Magnetometers

This page is very much a work in progress!!

The Earth is surrounded by a distorted donut-shaped magnetic field called the magnetosphere. This field is caused by the rotating liquid metal core of the Earth. As this magnetic field expands out into space it meets a constant stream of charged particles that originates from the Sun called the Solar Wind

The sun also experiences storms on it's surface called solar flares. These eject charged particles. When these hit our magnetosphere they cause fluctuations in it's strength and also, more noticably, cause Aurora to happen.

By using a magnetometer you can notice the sudden fluctuations in the Earth's magnetic field and know that there is a good possibility of an aurora being present. You can also see the daily changes in the magnetic field that are caused by the sun.

So, a magnetometer is an instrument used to investigate a magnetic field, which can be considered as a force that has the qualities of strength and direction. A magnetometer can be used to determine these particular qualities of a magnetic field, and make them visible to an observer. 

Introduction

The original inspiration for this project was a Sky & Telescope article on a "jamjar magnetometer". Consisting of a suspended magnet with an attached mirror, it was capapable of showing fluctuations in the local magnetic field that might indicate an aurora.

In many ways this project is about resurrecting the technology of more than a century ago. Many of the techniques, ideas, and theories are to be found in the scientific instruments of the 19th century. 

It is a natural conceit to think that technology from yesteryear is less sophisticated than technology of today. In some cases this may be true, but often what we take to be an advance is simply just a different way of doing the same job. Often it seems an improvement because it gets around some issue that an older method might have possessed, but eliminates one set of problems by replacing them with different ones. 

An example or two of this sophistication can be found in such instruments as the Moll Galvanometer or the Kater Pendulum. The Moll galvanometer movement was so sensitive that the user could see deflections on the scale caused by brownion motion of air molecules acting on the galvanometer suspension. The Kater pendulum was a device used to determine the local gravitational constant.

Constructing Magnetometers.

I've made 4 versions of this magnetometer, with varying degrees of success. 

Number 1
This was a "jamjar" version. The magnet was the magnet and the iron surround from an 8 ohm speaker, suspended in a fizzy-drink bottle. I didn't have a laser pointer at that stage and used a penlight torch with average success. 

Number 2

This was made when I discovered how powerful the magnets from old hard drives were! The suspension fibre was made by taking a short length of polyester string, untwisting it and splitting off thinner and thinner threads. This did not use the torsion of the fibre so it was effectively a sensitive compass needle. The supended magnet was housed in a wooden box that had an open front and back. Each opening was covered up with a sheet of mylar (used to photocopy transparencies for use in an overhead projector) that was stuck on with blu-tack, making it easy to get to the working parts. This magnetometer used liquid dampening, and the mirror was a segment of the hard drive platter. It had a mirror-bright surface and was very light!

Strangely enough I was never able to really see the daily changes in the magnetic field due to the sun, as it appears to move across the sky until...

Number 3
No 3 was just a variation on No 2. I made use of a thicker suspension fibre and twisted the fibre to align the magnet east-west. In this setup the magnet is most sensitive to changes in the strength of the magnetic field. The following graph is a days reading (From approx 9am to 9pm) and shows the "hump" that occurs as the sun appears to "drag" on the earth's magnetic field.

The suspension fibre was 0.4mm monofilament fishing line, which turned out to be not such a good choice in the end. It was as though the torsion in the filament would gradually "relax" over the course of time, requiring the fibre to be twisted up each day. This was an obstacle to any long term monitoring of the magnetic field. (As an aside, I originally bought the monofilament for making a simple dulcimer - this stuff seems to stop stretching when it gets near it's breaking point)

If the filament needs to be near it's breaking point in order to show true torsion, then maybe a thing metal filament might better.

Number 4

When I was thinking about the complexities of the design, I decided to use the No2 Magnetometer and make a mockup tunnel from cardboard. The biggests drawbacks were alignment of all the components and getting enough contrast so that the webcam could pick up the scale through the telescope.  So, after some pondering, I decided to go back to design No2 but revamp the case for it, and try to incoroporate the laser pointer with a webcam.  

Making the Magnetometer.

Appendix 1 - Some thoughts on analogue magnetometer design

Some basics... 

If a detector is constrained to rotate about one axis then it will react only to that component of the force. So for example, a hanging magnet rotates about the x-axis and so it will really only show the x-component of a force. 

So far I have found three ways that the strength can be determined with a suspended magnet.

Suspension Design. 

We're going to come across the word torsion in talking about the suspension filament. From what I've been able to read about, it's also a term that is used rather loosely. Take a weight and tie it to a piece of string. Hang this from somewhere. The weight will spin around, eventually oscillating from clockwise to anticlockwise and back again until eventually it settles down. A state of equilibrium  has now been reached between the natural elastic nature of  the string to remain "twisted up"  and gravity (acting through the weight) that has tried to "untwist" the string.  If we twist the weight on the string one rotation and let it go, it will eventually pivot back to it's starting position. The external force we have applied to the weight is being countered by the internal stiffness of the fibre. This internal, resisting force is the torsion of the fibre.

Now there are some designs for magnetometers on the net that are called torsion magnetometers that make no use of this property at all. Or they refer to torsion, yet the design does not utilise this property of the suspension fibre. 

In Design (a) the magnet hangs from an extremely fine fibre that has very little internal torsion. The magnet moves freely about the vertical axis in response to changes in the direction of the magnetic field, as the internal torsion of the fibre is weak in comparison to the strength of the magnetic field. The magnet is least sensitive to changes in the strength of the magnetic field, but most sensitive to the direction of the magnetic field

In Design (b) the magnet hangs from a thicker fibre that has a higher level of internal torsion. The fibre can be twisted about the vertical axis, dragging the magnet with it. When the magnet points east-west rather than north-south it is most sensitive to changes in the strength of the magnetic field (i.e: the internal torsion of the fibre balances the twisting force of the magnetic field). A slight increase in torsion will suddenly overcome the balancing force of the magnetic field and the magnet will flip about to line up with the north magnetic pole. 

Design (c) is a bifilar suspension. The stiffness of the suspension can be adjusted by changing how far apart the fibres are. 

In Design (d) the magnet is connected to a suspension fibre with very littel internal torsion, that is held taught in the vertical axis. The magnet still moves freely about the vertical axis, but is resistant to other indirect forces. This makes for a robust suspension. Provided the magnet is balanced, then changing the vertical inclination of the fibre will have no effect on the balance of the magnet. This suspension can work in the horizontal axis and function like a dip needle. 

Magnet

The deflections of the magnet can be as tiny as 5' of arc. The traditional way of making such minute variations visible, is to attach a mirror to the magnet and reflect a beam of light onto a far wall, creating what is called an "optical lever". If you can imagine the beam of light is a giant pointer and the distance from the mirror to the wall is 2 metres, then if the magnet twists a quarter of a degree, the end of the pointer will move a distance of 8mm. In fact it will be twice that because if a mirror is rotated x-degrees, then a beam of light bouncing off the mirror is rotated 2x-degrees. So the end of our "pointer" (the spot of reflected light on the wall) will actually move 16mm. 

There are a couple of caveats to this design. The light source must be sufficiently bright in order to be seen clearly. Possibly the room the magnetometer is in must be darkened to make it visible. Small laser pointers can do the trick here.

A variation on this is to have the mirror reflecting an image from a scale to a small telescope. The amplifying factor is then a result of the magnifying power of the telescope.

The optical lever is extremely sensitive to vibrations and physical disturbance, so the magnetometer needs to be on a solid footing of some kind. 

Appendix 2 -  The Kew Magnetometer

Bibliography.

Laboratory instruments : their design and application. by Elliott, Arthur,  London : Chapman & Hall, 1959.  2nd ed.
A manual and handbook originally published in the 1930s that goes into good depth about the construction and theory of "analogue" instruments. This was found in the stack of the local library.

Sky and Telescope
Magazine article that discusses the construction of the prototype "jamjar" magnetometer.

Magnetic Compasses and Magnetometers,   Hine, Alfred,   London: Adam Hilger Ltd ,  1968.
The best, most in depth book that I have come across that deals with the mechanical specifics and mathematics of mechanical compass and magnetometer construction. Full explanation of the design and use of the Kew Magnetometer. Includes chapter on inductor instruments (ie: flux gate compasses and magnetometers.) Many line drawings and references to papers on construction and design.. Reads very much like Sidgewick's Amateur Astronomers Handbook. This book is must read. 

This site last revised 6^{th :-)} Jan 2011