Click here to purchase the entire book in PDF format. The simple story Once upon a time when you were a kid, you probably did a science experiment where you made a compass using a tray of water and a sewing needle. The only work involved in the experiment was to turn the needle into a magnet by stroking it over and over with a permanent magnet. The lesson here is that if you put a piece of iron in a magnetic field, you can turn it into a magnet. Let's make soup! Unfortunately, not a good French onion soup, or a seafood chowder, however... We'll make a slurry of little needles (usually called a magnetic oxide because it's made out of things like ferric oxide, for example) suspended in a non-magnetic liquid binder. We'll then pour this goo on a wide sheet of polyester and let it dry so that the little needles are stuck to the sheet. Just before we dry it, we run the polyester with the goo on it through a really strong magnetic field so all the needles are pointing in the same direction - just like little compasses. Eventually, we're going to roll up this sheet, so we'll put a carbon coating on the back of it so that it's slippery and it doesn't build up static electricity. Then we take the wide sheet and slice it into strips ranging from 0.125 to 2 inches wide. A cross section of this stuff looks like Figure 6.53 Figure 6.53: A cross section of a typical piece of analog tape. The thickness of the three layers are drawn to scale following the physical dimensions of Ampex 456 tape with a magnetic oxide thickness of 13.97 $\mu$m, a polyester base thickness of 36.07 $\mu$m and a back coating of 1.27 $\mu$m [Woram, 1989]. The important thing about the magnetic oxide coating on the polyester is that it is pretty easy to magnetize. If we put it in a strong magnetic field and then take it out, it will maintain that field in the coating, just like the needle that we magnetized to make a compass back when we were kids. So, how do we magnetize the coating? We already know two useful things from Section 2.6: the first is that if we run current through a wire, we get a magnetic field around it. The second is that, if that wire is coiled around an iron bar, the iron bar will act like a magnet. So, let's take an iron bar and bend it so that the two ends almost touch each other. We'll also coil a wire around it so that the whole thing looks like Figure 6.54. This is a very basic model of a record head of an analog tape recorder. (Actually, it also works as a playback head.) Figure 6.54: A simple model of a record head for an analog tape deck. When we put current in the coil, the iron bar it's coiled around temporarily becomes a magnet. This, in turn, causes magnetic lines of force to go from one end of the bar to the other across the narrow gap that we created (seen at the top in Figure 6.54. If we look at a close-up of those magnetic lines of force, we'd see something like Figure 6.55. Figure 6.55: A close-up of the gap in a recording head, showing the magnetic lines of force in red, and the tape sitting on the head, magnetic coating down. Notice that some of the magnetic lines of force extend out from the head and into the magnetic oxide. In its most simple form, if we send an audio signal into the coil of wire wrapped around the tape head, we'll cause the magnetic field to change in strength and polarity at the gap of the record head. If we leave the magnetic tape sitting on the head while this happens, we'll be causing that magnetic field to be stored on the tape. If we want to keep the magnetic field stored on the tape, then we'll move it away from the head before the next signal comes in. So, we move the tape continuously across the head while the magnetic field changes (caused by changes in the current in the coil which, in turn are caused by changes in the audio signal). As the tape moves away from the gap (from left to right in Figure 6.55, the magnetic field that was imposed on it by the gap of the record head is maintained and we have a recording of our signal. Then all we have to do is to figure out how to play it back. This is where things get really easy. Remember that our tape is now basically a permanent magnet. If we put it next to an iron bar, then the iron bar conducts the magnetic lines of force. If the iron bar has a coil wrapped around it, and the magnetic lines of force going through the bar change, then we induce a current in the coil that is proportional to the change in the strength of the magnetic field. Therefore, if we continuously move the tape across the head gap, we continuously change the magnetic field and therefore generate a current in the coil that is proportional to the magnetic field on the tape, which, as you probably remember is proportional to the original audio signal. Consequently, we get a signal out of the coil that is representative of our original signal. Figure 6.56: A close-up of the gap in a playback head showing the magnetic field on the tape cutting across the gap of the head. Changing this magnetic field will induce a current in the coil wrapped around the head.