Abstract

Objective

The goal of this project is to determine the maximum "recording density" for storing digitized information using a grid of bar magnets. You'll learn about how information is digitized, and how the digitized information is stored magnetically. The project helps you to understand the limitations on information density recorded on magnetic materials.

Introduction

Today, magnetic disk drives are used to store and retrieve information for many different applications. Digital video recorders (DVRs), MP3 players, and gaming systems are all examples of popular products that use a disk drive to store and retrieve information. Some other applications that you may not be familiar with are global positioning systems (GPS), banking systems, and automobiles.

How are all these different kinds of information stored on magnetic disk drives? How much data can fit in a given amount of space on a disk? How is data erased from a disk? This project will help you answer these questions as you learn how magnetic materials are used to store information.

Information like words, music, or movies is translated into a format that can be saved onto various permanent storage devices, like a magnetic disk drive. This translation is called digitization, which means that the information is converted into a stream of numbers. The smallest unit for digital information is called a bit. A bit can be either 0 or 1, that's it. By stringing together a series of bits, larger numbers can be represented. For example, a byte is a sequence of 8 bits. A byte can encode 2⁸ (= 256) unique values. If you want to learn more about encoding information in bits and bytes, see the Science Buddies project Bits, Bytes, and Bases: Write a JavaScript Binary/Decimal/Hexadecimal Converter.

Letters, numbers, and other symbols for printed text are digitized using a standard code, called ASCII (ASCII is an acronym for American Standard Code for Information Interchange). Each character is assigned a unique code. Click here for a Table of 7-bit ASCII Character Codes. Using the ASCII table, what sequence of bits would correspond to the word "digitize?"

In this project, you will digitize a short piece of text (e.g., the name of your favorite band) using the ASCII representation of the text. Next, you will use bar magnets to represent the individual bits of the digitized text. The orientation of the magnet will determine whether it represents a 0 or a 1. You'll see how close together your magnets can be packed while still preserving your stored information. Finally, you'll see how easily you can erase your stored information with a permanent magnet.

Terms, Concepts, and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

• bit,
• byte,
• ASCII,
• magnet,
• bar magnet.

Questions

• Before MP3 players and DVRs, what was used to store and retrieve music and video?
• What are other examples of storage and retrieval methods or systems?
• Why do you think disk drives devices like iPODs and DVRs are now replacing tape-based devices like Walkmans and VCRs?

Materials and Equipment

To do this experiment you will need the following materials and equipment:

• a print-out of the ASCII code and binary code for 26 letters,
• plastic/glass tray with square/rectangular compartments,
• a few dozen 1 inch bar magnets,
• paper and pencil,
• ruler,
• one big horseshoe magnet.

Shop for Supplies at Science Buddies Online Store

Science Buddies has compiled some suggestions for harder to find items in our Amazon store. The store does not include every item for every project, but it does include items that we feel work for the projects on our website. If you have comments or would like us to add items to the store, please contact us at scibuddy@sciencebuddies.org.

Experimental Procedure

In this experiment, you will digitize a short piece of text (e.g., the name of your favorite band) using the ASCII representation of the text. Next, you will use bar magnets to represent the individual bits of the digitized text. The orientation of the magnet will determine whether it represents a 0 or a 1. You'll see how close together your magnets can be packed while still preserving your stored information. Finally, you'll see how easily you can erase your stored information with a permanent magnet.

1. You'll need a word or phrase to digitize. Pick the name of your favorite musician or band, your favorite song, or some other word or phrase.
2. Make a table to translate your chosen word or phrase into the binary ASCII code. For example, if we chose "Jem" as our favorite musician, here is how we would digitize her name:

   J	e	m
   0100 1010	0110 0101	0110 1101

3. Now use individual bar magnets to represent each bit of the coded word or phrase. We'll say that a magnet with its N pole facing right is a 1, and a magnet with its N pole facing left is a 0. (To make your code easier to see, you may want to color the N half of each magnet.) Place one magnet in each compartment of your plastic tray, arranging them according to the binary code for your word or phrase.
4. What is the information density of your recording? How many bits per sqaure inch? Just for comparison, a 1990 hard disk could store 1 billion (1,000,000,000) bits per sqaure inch, and a 2006 hard disk can store 100 billion (100,000,000,000) bits per square inch.
5. Gently tap your tray of magnets. What happens to the arrangement? Are some of the magnets attracted (or repelled) by their neighbors? Did something like this happen to your tray (see Figure 1):

   Figure 1. Neighboring bar magnets with opposite polarity are attracted to one another when tray is tapped.

6. If the magnetic material on a recording surface could move around like your bar magnets, how do you think this would affect the durability of the recording?
7. For the next part, you'll be working with the binary code for a single letter. Choose one letter from your word or phrase to use for this.
8. On a piece of paper, draw 8 squares, about the same size as the compartments in your tray. Arrange the magnets on the paper squares to represent the code for your chosen letter.
9. Do the magnets interact with each other now that there is no longer a wall separating them?
   1. If the magnets do not interact, try again with smaller squares. How small can your squares be without the magnets interacting with their neighbors?
   2. If the magnets do interact, try again with larger squares. How large do the squares need to be so that the magnets are not disturbed by their neighbors?
10. What is the highest recording density you can achieve when you place the magnets on a piece of paper (in number of bits per square inch)?
11. Again arrange your bar magnets on the paper to encode your chosen letter. Take the horseshoe magnet and, holding it a foot above the paper (measure with the ruler), pass it over the bar magnets. Did any of the bar magnets move?
    1. If the bar magnets did not move, lower the horseshoe magnet slightly and try again. At what height does the horseshoe magnet move the bar magnets?
    2. If the bar magnets did move, raise the horseshoe magnet slightly and try again. At what height does the horseshoe magnet first stop moving the bar magnets?
    3. What does this tell you about erasing data stored on magnetic recording media?

Variations

• For more science project ideas in this area of science, see Electricity & Electronics Project Ideas.

Credits

• Written by Susan Chang, JJ Martin,
• Sophie Fujioka, Sherill Rylie,
• Maureen Klatt, Hong-Ying Wang,
• Hui Kathy Liu, Stella Wu. 

Edited by Andrew Olson, Ph.D., Science Buddies

Last edit date: 2006-04-19 11:17:00