Lab #9: Magnetic Memory
Introduction
Magnetic memory is everywhere. It’s still the storage medium of choice in computers (though other forms of memory, such as flash storage and optical storage are increasingly competitive) because of the storage density (amount of memory), the write speed (speed of storing information to memory) and the read speed (speed of accessing information from memory). It’s also the predominant mechanism of information storage in credit cards, bank cards, etc. (though bar codes are becoming more common).
As is perhaps obvious, magnetic memory consists of little tiny magnets, whose orientation of N poles and S poles stores binary information. Writing to magnetic memory consists of using an electromagnet to orient the magnetic moments in the computer disk or the magnetic strip. Reading from magnetic memory is done using a number of techniques. Today, we’ll see how to read magnetic memory using the ideas of changing magnetic flux and Faraday’s Law of induction.
Equipment
In the first part of the lab, you’ll use iron powder to observe some of the magnetic pattern associated with the magnetic strip on your Bucknell ID (or any other card with a magnetic strip). The iron powder is available in a little shaker at your station. The powder is a little messy, so you can use rubber gloves as you choose. There are also magnifying glasses available for you to observe the pattern.
In the second part of the lab, you’ll use the small 3200 turn coil attached to the LabPro interface. Using LabPro, you can measure voltage vs. time, as needed when considering Faraday’s Law. The coil is already mounted and interfaced via alligator clips and banana plug leads to the voltage probe on the LabPro interface. You’ll use the program Lab #9.MBL, accessible under the PHYS141 folder, and in the Phys144 folder. You’ll also use the strong cylindrical bar magnets like the ones in your kit. You’ll mount the magnets on a steel pizza pie plate, and place the plates on the turntables from our rotations lab in order to spin them. You’ll also look at a real magnetic hard drive to see the rotating platter as well as the read coil.
Part I: Magnetic Strip
Theory
Information in your bar code is stored in the pattern of magnetic domains. We can imagine that we have “striped” domains, where the moment can either point up (away from the card) or down (toward the card). We can associate this with N poles and S poles of magnets, and also see how this can be turned into binary digits: for example, N pole up could be the binary digit (bit) 0, and S pole up could be the binary digit 1.
This information is encoded by electromagnets that write the binary digits by orienting the domains so that they are up or down in the right pattern. This information is then read via Faraday’s law: when the card is swiped through the reader, the moments pass by some coil. The changing magnetic flux caused by the motion of the individual magnetic bits induces a potential difference that is corresponds to the binary digits stored in the strip. In the next part of the lab, we’ll see how this information is read.
Observing the Magnetic Strip Pattern
You may want to wear gloves for this part. The iron powder isn’t dangerous, but it is a little messy. There is a sink and paper towels at the front of the room for any cleanup.
One person in the group should take out their Bucknell University ID (BUID) or other card with a magnetic stripe. Place the card on the weighing paper with the magnetic strip face up. At your station should be a glass vial with some iron powder in it; you won’t need to open the vial as it is ready to use as is. Gently shake some of the iron powder onto the magnetic strip until the magnetic strip is lightly covered in powder. Then, holding the card firmly, gently tap it onto the weighing paper so that any excess powder falls off the card onto the paper. Repeat this tapping until no excess iron powder falls off the card.
1) Now, carefully examine the magnetic stripe. You may need to hold the card so that the light reflects off at some good angle. What do you observe? Use the magnifying glass to aid in your observations. Briefly describe and draw a rough sketch of what you see (your description and sketch don’t have to be very detailed; they should just indicate the main features of what you see). Is there any noticeable difference between the left part of the strip and the right part of the strip?
2) Describe the motion when your card is swiped through a card reader. Why do you need to swipe the card at all? Based on your observations, why is the direction you swipe the card important?
Cleanup
Let your instructor know when you are done so that you can try to recover the excess iron powder. You can wipe the powder off your card with a paper towel, and wash the card in the sink.
Part II: Magnetic Hard Drive
Theory
As discussed before, information is encoded in magnetic materials by the pattern of magnetic orientation in the domains. For this lab, we’ll say that when the N pole is up, that corresponds to the binary digit 0, and when the S pole is up, that corresponds to the binary digit 1. While in actually magnetic storage media, the information is written to domains via an electromagnet, in this part of the lab, we’ll just orient cylindrical bar magnets by hand, mounting them on a steel pizza pie plate. In actual magnetic media for data storage, the magnets are microscopic: in a magnetic strip the magnets are in the strip, and in a magnetic disk (floppy or hard drive) the disk itself is made up of, or covered with, magnetic material.
Binary
As discussed in class, computers “talk” in binary: 0’s and 1’s. The fundamental unit is called a binary digit (bit), and 8 bits make up a byte. For this lab, we’ll encode letters using a non-standard method. We won’t make a distinction between upper case and lower case, and we’ll associate each letter of the alphabet with its corresponding number in numerical order. We’ll then convert that number to binary:
A / B / C / D / E / F / G / H / I / J / K / L / M / N / O / P / Q / R / S / T / U / V / W / X / Y / Z1 / 2 / 3 / 4 / 5 / 6 / 7 / 8 / 9 / 10 / 11 / 12 / 13 / 14 / 15 / 16 / 17 / 18 / 19 / 20 / 21 / 22 / 23 / 24 / 25 / 26
So we can see that to encode all the letters of the alphabet, we need 26 numbers (note, we could have started counting at 0 and end up with Z à 25, but that’s still 26 numbers).
3) What is the MINIMUM number of bits that you need to write the decimal number 26 in binary?
4) Using the scheme above, convert your initials (just first and last, don’t include your middle name) into two decimal numbers. Then, convert those numbers into binary.
You should see that you need 10 bits total to encode your initials using the scheme we have invented for this lab. Again, note that this is a non standard scheme, but it gives you the idea of what is going on. How do we convert this information into a magnetic representation? We’ll say that 0 corresponds to N, and 1 corresponds to S. We can easily write that pattern of N and S to magnetic media using an electromagnet. Or as in this lab, we can set up that pattern of N and S by hand on the steel pie plate.
Reading Magnetic Information
5) But how do we read magnetic information? If you were given a disk with 10 magnets arranged on it, what’s an easy way you could tell the pattern of N poles and S poles?
But this scheme won’t work well for a computer trying to read information from microscopic magnets. A magnetic information retrieval system uses a “read” head to “see” the pattern. One way to do this is by spinning the disk (or moving the read head) and measuring the electric potential difference induced by the changing flux caused by the microscopic magnets. If the microscopic magnet has its N pole up, that will induce a different signal than if the microscopic magnet has its S pole up. Note that this “inductive” (called this since it uses the induced potential difference caused by a changing flux) method is not the only way to read information stored magnetically. In a few weeks, we’ll discuss another method that uses light to read this information (as in magneto-optical devices that read “burned” CD’s and DVD’s).
6) For magnetic storage devices that use Faraday’s Law, the read head consists of a coil, and the magnetic media is coated onto a disk (“platter”) that spins very rapidly. Have your instructor show you a real magnetic hard drive, and identify the read head and the platter. Write down some comments about your observations.
Next, we’ll start to construct a magnetic hard drive of our very own! It won’t be very fast, and it won’t hold much information (just up to 10 bits, or enough for your initials using our scheme from before). But it will in principle be just like any other magnetic hard drive that uses Faraday’s Law to read information from the magnetic media.
A Toy Magnetic Hard Drive
At your station, you will find a steel pizza pie plate. The pie plate is marked at 10 places, roughly 36o apart. You can place a magnet at each of those places, and thus construct a 10 bit hard drive. You also have 10 magnets at your station. The pie plates fit nicely on top of turntables that rotate, so this is analogous to the platter in a magnetic hard drive.
You’ll also see a mounted coil. This coil is analogous to the inductive read head in a typical hard drive of the type we have discussed. If you follow the leads, you’ll see that they connect to the LabPro unit at your station. Please keep the toy drive components on the end of the bench that they are at, as we don’t want the magnets too close to the computer.
7) Place ONE magnet, N pole side up, on the red spot on the platter. Note that the spots are roughly in the correct position. The exact positioning of the magnets doesn’t matter too much, though when the platter is spun, the magnets should all basically move directly under the read head coil. Put the other magnets aside for now. Adjust the vertical height of the read head coil so that it is as close as possible to the magnets while leaving enough room for the magnets to pass underneath. If you don’t have your N pole marked magnet from your kit, find someone who does so that you can figure out the N pole of the magnet you’ve placed on your platter.
8) Turn ON the power to the LabPro unit (at the power switch). Log on to the computer. You can access the appropriate program by going to the PHYS141 folder on the desktop, and then going into the Phys144 folder and double-clicking on Lab #9.MBL. This will bring up a graph of Potential (in volts) vs. Time (sec). To collect data, you simply hit collect. You’ll often be changing the horizontal scale: to do this, click on any number on the horizontal scale and change it as needed. Don’t forget to change it back before taking more data. The default should start at 0 and end at 15 seconds.
9) Make sure that you only have one magnet, N pole side up, on the red spot on the platter. Adjust the position of the read head coil so that when the platter rotates, the magnet passes directly under the read head. ALWAYS ROTATE THE PLATFORM CLOCKWISE AS VIEWED FROM ABOVE. Slowly spin the platform clockwise as viewed from above. Press Collect or simply hit Enter and obtain the voltage vs. time graph as the magnet moves underneath the read head. I recommend that you start collecting data and then you move the magnet.
10) Change the horizontal scale so that you only see the interesting region (when the magnet was approaching, moving under, and moving past the read head. CAREFULLY sketch the pattern you observe in the space below. Make sure to label axes and place some numbers on them as well.
11) Repeat step 10, but this time, move the magnet a little bit faster. What is different about this graph compared to the one you sketch above? Repeat again, but again move the magnet even faster. Briefly comment on your results. There’s no need to sketch unless it helps with your comments.
12) Repeat step 10, but this time flip the magnet so that the S pole end is up. Again, CAREFULLY sketch the voltage vs. time graph from the data collection program in the space below.
13) Compare the sketches from step 10) and step 12) carefully. What is different about the two?
14) Return this magnet so that it again has the N pole facing up. Place a second magnet, N pole also up, in one of the adjacent blue spot as you go clockwise next to the first magnet. So now, you should have two magnets, both oriented with N pole up. Making sure you turn it clockwise, rotate the platter so that the first magnet and then the second magnet pass underneath the read head coil. Obtain the voltage vs. time using the data collection program (make sure you’ve set the time scale back to 0 to 15 s). Adjust the horizontal scale so as to show the region when the two magnets passed underneath the read head coil. As before, CAREFULLY sketch the voltage vs. time in the space below.
15) Repeat step 14), but this time have the second magnet have its S pole pointing up. Again, CAREFULLY sketch the voltage vs. time obtained from the data collection program in the space below.
16) Do this again, but this time have THREE magnets, oriented N N N. Then do it again, with N S N. Then again with N S S, etc. Try different permutations of the three magnets. Sketch the various patterns below.
17) Draw some general conclusions. What do you look for in a pattern that indicates that the N pole is up (ie that the binary digit is 0)? What do you look for in a pattern that indicates that the S pole is up (ie that the binary digit is 1)? What does the pattern look like if you have two N poles next to each other? Two S poles? A N pole followed by a S pole? A S pole followed by a N pole?