. Even common stainless steel is non-ferromagnetic.

How does a dishwasher machine work?

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How does a dishwasher machine work? — WW, Bochum, Germany

A dishwasher is really a number of simple machines that work together to clean dishes. These machines are controlled by a mechanical or electronic timer and include an electrically operated water valve, a water level sensor, one or two water pumps, a thermostat, an electric heating element, one or more rotating spray nozzles, and a fan.

The cycle begins when the timer sends electric current through a coil of wire in the water valve, making that coil magnetic and pulling the water valve into its open position. Water flows then flows from the high pressure in the water line to the atmospheric pressure in the cleaning chamber. When the water sensor detects that the dishwasher is adequately filled, it shuts off current to the valve and the valve closes.

The thermostat measures the water temperature and may delay the start of the cycle if the water is too cool. If so, it directs electric current through the heating element, where that current’s energy is converted into thermal energy and transferred to the water. When the water is hot enough, the cycle continues.

During the cleaning cycle, one or more pumps operate. They add energy to the water and increase its pressure. This high-pressure water flows slowly to the rotating nozzles and then accelerates to high speeds as it enters the narrow openings and sprays out into the low-pressure cleaning chamber. As the high-speed water collides with the dishes and slows down, its pressure rises again and begins to exert substantial forces on the food particles. The food particles are pushed off the dishes and fall into the bottom of the dishwasher. Soap added to the cleaning water forms tiny spherical objects called micelles that trap and carry away fats that would otherwise not mix with water. At the end of the cycle, the water, food particles, and fat-filled soap micelles are pumped down the drain.

The cleaning cycle may repeat with fresh water and is then followed by a rinse. A soap-like surfactant may be added to the rinse water to lower its surface tension and prevent it from beading up on the dishes. When the pumps have removed the last of the rinse water, a fan begins to blow air over the dishes. The heating element may heat this air to assist evaporation. The water molecules leave the surfaces of the dishes and become gaseous water vapor. The dishes are left clean and dry.