Although the Earth’s magnetic field is reliable enough for navigation and is also essential for blocking harmful solar emissions and for improving radio communications, it’s not a uniform strength everywhere on the planet. Much like how inconsistencies in the density of the materials of the planet can impact the local gravitational force ever so slightly, so to can slight changes impact the strength of the magnetic field from place to place. And it doesn’t take too much to measure this impact on your own, as [efeyenice983] demonstrates here.
To measure this local field strength, the first item needed is a working compass. With the compass aligned to north, a magnet is placed with its poles aligned at a right angle to the compass. The deflection angle of the needle is noted for varying distances of the magnet, and with some quick math the local field strength of the Earth’s magnetic field can be calculated based on the strength of the magnet and the amount of change of the compass needle when under its influence.
Using this method, [efeyenice983] found that the Earth’s magnetic field strength at their location was about 0.49 Gauss, which is well within 0.25 to 0.65 Gauss that is typically found on the planet’s surface. Not only does the magnetic field strength vary with location, it’s been generally decreasing in strength on average over the past century or so as well, and the poles themselves aren’t stationary either. Check out this article which shows just how much the poles have shifted over the last few decades.
The hack here is not “measuring variances in Earth’s magnetic field”, it’s how to measure the magnetic field with nothing but a compass and another magnet.
It produces a measure of the Earth’s field, relative to the (presumed known) magnetic field from the test magnet.
That said, how do you get an absolute number for Earth’s field without knowing the field strength produced by the magnet?
Alternatively, how do you measure the magnetic field of the magnet? (there are lots of ways to do it, but none mentioned in the post)
Looks like magnets all the way down.
Helmholts coils, a good power supply, and a sensitive multimeter work well. Using a procedure similar to a Wheatstone bridge, a field measured by a magnetometer can be zeroed in different orientations to remove the Earth’s field influence. The Earth’s field field strength can be calculated using the physical dimensions and measured currents and voltages from the coil. Back in my college days, one of my senior design projects was a fluxgate gradiometer and I used that method to calibrate the fluxgates in the array to within 1% of a local lab’s calibrated sensor with some stuff from Radio Shack. Fun times and quite cheap.
It’s “Helmholtz”
That’s all standard stuff, but doesn’t address the actual problem: The OP “measures” Earth’s field by comparing it with the field from another magnet. So you’re left with the problem of knowing the what the field is from that magnet, and that one is highly variable depending on where you measure it.
It’s an interesting exercise, but made the question of measuring Earth’s field much harder.
Yes, I know it’s Helmholtz. S is close to Z and my autocorrect doesn’t appear to like either. Easy to miss.
As for magnet-only measurements, I’ve seen some methods described in old physics books of mechanically measuring the torque of a few magnets on the ends of long pivots against each other under bell jars in various orientations to minimize external influence and determine deltas and then use that to derive an absolute value. Now that I think of it, that might be a fun exercise for my 10th grade physics class.
You seem pretty knowledgeable for a 10th grader.
;-)
@Bryan Cockfield said: “To measure this local field strength, the first item [efeyenice983] needed is a working compass. With the compass aligned to north, a magnet is placed with its poles aligned at a right angle to the compass. The deflection angle of the needle is noted for varying distances of the magnet, and with some quick math the local field strength of the Earth’s magnetic field can be calculated based on the strength of the magnet and the amount of change of the compass needle when under its influence.”
Instead of that, will this work? So Earth’s magnetic field at its surface ranges 0.25 to 0.65 Gauss. That translates to 2,500 to 6,500 Tesla (1 tesla = 10,000 gauss).[1] Today, inexpensive MEMS (Micro-Electro-Mechanical-System) devices for detecting and measuring magnetic fields (magnetometers) can measure magnetic fields down to less than +/- 1 Tesla.[2] For example, the MMC5603NJ 3-AXIS AMR (Anisotropic Magneto-Resistive) magnetic sensor made by MEMSIC Semiconductor Co., Ltd. in China [3][4] can measure down to +/- 3 mT (milli-Tesla) with a dynamic range of +/- 30 Gauss and 20 bits of resolution, plus it costs only $0.67 each in unit quantity with 17,377 currently in-stock @ Digi-Key.[5][6]
References:
[1] Tesla (T) to Gauss (G) Converter Calculator
https://converterr.com/magnetic-flux-density/tesla-to-gauss
[2] MEMS Magnetic Field Sensor
https://en.wikipedia.org/wiki/MEMS_magnetic_field_sensor
[3] MEMSIC Semiconductor Co., Ltd.
https://www.memsic.com/
[4] MMC5603NJ @ MEMSIC
https://www.memsic.com/magnetometer-2
[5] MMC5603NJ @ Digi-Key
https://www.digikey.com/en/products/detail/memsic-inc/MMC5603NJ/10452796
[6] MMC5603NJ Datasheet (PDF) @ MEMSIC
https://www.memsic.com/Public/Uploads/uploadfile/files/20220119/MMC5603NJDatasheetRev.B.pdf
Kinda got your Tesla-Gauss conversion backward. 6,500 Tesla is a hundred times stronger than any (static) field humans have ever created.
If one insists on changing to some preferred unit, one really should have a good idea of what is a sane range for those preferred units!
Earth’s field is commonly quoted in nanotesla, maybe to make the numbers nice integers: Typically 45 000 +/- 20 000 nT. Often just the variance from the local mean is quoted.
Earth’s field is a big deal for space weather observations: https://www.spaceweatherlive.com/en/auroral-activity/magnetometers.html
And Hamsci plays with it too: https://hamsci.org/magnetometer
It’s nice to do it from first principles, but you can make a fantastic device to record the changing earth’s field using a $3 MMC5983 sensor and a microcontroller (e.g. STM32 Nucleo) with I2C interface.
I’ll have to do a public write-up of the project one day.
Please do!
fun fact: there’s been work on using the magnetometer in your phone to measure the magnetic fluctuations for indoor localization. MGPS.
https://europe.naverlabs.com/blog/magnetic-sensor-based-localization-and-deep-learning/
0.49 Gauss is 49 microteslas