Tape Articles Intro- Magnetism and Electrons

December 2000
by William J. Vermillion. All rights reserved

Next: Recording and Playback Heads

A while back on the SCO list there was some discussion on tape, and I took the time to show why the newer tape drives with their higher data transfer rates had less wear on the drive and tape than did the older lower capacity drives.

Tony suggested I expand on this as he said I am good at explaining things.

So this is a description of how some of the current tape drives, and immediate predecessors work.

Because of the processes involved and to make sure that everyone has at least the level of knowledge to understand this, I'll start by explaining the basic fundamental concepts that relate to this technology.

We have the electron - the part of an atom which spins around the nucleus just as the planets spin around the sun, and the magnetic domain - just like the north pole, or a simple child's magnet.

The atom is the basic building block. Some materials [elements] have electrons which are easy to dislodge and move over to another atom. [Using the analogy of the planets these electrons would be those furthest from the sun.]

A material with easily movable electrons is called a conductor. A material which will not let go of it's electrons is an insulator. One of the most commonly used materials for conductors is copper [atomic symbol CU]. Glass is an example of an insulator.

When a conductor [from now on we'll just call it a wire] is passed through a magnetic field, or conversely a magnetic field passes a wire, then some of the outermost electrons will be move from one atom to another. And electrons from another atom will take their place.

Knowing that moving a wire in an magnetic field, or moving the magnetic field in relation to that wire causes the electrons to move, we now need to understand the consequences of doing this.

If we move a wire into the magnetic field the electrons will flow in one direction or another, depending on the orientation [north or south] of the magnetic field.

At the lowest level a magnetic particle has a north pole and a south pole. A magnetized bar of metal has ends that has the same north/south orientation. These are called bar magnets. If we bend the bar in a loop we now have the typical horseshoe magnet with north and south poles across the gap at the end of the U shape.

Now let's take a wire and make it into a loop. We will place this loop perpendicular to the U of the magnetic and at the opening of the magnet. Thus if we rotate the loop one side of the loop will enter the U and while the other side exits.

The direction the electrons flow depends on the orientation of the magnetic field. If as the top of the loop enters the magnetic field [well designate this north for this example] the electrons would want to flow into the wire from the outside, then the electrons at the bottom of the loop [nearest the south pole] would want to go in a direction from the wire to the outside.

Looking at it this way you can see that electrons go in one end and out the other. All well and good. Now as the loop moves through the first quarter of a revolution, it will enter the domain of the south pole of he magnetic the electron movement will slow, stop, and then reverse direction.

If we make the loop bigger - by coiling up this one length of wire - we will generate more electrons - which translates into high current.

If we spin this loop at 60 times each second then we basically have generated 60 cycle alternating current. [It is called alternating because the electrons reverse direction.] The speed at which they reverse defines the cycle. In the US the the current is 60 cycles per second. [60 cycle AC or just AC] In Great Britain it is 50 cycles per second.

At one time all measurements included a time domain, eg seconds, days, months, etc. Today the term Hertz - abbreviated Hz - is used to define a 'cycles per second'. So 60 Hertz is the same as 60 cycles per second.

If you ask yourself that if moving a wire inside a magnetic field causes electrons to move, would putting electrons inside a wire which is inside a magnetic field then cause the wire to move, the answer would be yes. That is exactly how we make an electric motor.

In either of the above examples we could have held the wire stationary and moved the magnetic field. The end result is the same whether we moved the conductor or the magnetic field. Some motors are built where what we think of as the outside of the motor moves while the center part - the shaft - stays stationary.

The last part of the this basic understanding is that we can also create a magnetic field with electrons. This is called an electro-magnet. That is something which is magnetic only when we pass an electric current through it.

Take a plain old nail, wrap some wire around it, attach each end of the wire to a flashlight battery, and you will have created a magnet. If you are the experimenting type do NOT try this with anything larger than a flashlight battery. Working with electricity above the level of a flashlight battery demands knowledge and care.

If you understand the above then that's all you need to know about electricity and magnetism to understand the basics of how we use magnetic media in the computer world, whether it be hard disks or tape drives.

To summarize these fundamentals.

1. There is interaction between electrons and magnetic fields.
2. We have the ability to make electrons move by moving a conductor in a magnetic field or moving a magnetic field past a stationary conductor.
3. We can cause conductors to move with respect to the magnetic filed by passing electrons through the wire.
4. We have a method to generate a magnetic field with electrons.

Now - on to how we make all this work together.

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