Category: tape recorders

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.

I am interested in finding out if and what materials affect magnetic fields. — HLD, Jacksonville, FL

Magnetic fields are associated with lines of magnetic flux, invisible structures that stretch between north and south magnetic poles or that curve around on themselves to form complete loops. Unless a material has its own north or south magnetic poles, it can’t terminate the magnetic flux lines and can have only small effects on magnetic fields. The few materials that do affect magnetic fields substantially are ones such as iron or steel that are intrinsically magnetic and that can easily develop strong north and south magnetic poles. These magnetic materials can significantly shift the paths of the magnetic flux lines. If you put an iron or steel box in a magnetic field, the flux lines will tend to travel through the walls of the magnetic box. As a result, there will be few magnetic flux lines inside the box and almost no magnetic field. This effect is used to shield sensitive equipment such as the picture tubes in televisions from magnetic fields.

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.

In our busy trial court we have preserved the original cassette tapes since 1989. They are kept in a relatively constant room temperature environment in our modern courthouse. Should we take any further precautions to extend the life of these tapes, considering the possibility that they may need to be replayed one day, such as in the retrial of a death penalty case that is reversed a decade after trial? I’ve heard of the practice of unwinding and rewinding tapes for this purpose, but haven’t attempted it yet. The time involved is daunting! What is your opinion? — JD, Bryan, TX

A magnetic recording tape is usually a Mylar ribbon, coated with a thin layer of plastic that’s impregnated with tiny permanent magnets. As long as it’s store away from heat and moisture, the Mylar film itself shouldn’t age. However, the layer of permanent magnets can change slightly with time. When a tape is left tightly wound on its reel for a long time, the magnetic layers can begin to affect one another—the magnetic fields from one layer of tape can alter the magnetization of the layers above and below it. The result is that sounds from one layer of tape can gradually transfer themselves weakly to the adjacent layers, creating faint echo effects. The solution to this problem is to unwind and rewind the tape, so that the layers shift slightly relative to one another. But while these echoes may be annoying in a recording of classical music, they probably aren’t important in a recording of a noisy courtroom. Unless I hear otherwise from someone reading this note, I wouldn’t worry about unwinding and rewinding your tapes. The slight imperfections that will result from transfers between layers shouldn’t affect their utility in later trials. Properly stored, I’d expect the tapes to outlive everyone involved with the trials, even without any unwinding and rewinding.

Please explain the concepts of magnetism pertaining to ferromagnetism, diamagnetism, and paramagnetism. – SC

A ferromagnetic material is one that contains intrinsic magnetic order. Iron, for example, is a ferromagnetic material—meaning that if you were to examine a microscopic region of the iron, you would find that it was highly magnetic. The magnetism in a ferromagnetic material is often hidden by a domain structure, in which microscopic magnetic regions or “domains” all point in random directions to give the material no apparent magnetism. Only when you expose the ferromagnetic material to a magnetic field does its magnetic character suddenly reveal itself. A ferromagnetic material becomes strongly magnetic when it’s exposed to a magnetic field.

A diamagnetic material is one in which the electrons begin moving when it’s place in a magnetic field. These moving electric charges create a second magnetic field that partially cancels the original field. A diamagnetic magnetic field partially shields itself from magnetism when it’s exposed to a magnetic field.

A paramagnetic material is one in which individual magnetic electrons respond magnetically to any external magnetic field. It becomes weakly magnetic when it’s exposed to a magnetic field. Unlike a ferromagnetic material, a paramagnetic material has no intrinsic magnetic order before it’s exposed to an external field.

In plain English that a child can understand, how does a magnet work? — EK, Dale City, VA

There are several way in which objects in our universe can push or pull on one another and one of these ways is through electric or magnetic forces. Two objects that have electric charges are observed to push or pull on one another and two objects that have magnetic poles are also observed to push or pull on one another. That’s simply the way our universe works. With electric forces, things are relatively easy—when you pull a sock and shirt out of the dryer, the sock may well stick to the shirt because friction has given the two different electric charges (one is positively charged and the other negatively charged). By playing around with electrically charged objects, you can convince yourself that (1) there are two different types of electric charge—normally called “positive” and “negative”—and (2) that like charges repel while opposite charges attract.

With magnetic forces, there is an annoying complication: magnetic poles (the magnetic equivalent of “charge”) always come in equal but opposite pairs. As with electric charges, there are two types of magnetic poles—normally called “north” and “south”—and like poles repel while opposite poles attract. However, you won’t be able to find a pure north pole anywhere; it always comes attached to a south pole (and vice versa). So any magnet you find will have at least one north pole and at least one south pole (while they typically have only one of each, they can also have many of each). The forces that these poles exert on one another are fundamental to our universe—I can’t explain them in terms of more basic phenomena because they are already basic except at a very abstract level. (In fact, electric and magnetic forces are intimately related to one another and it is actually electric charges that are creating the magnetic poles that you observe in a magnet.) If you play around with several magnets for a while, you should be able to convince yourself about the existence of two different poles and that like poles repel while opposite poles attract. You should also notice that the magnets push one another directly toward or away from them (the forces between poles are parallel to the line separating them) and that the forces become stronger as the poles become nearer (the force is inversely proportional to the square of the distance separating the poles).

As for how a permanent magnet works, it’s made from a material that contains ordered electrons. Electrons are intrinsically magnetic and, in a few special materials, that magnetism as organized so that the overall materials are themselves magnetic. Each electron has its own north and south pole, but together they give the material a giant north and south pole.

Would extreme temperatures affect the strength of a magnet? — PL, Columbus, OH

Yes! High temperatures disorder materials and destroy magnetic order. Permanent magnets can be demagnetized by heating them, often to surprisingly modest temperatures. Many household magnets can be spoiled by putting them in a hot oven. Even electromagnets will lose most of their strength at very high temperatures because they rely on iron and iron undergoes several phase transitions at high temperatures that destroy its magnetic order. You can show that iron loses its magnetism at high temperatures by heating a steel nail red hot with a propane torch and then trying to pick it up with a magnet. Be careful not to burn yourself. The hot nail won’t stick to the magnet because it won’t have any magnetic order. Once the nail cools, its magnetic order will reappear.

Is there any substance that can stop magnetic fields — K, Mendenhall, MS

Magnetic fields are related to what are call magnetic flux lines. These magnetic flux lines extend unbroken from north magnetic poles to south magnetic poles. Where the flux lines are close together, the magnetic field is strong. Thus to avoid magnetic fields, you need to keep magnetic flux lines away. Because magnetic flux lines can’t be broken, they can’t simply be made to disappear. To “stop” a magnetic field in a particular region of space, you have to either terminate the flux lines at a magnetic pole or you have to divert the flux lines away the region that you’re interested in. The first strategy has a problem: no isolated magnetic poles (so-called “magnetic monopoles”) have ever been found. That means that every north pole you find has a south pole attached to it. Thus you can’t simply end the flux lines with magnetic poles because for each flux line you end with a south pole, you’ll start a new one with the attached north pole. But the second strategy is reasonable. There are many materials that divert magnetic flux lines. One of the most important of these is a metal called “mu metal,” an alloy that’s made from nickel, iron, chromium, and copper. Mu metal attracts flux lines. It draws flux lines through itself so that if you were to wrap yourself in a layer of mu metal, any magnetic flux lines that would have gone through you (and thus exposed you to magnetic fields) will go through the mu metal instead. Mu metal and similar alloys are used routinely to shield objects that can’t tolerate magnetic fields.

Why does my voice sound different to me when I listen to a recording of myself?

When you hear yourself speak directly, much of the sound that reaches your ears travels to them through the bones and tissues of your head. This type of sound conduction tends to emphasize the low frequencies in your voice so that your voice sounds lower to you than it does to other people. When you listen to a recording of your voice, you are hearing your voice as other people hear it, without the modifying effects of bone and tissue conduction. Everyone else listening to the tape thinks that your voice sounds normal but you think it sounds higher than normal.

How does magnetic recording work?

During the recording process, an electromagnet in the recording head magnetizes the surface of a specially coated tape. This tape is coated with a thin layer of plastic that’s impregnated with tiny cigar-shaped magnetic particles. As the tape moves past the recording head, the head magnetizes these particles back and forth to a certain depth, according to the audio signal reaching the recorder from the microphone. The higher the pitch of the sound, the more frequently the direction of magnetization reverses. The louder the volume of the sound, the deeper the magnetization extends into the layer. During playback, this magnetized layer moves past the playback head and induces electric currents in it. These currents are then amplified and used to reproduce the sound. A much more detailed discussion of this process appears in my book.