The Bridge V Learning 2(clone) - Page 51

These plans will make a very very sensitive magnetometer. This magnetometer will go crazy if someone downstairs sets up an ironing board, or if a car drives near your house (within 50m or so!). If the car parks, the local magnetic field is permanently displaced and the magnet will not return to its original position until the car leaves. Turning a TV off nearby causes it to move a lot. Sometimes you will see unexplained magentic activity. It will also show fluctuations in the earth's magnetic field which are most obvious during geomagnetic storms.

A mirror is a piece of clear glass with a reflective coating. If you used the mirror as it is for reflecting the laser light, you would have problems with refraction as the light passed through the glass and it would be hard to get a reading of the magnetic field movements. For this reason we need to reveal the naked silver surface of the mirror. On the back side of the mirror will be a grey\brown coating. Removing this will reveal the surface of the mirror. The acetone will dissolve this coating. Once the coating is removed cut (using pliers or something and wear glasses!) the mirror into two small square pieces, the same size as the magnets.

Take a length of the fine nylon and stretch it out on your desk. Use the magnets to clamp onto the middle of the thread. This is easier said than done as the magnets are so strong the will slam (wear glasses, the magnets can disintegrate!) together and chuck off the thread. Once you've got the thread going between the two magnets take the two small pieces of mirror and glue them (glass) back to back just above or below the magnets. Use the rubber glue as this will not melt the fine thread and can be easily taken apart if you make a mistake.

Once you've got this set up which might take a while, thread the nylon through the bottom and tops of the jar, through the cotton bud tubes. Secure the thread at each end with blu tack and screw on the lid. Adjust the thread until the magnets and mirrors hang in the middle and so that the thread is tense.

Make a mount to hold the coin by making a roll of stiff card, like a business card and push into the hole you made in the jam jar. The idea is that the face of the coin is a few cm from the magnets and can be pushed in and out to adjust the damping effect.

At this point you are almost ready to go. You'll be glad after all that! You need to find a place which doesn't have much metal around or magnetic disturbances (easier said than done!!) and using a mount, point the laser at the mirrors. Hopefully the laser will reflect off the mirrors onto a nearby wall, from where you can see a dot that will move at the slightest magnetic disturbance.

The magnets will automatically align north-south. During a magnetic storm, the light on the wall will appear to drift, over the space of several minutes or hours back and forth. For example, in my room with the wall a couple of meters away it appeared to move over the space of 30 -40cm. This was during strong storms a few years ago.

If the light keeps jumping about too much (due to being so sensitive!) dampen the movements by pushing the coin closer.