The Beeb Body Build course 134
Mike Cook builds a Solar Magnetic Storm Telescope

Did you know that the Earth's magnetic field varies? I am sure that you know about compensating for magnetic north when taking map readings but did you know that its strength can vary over time periods as short as one hour? The reason for this lies far away, on the surface of the Sun. Now the Sun sends out a continuous stream of charged particles known as the Solar Wind and when these reach the Earth they can have measurable effects. The charged particles get caught up in our magnetic field and cause current to flow in the ionosphere, this current, amongst other things, causes small variations in the magnetic field we experience on the surface.
This ionospheric current has other effects as well, the most spectacular being auroras seen usually at the poles. However, large auroras can be seen as far south as the south coast of England. The other major effect of ionospheric current is the altering of radio propogation. What happens is that the ionosphere will reflect radio signals at higher frequencies than it normally does. This leads to disruption of local services as distant stations that normally can't be heard come through loud and clear. It also gladdens the hearts of radio amateurs and CB enthusiasts as it means that they can contact stations much further away. There are other factors that affect radio communication, the main one being tropospheric ducting due to high pressure weather fronts.
So if we could monitor the variations in the Earth's magnetic field we could be warned of these happenings as well as giving an indication of Solar magnetic storms.
Traditional equipment sensitive enough to measure magnetic fields is quite expensive but with a computer and a good junk box we can make a very sensitive magnetometer. The whole project hinges on the use of a compass needle, this is a cheap, sensitive indicator of the Earth's magnetic field.  Imagine another magnetic field placed at right angles to the Earth's by means of an electro magnet. As we increase the current in the electro magnet it's strength increases. When it matches that of the Earth, the compass needle will be being pulled equally in two orthogonal (at right angles) directions. The result is that the needle will sit exactly between the two at 45 degrees. Now at this point we can measure how much current we have to put into the electro magnet to achieve this field and hence we have a measure of the field itself. There are some further complications but I will come to them later.
The basic setup is shown in figure I, this uses a compass in a perspex mount filled with oil for damping. At 45 degrees a photo diode is placed under the compass and an Infra red LED placed directly above on the other side. To the East of the compass I placed the coil out of a 5 volt miniature relay, I removed the plastic cover and snipped off the moving parts to just leave the coil. The whole arrangement was placed on a piece of vero board with a small square hole cut out so that the photo diode can be mounted flush to the board. Hot melt glue was used to fix the compass to the board. I used the flexible type so that I can recover the compass after the project. Due to stray light affecting the reading from the photo diode it is best to place the whole thing in a non metallic box although it can be set up in the daylight.
The idea was that I would feed the electromagnet from the output of a digital to analogue converter (D/A) and measure the output from the photo diode using an analogue to digital converter. These two devices are conveniently situated on the Body build I2C interface we built in June 91, or you can use the Andi-Odule from Ian Copstake.
However, initial tests showed that I did not have much control just using the D/A as the needle swept through the detector with just a change in D/A output of one value. Therefore I had to reduce the size of each D/A step. I did this by feeding the bulk of the current to the magnet from a fixed source and a small part of it from the D/A. This meant that the magnet was always on but I had fine control over its strength.
The electrical circuit is shown in figure II, note that the A/D converter is working in differential mode, this helps nullify any mains pickup from the connecting leads.
As the compass needle did not fully cover the photo diode  light was always leaking round the side. This meant I needed to adjust the LED current so that the diode was not swamped with light while the needle was over it. Do this by monitoring the detector voltage with the needle out of the way and getting to the point where the voltage has just reached its minimum, then back it off a little so that it is just a few values larger. Note here that as we are using the differential inputs of the A/D you will get a reading of 128 when the detector is shorted out or the two inputs are at the same potential, as the needle covers the detector this value will rise. If it falls then you have got the two inputs the wrong way round so just swap them over.
The standing current in the relay coil should be adjusted such that you get the needle over the detector when the D/A is feeding in half power. Don't forget to align the whole arrangement north / south first with no current flowing.
Now what makes an instrument like this possible at all is the fact that we have a computer not only monitoring it but controlling it as well. The program MagMon is on the subscription/cover disc and is split into a setup part and a measurement part. The measurement part of the program does quite a simple job, sweep the range of magnet strength and note the number you are sending to the D/A at the peak. This takes a few seconds but it is not possible to track the peak, this is because of hysteresis in the core of the relay. You will see from the screen shot that there are in fact two peaks, one when the field is being increased and the other when the field is being reduced. This is because you don't get the same magnetic field from the coil for the same current, it depends on where the current has been. If nothing else this experiment is a perfect illustration of this effect. Certain types of material have different hysteresis characteristics this could be shown if you changed the coil's core. If you have an air core then there would be no hysteresis and so it would be possible to track the peak and get field readings more rapidly.
There is another cause of these twin peaks and that is a bit more complicated. The relay is very close to the compass needle and so the magnetic field is not linear, it turns out that you can get two equilibrium points for any one coil current. You can try this out yourself, just set the current to give a half way deflection of the compass needle and use a magnetised screwdriver to disturb it. You will notice that when this is removed the resting point is different depending whether you moved the needle towards north or the relay. Nevertheless this does not distract from the validity of the readings you get when sweeping the field.
The curves obtained are very repeatable, so much so that I have made each pass plot the curves in a different colour so that you can see something is happening. To minimise noise I take each reading 9 times and average the results, well I don't really take the average but let the computer's graphics drivers do it. That is each reading is summed and that gives a number big enough to use as graphics coordinates. Note that this just gives a smooth plotted value for the screen, the real field is indicated by the power being put into the magnet at the peak. Either of the peak values can be used as a measure of magnetic field and I have found it quite sensitive. Whilst working on the project I found that there were sudden large shifts in the curves, I finally realised that this was due to the different positions of my computer chair.
Calibration of the magnetometer would make an interesting addition to this project so you could tell exactly what variations in field you could detect. Remember over the short term the Earth's field is constant so you can use the instrument to measure weak fields as well. For example you can plot the effect of a magnet at various distances and see the inverse cube law that magnetic fields have. Because of its sensitivity to modern electronic equipment, if you are using this for field monitoring, you need to site it away from any source of interference, a good place might be in the roof space of your house or school.
If you do any modifications or changes I would love to here from you but until next month I will say goodbye.