DIY Torsion Magnetometer

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By Alexander Avtanski
Images by the author

The magnetic activity of the Sun shows cyclic variation with a period of about 11 years. The next solar maximum is predicted to be around the middle of year 2013 and things are starting to heat up. Solar events such as coronal mass ejections (CMEs) affect Earth's magnetic field and can cause magnetic storms, so if you are interested in capturing a magnetic storm or two now is a great time to build your own magnetometer.

There are a number of different magnetometer designs – each with its advantages and disadvantages. For DIY projects, a magnetometer of torsion design is often the best choice – it is cheap and relatively easy to construct while providing excellent sensitivity and reliability.

The following diagram illustrates how a torsion magnetometer works:

[http://avtanski.net/projects/magnetometer/media/torsion_magnetometer.png]

A mirror with an attached magnet is set hanging from a thread (torsion spring). Light is bounced from the mirror and position of the reflected spot is recorded. The changes in the spot position correspond to changes in Earth's magnetic field direction.

Variations in Earth's magnetic field are usually very small and the movement of the reflection is just a few millimeters. Therefore, it's important to make sure the mirror is enclosed and protected from air movements. Vibrations can also affect the mirror – to reduce the effect of any such disturbances a damping mechanism is necessary.

After a couple of months of experimenting I got to a design that works pretty well:

Torsion magnetometer

 

 

[http://avtanski.net/projects/magnetometer/media/overview.png]

The main building material here is PVC pipe - it is easy to find, comes in all sizes and shapes, and it is simple to work with.

The mirror cell is constructed from a small magnet (even a shower curtain magnet will do) with two round mirrors balanced on each side. (Two mirrors are recommended instead of just one, because this makes the mirror easier to balance, and provides the option to position the magnetometer body either to the North or to the South of the magnet.)

Damping is provided by filling the lower part of the mirror container with liquid (usually water, but if you live in place where temperatures are below freezing and you plan to keep the magnetometer outside – antifreeze, alcohol, or even light oil). The liquid in kept in the container by a couple of plexiglass windows capping the side of the T-junction:

Mirror assembly

 

 

[http://avtanski.net/projects/magnetometer/media/overview_mirror.png]

(Alternative approach is to use magnetic damping, but I was not able to make this work to my satisfaction.)

The light is emitted from a laser, which is mounted together with the light sensor in the other end of the magnetometer. The sensor can be as simple as a couple of photoresistors positioned behind a semitransparent diffusion screen:

Laser/Sensor assembly

[http://avtanski.net/projects/magnetometer/media/laser_assembly.png]

[http://avtanski.net/projects/magnetometer/media/laser_assembly_screen.png]

The light spot bounced from the mirror is diffused by the screen – depending on the position of the light spot, one or the other of the photoresistors receive more light.

My magnetometer has been operating almost without interruption from December 2011. It has proven pretty reliable and requiring little maintenance. Hooked to a computer, it has been recording the changes in the Earth magnetic field for almost a year now.

Minor magnetic storm on July 15, 2012.

[http://avtanski.net/projects/magnetometer/media/2012_07_15_06_00_00.png]

Complete details regarding the construction of my magnetometer, pictures of each of the components, and near real-time data of the magnetic field fluctuations can be found on the project website, http://avtanski.net/projects/magnetometer.

This entry was posted in Astronomy, Experimentation, Instrumentation, Magnetism, Measurement, Physics, Projects. Bookmark the permalink.

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