Experiment – Long Period Magnetometer, Part 2

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By Timothy Raney…Bald Engineer Guy with Glasses

Last week, we discussed the theory very briefly. Now we’ll cover the equipment and procedures.

Long period magnetometer, laboratory stands, electric rotator and its DC power supply, clamps and a 3.2 kilogauss (0.32 tesla) alnico test magnet. Other items included gaussmeter (Model DCM from AlphaLabs, Inc.), laboratory jacks, a ~5mW red laser pointer and clipboard with graph paper. I calibrated the test magnet previously with the DCM gaussmeter – it produced a ~2 gauss flux at 1.8m. The gaussmeter was accurate to within +/- 2% per its specification sheet – its calibration is also traceable to the National Institute of Standards and Technology (NIST).

The procedures consisted of first placing the magnetometer on a stable horizontal surface free of ambient mechanical vibrations. Passing traffic or even walking nearby can transmit vibrations to the magnetometer. Achieving a “vibration free” environment is problematic. This fact is true for a heavy workbench on a concrete floor or similar situation. However, we can minimize the effects of ambient vibrations – a non-magnetic heavy mass on some kind of vibration-damping material works well. In this case, a 1.25” thick aluminum plate rested on a 1” thick dense foam rubber mat – this assembly reduced ambient vibrations to a manageable level.

Another option is a granite slab, e.g., a machinist’s surface plate on silicone rubber feet or 12” x 12” granite floor tiles glued together with silicone sealant. The point is to make a relatively massive base to dampen vibrations. I now have a ~50-lb. granite base purchased as “scrap” from a local headstone company – it works very well. The laser pointer was then mounted in a burette clamp on a laboratory stand and adjusted so its beam spot was centered on the magnetometer’s mirror. The trick here was to hold an index card in front of the mirror – it makes the beam spot easy to see and simplifies the alignment procedure. Otherwise, it is difficult to see the laser spot as it impinges on the mirror.

Importantly, the magnetometer sensor assembly vibrations were allowed to dampen-out over several minutes. If time is an issue, one can hold a piece of copper scrap near the sensing magnet – the induced eddy currents per Lenz’s law will dampen the oscillations much sooner. After performing these procedures, the reflected beam spot shows-up on the graph paper attached to a clipboard. I used a laboratory rotator to rotate the test magnet at ~125 RPM to produce a consistent time-varying magnetic field. The rotator was originally a laboratory stirrer. Any low speed gear would work too. Lastly, the test magnet was placed at a given distance from the magnetometer. The maximum separation between the magnetometer and test magnet was selected based on experience. In this case, I was fairly certain the magnetometer would not deflect at six (6) meters given the test magnet’s flux. Though this part of the process is largely trial and error given a newly built magnetometer with unknown characteristics.

Next week, we’ll conclude this paper by discussing the experiment and its spell-binding conclusion. See you next week!


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