Simple Magnetometer Science Project Using Water as Lens

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A magnetometer such as this can detect small changes in the Earth’s magnetic field. It can be useful to monitor these changes as they may indicate the occurrence of natural phenomena that can influence our lives. Certain kinds of solar activity for example, have the potential to disrupt communications and power systems. Large earthquakes are also known to produce magnetic changes prior to their arrival. (This project was originally intended to work in conjunction with the search coil magnetometer project for predicting earthquakes.)

Using a light shining on a suspended mirror with a magnet attached to it is one of the simplest and cheapest ways to make a magnetometer. By shining the reflected light onto a distant wall, tiny changes are magnified and become much easier to see.

The design given here is an interesting variation that suspends the mirror in a bottle of water. There are several advantages to this. Firstly, the bottle and water form a lens which can focus the light into a thin line, which means that a very bright light is not necessary. A single high brightness LED is enough. It also means that at shorter distances, one can still see movement clearly. And since it doesn’t need to be so long, this allows us to build it as one tidy unit on a wooden base, in this case 40 cm long. One only needs to place it somewhere away from magnetic or iron/steel objects, and point it north.

Another advantage is that the water provides good damping of the mirror’s movement. It doesn’t need to be attached to a large piece of card to obtain damping from the surrounding air. A relatively small mirror still provides suitable damping. (Damping doesn’t mean “making it wet”. It means applying some friction to reduce its tendency to swing freely back and forth.)

Lastly, it is not influenced by wind or air currents as it’s in a bottle.

The focusing of the light from the LED looks something like this:


Cut the base board. This one is a piece of 12mm pine board 40cm long and 11cm wide.

Two triangular blocks of wood were glued onto the end to hold the screen. Small sections of aluminum were cut and glued on so the screen could be just pushed into the slot. Make sure there’s space for the battery holder.

Next I cut circular holes in some thin plywood and made two supports from 12mm square pine. The holes need to be a little smaller than the bottle and the supports have to be carefully measured so that the plywood just touches the bottle when its screwed on.

The supports and bottom plywood were just stuck on with PVA wood glue.

Next I got a small piece of mirror that could just fit down the neck of the bottle. A piece of plastic was folded over the mirror and a disc magnet each side was enough to hold it on. A length of sewing thread was tied on and pushed through a small hole in the lid. The thread was tied at the top to a small piece of plastic tube and adjusted so the mirror didn’t touch the bottom of the bottle.

It was then filled with water and a piece of modeling clay was stuck on the top in case water came out the small hole. Then the bottle was screwed into its frame.

The LED mount was made from blank printed circuit board. It doesn’t have to be made like this; it was just that I had some handy.

This one used two series resistors of 12 ohms each – one on each side of the LED. A single resistor close to 24 ohms would also be OK, such as 27. Again, it was just that I had those available. The copper on the board was cut away with a sharp knife, and wires attached from underneath, leading back to the battery pack.

Before doing the final wiring of the LED, it was necessary to hook it up temporarily to find the best focal point. The holes I drilled in the base PC board were elongated slightly to allow for adjustment later.


Candle wax was used to hold the wires onto the wood. Rubber feet from the local DIY shop stopped it from slipping sideways while turning the light on and off.

This is the scale used. It was printed 82mm wide and shows plus or minus 2.5 degrees deviation from the center. In my location, traffic on the street gives me regular deflections of 0.5 degrees for cars and up to 2.0 degrees for big trucks and buses.

When finished, the next challenge is to find a place for it that doesn’t have much iron around. Things such as metal chairs being moved around the room will make it impossible to take readings. Likewise, electronic gadgetry such as cellphones often have magnets in them that will send it crazy. To actually record events such as solar storms, one needs to note down the reading every half hour for a whole day. Any deviation can then be compared with a professional magnetometer on this website:

Possibly there are other ways you can think of to take readings, such as a webcam. There are also designs around for a computer interface using photo-sensors. The first step is to make one and try it out…..

Have fun!

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