All classical magnetometers have their origins in the simple compass needle. As such, they were capable of showing changes in the direction of the magnetic field, but could tell us nothing about changes in the strength of the field. The invention of this "nulling magnetometer" is an ingenious solution to this problem
Imagine you are sitting at a river mouth. The tide has run out and you are watching the current flow out to sea. You spy a small boat that has broken free from it's mooring upstream. You watch as is lazily drifts down the river...
Although not strictly true, you can imagine how a compass needle is drawn northwards by the lines of magnetic flux that surround us to point north.
Now imagine that the tide has turned. The flow of the river begins to slow down, as it pushes against the increasing resistance of the incoming tide. You notice the the small rowboat from earlier on. Stuck between the river current and the incoming tide, it's no longer drifting downstream. In fact, it seems to be moving back upstream
This particular type of magnetometer works in the same way. The needle or magnet is balanced between the "flow" of the Earth's magnetic field and the repulsing force of the nulling magnets. However if you observe this device you will see that the needle does not sit still, it will shift and change in direction. As the nulling magnets provide a constant field, the changeability can only be a result of variations in the Earth's magnetic field. In fact our compass is now responding to changes in field strength, not field direction.
How so? If we use the river current and tide analogy, then you can imaging that a sudden surge of water coming down the river would help push against the incoming tide and give our runaway rowboat an extra shove out to sea.
Like many scientific discoveries, several people were mulling this idea over. The earliest implementation was devised by British scientist Samuel Hunter Christie, and consisted of a compass needle balanced between nulling magnets. However it was the renowned German polymath Karl Gauss who developed and publicised a different form of this device. He was also responsible for promoting and setting up a system of geomagnetic observatories across much of Europe.
For our purposes, we will use Christie's design. It's simple construction is easy for amateurs. We also avoid the technical and mathematical setup of Gauss's device (Gauss refined the construction and mathematics of the bifilar suspension, commonly used in magnetometers of the 19thC). It is also very sensitive. Gauss showed such devices to be seven to ten times more sensitive that the standard declinometer.
Converting a jam-jar magnetometer is the easiest way to try out this device. All you need are the nulling magnets. I used cheap magnets from a hardware store. Mount them with the same pole out. I used cardboard and double sided tape.
The setup is easy enough - Set up your jam-jar magnetometer in the usual way, but align the window you sight the magnets through E-W instead of N-S. You want to place your nulling magnets north (or south) of the magnetometer. Slide them towards the magnetometer until it’s sensing magnet faces E-W. In this position the sensing magnet will be most sensitive to changes in field strength. This can be a little fiddly, so experiment with the placement of the nulling magnet. DIAGRAM OF CHRISTIE CONFIGURATION
You will discover that this configuration is also very sensitive to small changes in position of the components - to this end you might decide to make something a little more robust like the following design:
This device can be read in the classical way by bouncing light (laser pointer) off a small mirror mounted on the magnets, onto a scale on the wall.
If you know how to use a hobbiest micro-processor platform like an Arduino or Raspberry Pi, then the addition of a cheap 3-axis magnetometer sensor allows you to measure the orientation of the sense magnet in your magnetometer. To get data into a PC you'll need record log files (The Arduino can use it's USB cable to talk to a PC and you can use datalogging software like Gobetwino). A Rasberry Pi also can ustilse a serial connection to talk to a PC.
A more tradtional approach is to use simple electronics to measure the magnet position in some way.
The above magnetogram was recorded over 3 days from the 30th April and shows diurnal variation. The homebrew magnetometer is the purple graph and underneath is a magnetogram recorded by professional equipment located in Hobart. This is a tidal effect on the magnetosphere caused by the sun and moon as the earth rotates. You can also see the homebrew equipment records the disruptions in the magnetic field for the 2nd/3rd
This magnetogram is a recording of a modest aurora. You can see the Sudden Storm Commencement as the sharp upswing in the graph.