Recording Earth field magnetometer

Results

The traces reflect the filtering that occurs when multiple samples are aggregated, and the average taken — in effect this tends to smooth the readings out. When single sampling was tried in this location, the readings, and subsequent traces are very spiky, and it becomes difficult to see trends. In particular, passing traffic becomes much more obvious, as do discrete events in the vicinity of the sensor. In effect, it increases the sensitivity of the instrument.

Figure 2: Moved Avo 8 multimeter

Moved Avo shows the effect of nearby magnetic sources being moved - in this instance, an Avo 8 multimeter was moved slightly about 4 feet from the sensor. The dramatic effect is easily visible as a step in the output waveform.

Figure 3: Minor solar event

The sharp dip at 19:20 was recorded as possibly being the start of a solar storm, but proved to be a nothing more than a 'blip'.

Figure 4: Car moving and domestic noise

This shows the effect of a Ford Ka being moved from its usual parking space in front of the house (about 100 feet from the sensor), and then replaced later. From the trace you can see the car made three journeys - one from approximately 08:00 to 08:45, another from 11:20 to 13:15 and a final journey from 15:15 to 16:30. Interestingly, an apparently larger Land Rover Discovery was parked in the same place during the period that the Ka was absent, but due to being made of aluminium, doesn’t show on the trace.

Teachers’ notes

Safety issues

Normal safety precautions should be observed.

Time required

Ideally this experiment should be left running for an extended period of time.

Target age groups

Age range: 16+
Science Key Stage(s): 5+

Curriculum comments

Whilst not having an obvious place in GCSE, AS/A2 or AVCE curricula, some students may be interested in the use of this instrument to detect archaeological artefacts or to see how solar flare eruptions affect the Earth’s magnetic field, its weather and radio reception. At degree level this instrument’s use would be more widespread, utilising its capacity as an extremely sensitive detector and recorder of minute changes in magnetic field strength. Additionally the construction, operation and calibration would be of interest and use to those embarking on post–16 electronics and electronic engineering courses.

Appendices

Mechanical construction

Sensor head

As has been discussed previously, the sensor is sensitive to temperature, but the sensitivity can be reduced with a combination of thermal inertia and insulation.

A temperature-sensing device is placed in contact with the FGM-3. The sensor assembly should then be suspended in a vacuum flask so that the two sensors are positioned centrally. Dry sand is then poured into the flask, which should be tapped very gently to settle the sand around the devices.

Use a piece of shaped polystyrene in place of the stopper to retain the sand. The connections can either be brought out along the side of the stopper, or through a prepared hole in the middle.

A box should be constructed from thick polystyrene insulation sheets and tape, with inside dimensions that will comfortably accommodate the thermos flask. When constructing the polystyrene box all the edges should be taped inside and out to make them sand-proof, and for strength.

The additional components, e.g. voltage regulator and decoupling capacitors that are required at the sensor head can be housed in a small plastic box which might include an LED indicating that the supply is reaching the unit.

The case can be secured to the sensor head with double sided servo tape to prevent the cables connecting to the FGM3 and internal LM35 from being accidentally tugged free.

An outline diagram of the instrument

Figure 5: Circuit diagram of magnetometer

The manufacturers data sheets should be consulted in conjunction with this diagram for additional components, eg. de–coupling capacitors, etc.

Using Excel to derive the calibration factor

The calibration formula in the form ‘y=mx+c’ may be derived as follows:

1. Allow the system for run for a significant number of samples, eg. 500
2. Use the view spreadsheet facility in PicoLog to display the output from the f/v, and the output from the temperature sensor
3. Copy the figures to the clipboard and paste them into a blank spreadsheet in Excel
4. Using the graphing facility, create an ‘xy (scatter)’ graph with the temperature on the horizontal axis, and the output from the sensor on the vertical axis
5. Add a trend line (Chart > Add Trendline)
6. By right–clicking on the displayed line, the options to format the trendline are available, including showing the formula, under ‘Options, Display equation on chart’
7. The equation will be in the form: y = 2.6022x - 101.80
8. Go into PLW Recorder, and then ‘Calculated Parameters’
9. Add a parameter and name it, ‘temperature corrected’
10. Select the output from the sensor as one parameter, eg. ‘A’, and the output
11. from the temperature sensor as the other, eg. ‘B’
12. Add the correction formula, eg A-(( 2.6022*B) - 101.80)
13. Select the additional data for display - this can be multiplied by the previously derived nT / mV figure for display in nT if desired, for example : (A-(( 2.6022*B) - 101.80)*1.6)
14. The displayed output shows deviation from the nominal ‘norm’ during the course of the sampling run.