Sorry that I’ve not posted in a week, but my laptop has been in the shop, getting the hard drive replaced. The destruction of the old drive happened last Sunday in a physics lab I was doing with my son. We were planning to measure the magnetic field of a coil as a function of current, distance, radius, or number of turns (but probably not all those variables, as that would have gotten tedious). We weren’t getting a noticeable reading from the magnetometer, so I got out a neodynium magnet to see if the magnetometer was working. It was—in fact the magnet saturated the magnetometer easily. When I went to turn off the program that was streaming data from the magnetometer, I carelessly put the magnet down on my laptop—right over the hard drive. That mistake turned out to be an expensive one.
The computer continued to work that day, but the next time I tried to restart it, it wouldn’t boot up. I took it down to the local computer shop on Monday, where I found out that the extended warranty had expired 6 months ago, so I had to pay labor as well as materials for replacing the disk, which ended up costing me $376 (as long as they were replacing the drive, I had them upgrade from a 500GB drive to a 1TB drive, since I was running out of disk space). When I finally got the computer back on Saturday, I spent most of the day restoring the system from my backup drive. Now I need to replace the backup drive, since Time Machine complains that the old drive does not have enough space to do a full backup. It looks like that will cost me another $100–$150 for a 1–2TB backup drive. I don’t have many choices of drive, since I need a Firewire 800 interface (my old MacBook Pro does not have USB 3 or Thunderbolt). So my moment’s carelessness cost me the use of my laptop for a week and about $500.
After having confirmed that the magnetometer was ok, we did a rough calculation of how strong the field from the coil should be, to see whether we ought to be detecting. (I know, we should have done that first.) We were running about 33mA through a coil of 5 turns with a diameter of about 4.4cm. Using the formula , with I=0.033A, R=0.022m, and N=5, I computed that the magnetic field right at the center of the coil (z=0m) should be 4.7µT, at 1cm (about as close as I could get the magnetometer) it should be 3.6µT, and at 2cm (where the measurements were being attempted) the field should be about 1.9µT. The magnetometer has a resolution of 0.1µT per count, but the noise level was high enough that counts of 20 (2µT) would have been barely detectable. I suspect that a lot of the noise was because we had not immobilized the magnetometer. According to the World Magnetic model, as displayed in Wikipedia, we should have about 49µT at a 60° inclination due to Earth’s field, so changes in orientation of 1° in the magnetometer would causes changes of about 0.9µT.
We’ll repeat the experiment (without having a strong magnet near the laptop!) using more current (say 300mA), more turns (40), and a smaller radius (a diameter of 1.25cm). With those values, we should be able to get a field of 1.2mT at the center of the coil, 180µT at 1cm, 32µT at 2cm, and 11µT at 3cm. We’ll also immobilize the magnetometer in my plastic-jawed Panavise, and make measurements by subtracting the field with the current off from the field with the current on. We may even double the signal by subtracting the field with the current in one direction from the field with the current in the opposite direction.