# Gas station without pumps

## 2012 November 20

### Physics lab with magnetometer

Filed under: home school,magnetometer — gasstationwithoutpumps @ 16:10
Tags: , , ,

Today we did not go over homework problems in our weekly physics time, but did a lab instead.  I rewrote the code I had for using the Freescale MAG 3110 magnetometer chip on the Sparkfun breakout board, so that I could zero-out the measurement for easier differential measurement.  What we did then was to make a big loop of wire (26 gauge magnet wire—the same wire we used for our Young’s modulus experiment) from the floor to the ceiling beams, holding it to the floor with a couple of books, so that we had a straight vertical wire in the middle of the room, several feet from other wires.  We put the magnetometer near the wire, recording the distance and the orientation of the sensor, then zeroed the reading and measured the magnetic field 3 times. We then put a current through the wire (about 200 mA), measuring the current and the magnetometer reading (again, 3 sets of x,y,z values).  We did this for several distances and magnetometer orientations.

We also did one pair of measurements without the wire, zeroing the sensor in one orientation, then turning it around 180° and making another measurement.  This should give us an independent check of the units that the sensor readings are in, since we expect this measurement to be twice the strength of the Earth’s magnetic field (according to the World Magnetic model, as displayed in Wikipedia, we should have about 49µT at a 60° inclination, so the horizontal component should be about 24.5µT, and our measurements should show about a 49µT difference).  Of course, the rotation may not have been an exact 180° horizontal rotation, so we can’t really use this measurement to calibrate the sensor.

The manufacturer’s data sheet claims that the resolution is 0.1µT, but we recorded the sum of 10 successive readings, so our units are 0.01µT.  The repeatability of the measurements was not too bad—probably around ±0.5µT (I’ll have my son compute standard deviations for each set of 3 readings).  We only measured out to 5cm from the wire, since at that distance the field we were measuring seemed to be buried in noise.

My son’s task is to take the recorded field measurements and plot field strength as a function of distance for the measurements we made (probably correcting for differences in current, if those are large enough to matter).  He should also compute the expected magnetic field around a long wire for that field. There are several measurements at one distance, as we tried to verify that we were reading the orientation of the chip correctly—that distance might be a particularly good one for comparing the measured and computed field strengths.

Next week, we’ll try to do a little homework comparison, but we’ll also do a lab winding a helix of wire and measuring the field around it.  We’ll use the computational problem (18P79) to compute the expected field in different places, and try measuring the wound solenoid in corresponding locations.  This means that in setting up the program we’ll have to make the number of turns, the radius of the solenoid, its length, and the current through the solenoid all easily changed, to match the simulation to the coil that we wind.

## 1 Comment »

1. […] week we did measurements of the magnetic field around a single wire, and I had planned to “do a lab winding a helix of wire and measuring the field around it.  […]

Pingback by Magnetic fields with no lab « Gas station without pumps — 2012 November 27 @ 15:21

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