How did wire recorders work?

How did wire recorders work? — MW, San Diego, CA

The original recording scheme invented by Poulson used a wire as the recording medium, rather than a tape. It recorded audio information as the magnetization of a steel wire in much the same way that a modern tape recorder records audio information as the magnetization of iron particles on the surface of a plastic tape. Both devices record the air pressure changes associated with sound as magnetization changes in a magnetizable surface—the higher the air pressure, the deeper the magnetization in a particular direction; the lower the air pressure, the deeper the magnetization in the opposite direction.

When making an electromagnet, why does a hard core stay permanently magnetized w…

When making an electromagnet, why does a hard core stay permanently magnetized while a soft core does not? — CD, Houston, TX

Iron and steel are intrinsically magnetic materials, meaning that at the atomic scale they exhibit magnetic order and have magnetic poles present. Most materials, including copper and aluminum, have no such magnetic order—they are nonmagnetic all the way to the atomic scale. But while it is composed of magnetic atoms, a large piece of iron or steel normally doesn’t appear magnetic. That’s because a large piece of iron or steel contains many tiny magnetic domains. Although each of these magnetic domains is highly magnetic, with a north pole at one end and a south pole at the other end, the metal appears nonmagnetic at first because these domains point equally in all directions and their magnetizations cancel one another. Before the magnetic character of a piece of iron or steel will become visible, something must align its magnetic domains.

In an electromagnet, an iron or steel core is surrounded by a coil of wire. When you run current through that coil of wire, the magnetic field of the current causes the core’s magnetic domains to change sizes—the domains that are aligned with the field grow at the expense of the domains misaligned with the field and the whole piece of iron or steel becomes highly magnetic. When you stop current from flowing through the coil of wire, the domains may return to their original sizes and shapes and the iron or steel may become nonmagnetic again.

The abilities for magnetic domains to change sizes depends on the chemical and physical properties of the metal, particularly its crystalline structure. In some magnetic materials, the domains change size extremely easily. These materials are considered to be “soft”—they magnetize easily in the presence of a magnetic field and demagnetize easily when that field is removed. Most electromagnets are made from such soft magnetic materials because it takes only a small current in a wire coil to magnetize the electromagnet’s soft core and that core quickly becomes nonmagnetic when you stop the current from flowing.

But in other magnetic materials, the domains don’t change size easily. These materials are considered to be “hard”—they are both difficult to magnetize and difficult to demagnetize. You must put lots of current through the coil of wire around a hard magnetic material in order to magnetize that material. But once you turn off the current, the material will retain its magnetization and it will be a permanent magnet.

How long will a magnetic tape stay magnetized? Won’t it lose its magnetization v…

How long will a magnetic tape stay magnetized? Won’t it lose its magnetization very fast, like we saw with the iron nails?

At room temperature, a magnetic tape will remain magnetized for years and years. It is made of much harder magnetic materials than the nails are made of and it is much harder to demagnetize than the nails. In effect, it is covered with tiny permanent magnets and you have seen permanent magnets that remain magnetic for decades or centuries.

When you talk about the magnetic tape and recording, is it the pressure or frequ…

When you talk about the magnetic tape and recording, is it the pressure or frequency that is being recorded? Are pressure and frequency interrelated?

Sound consists of pressure fluctuations. The stronger those pressure fluctuations, the louder the sound. The rapidity with which the air goes between a pressure increase and a pressure decreases determines the frequency of the sound and the pitch that we hear. So the extent of the pressure fluctuations, their amplitude, determines the sound volume while the number of pressure fluctuations each second, their frequency, determines the sound pitch. The tape recorder detects both and records both. The louder the sound, the deeper the recorder magnetizes the tape. The higher the frequency of the sound, the more often the tape recorder reverses the magnetization of the tape’s surface.

Can an object conduct electricity but be nonmagnetic? Are these independent prop…

Can an object conduct electricity but be nonmagnetic? Are these independent properties? You said during lecture that copper is nonmagnetic but doesn’t it conduct electricity?

Electric conductivity and magnetism are pretty much independent properties. There are good conductors that are magnetic (iron) and good conductors that are nonmagnetic (copper). There are also insulators that are magnetic (iron oxide) and insulators that are nonmagnetic (glass).

How does a magnetic tape record the difference in timbre or sound quality of the…

How does a magnetic tape record the difference in timbre or sound quality of the sounds? How does it represent a piano versus an electric guitar? Also, how does more than one tone get recorded (e.g., an entire band or symphony)?

Even a single instrument playing a single note produces a complicated sound. The air pressure fluctuations produced by the instrument aren’t as simple and smooth as you might think. While the instrument may produce mostly the fundamental tone—the main pitch associated with the note being played—it also produces other tones that are usually integer multiples of the fundamental tone. These higher pitched “harmonics” contribute to the sound we hear and allow us to determine what instrument is playing that sound. We also hear the temporal shape of the sound—the sound envelope. A piano produces a sound that starts loud and gradually becomes softer while a violin produces a sound that starts soft and gradually becomes louder. An electric guitar offers its player even more control over the pitch and sound envelope. The tape recorder detects the pressure fluctuations associated with all these tones and volume changes and records them all as the magnetization of the tape’s surface. When many instruments are playing at once, the pressure fluctuations are even more complicated and they add together to create a complicated pressure pattern at the microphone. Nonetheless, the recorder simply detects the air pressure changes at the microphone and records them on the tape, and that’s all it needs to do to keep an accurate record of the sound. When the magnetization of the tape is used to reproduce sound, you again hear all the instruments playing.

How does heat affect magnetism?

How does heat affect magnetism? — MC, Capitol Heights, MD

The magnetism we associate with a permanent magnet or with steel’s response to that permanent magnet involves the careful ordering of tiny magnetic electrons within the materials. Just as heat tends to destroy all forms of order in a newspaper when you put it in the fire, so heat tends to destroy the magnetic order in a permanent magnet or in steel when you bake them. Many permanent magnets lose their magnetism when heated to oven temperatures and even steel becomes non-magnetic when heated red-hot.

How do you demagnetize a magnet?

How do you demagnetize a magnet?

A permanent magnet was magnetized when it was first made out of metal. It did have microscopic regions of magnetic order—magnetic domains—but those regions all pointed in random directions and the magnet didn’t have any overall magnetic poles. To give it poles, it had to be magnetized. It was placed in a very strong magnetic field so that its domains grew or shrank until most of them were aligned with the magnetic field. The magnet acquired overall magnetic poles for the first time. When the field was removed, the domains remained as they were and the magnet permanently retained its new magnetic poles.

If this same magnet were reversed and then placed in that strong magnetic field again, it would become remagnetized in the opposite direction from before—its domains would grow or shrink until most of them were aligned with the magnetic field again. The magnet’s north poles would become south poles and vice versa. Finally, if the magnet were wiggled back and forth in that strong magnetic field and gradually removed from the field, its domains would grow or shrink almost randomly. The magnet’s magnetic domains would become randomized and it would end up with no overall north or south magnetic pole at all. It would be demagnetized.

Do regular magnets lose their magnetism or do they stay magnetized always? What …

Do regular magnets lose their magnetism or do they stay magnetized always? What about electric magnets, like the ones used in wrecking yards? — KM, Delta, British Columbia

Permanent magnets are made from materials with two important magnetic characteristics. First, these materials are intrinsically magnetic, meaning that some of the electrons in these materials retain their natural magnetism. While electrons are always magnetic, that magnetism is lost in most materials because of complete cancellations—each magnetic electron is paired with another magnetic electron so that they cancel one another perfectly. However, there are some materials in which the cancellation is imperfect and these materials (including iron, cobalt, nickel, and many steels) are the basis for most permanent magnets.

Second, the materials used in permanent magnets have internal structures that make the magnetic electrons align along particular directions. Once the electrons are aligned along one of those directions, they stay aligned and the material exhibits strong magnetic characteristics. It becomes a “permanent magnet.”

A permanent magnet remains its magnetization as long as nothing spoils the alignments of its magnetic electrons. These electrons can be knocked out of alignment by vibrations, heat, or other magnets. If you hit a permanent magnet with a hammer or heat it in the oven, you will change and perhaps destroy its magnetization. This magnetization can be recovered by exposing the permanent magnet to the magnetic influences of an electric current. In fact, permanent magnets are originally magnetized by placing them near electric currents that align their magnetic electrons. Moreover, even a material that doesn’t have the internal structures needed to keep its electrons aligned along a particular direction will become magnetized temporarily by placing it near an electric current. That’s how a wrecking yard magnet works-an electric current temporarily turns a large piece of iron into a strong magnet.

Magnets stick to metal, but can you make a magnet repel metal? – M

Magnets stick to metal, but can you make a magnet repel metal? – M

Yes, but not in the way you’re thinking of. When you bring a magnet near a piece of steel, the intrinsic magnetic character of that steel causes it to become magnetic in such a way that it attracts the magnet. There is no way for the steel, or another similar metal, to become magnetic in such a way that it would repel the magnet.

However, if the metal is already magnetized it can repel an approaching magnet. A more interesting case is when a magnet approaches a normally non-magnetic metal at high speeds; in which case electric currents begin to flow through the metal and these currents do repel the approaching magnet.