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Name: Loretta M.
Status: educator
Age: 50s
Location: N/A
Country: N/A
Date: 6/30/2004

Magnetic Blankets use 1000 Gauss barium Ferrite Magnets under a woollen under-blanket. This is the flux measurement on the surface of the magnet. A few centimeters away from these magnets there appears to be very little magnetic effect (I understand that flux works according to the formulae of volume - strength 4/3 pi r cubed. A DC Gauss Meter is $5000 plus and this is out of my budget. Is there any way for me to measure the topography of a magnetic blanket with typical 2cm 1000 Barium Ferrite Magnets as used in Magnetic Therapy Blankets.

Depends on what resources you have to work with. If you have access to a voltmeter with few-millivolt resolution, you could just buy a Hall probe. You can find several by searching the Internet for the phrase "hall probe". (I found one for $250 at Also there are hall-probe chips, which put out a voltage proportional to field. These are less expensive, but you have to provide a (typically) 5V power supply (four LiMH batteries puts out 4.8 volts, which should be fine) and a voltmeter.

Tim Mooney


Magnetic sensor IC's are rapidly becoming more available. An amateur-grade DC magnetometer should no longer cost $5000, maybe not even $500.

Linear magnetic sensor chips go down to about $10. (It must say "Linear", not "switch" or "latch".) So if you can wire up some 3-pin electronic devices for yourself, it gets very inexpensive.

I recently bought and tried one, a Melexis MLX90215LVA-BC03 Precision Programmed Linear Hall IC, rated 10 milliVolt/milli-Tesla. (Go to , Parts search "mlx90215".) It has been nice and useful, although I wired it backwards the first time, and degraded it's repeatability. Be prepared to lose maybe one device, while you get familiar with using it. I have the chip mounted on the end of a stick, pretty much a personal magnetometer probe. It is just a 4-pin plastic transistor package, only 3 pins used. (It is supposed to be re-programmable, but we do not want to mess with that. Never use the 4th pin: "pin2", "Test". Cut it short, about 1/8 inch, and insulate it. ) So it is about as easy to wire up as sensor chips get. You only need a 9v battery, a 5v regulator (78L05), and a low cost digital voltmeter ($5-$30).
  +5v      |       |            
  regulator| 78L05 |             
           |       | 
    on/off |_1_2_3_|  
     switch  | | |         (+5v)  __________                     
 (+)---/ *--/  | \----*-----*-----|1   MLX  \    voltmeter    
|    |         |      |           |   90215  |  ___________   
| 9v |         |  [10K-ohm]   cut-|2  -BC03  | |           |  
|batt|         |      |           |  magnetic| | [#1.999v] |  
 (-)-----------*------)-----*-----|3  sensor | |           |  
               |      |   (0v)    |     IC   | |   D.V.M.  |  
           [10K-ohm]  |       |---|4_________| |           |  
               |      |       |----------------->(V)       |  
               -------*------------------------->(Com)     |  
This circuit reads 1 count or 1 mV per gauss. It reads 0.0V in zero magnetic field. A 1K-ohm potentiometer, added between the 10K-ohm resistors, would allow adjusting the zero-field reading to 0.000v.
There might be someone on the Internet providing this as a completed meter or kit. Sorry, I have not done that shopping yet. I would search for combinations of words including: magnetic, probes, sensor, Hall, Hall effect, GMR (Giant Magneto-Resistance), magnetometer, kits, inexpensive, educational, amateur, hobbyist.

What the chip does: At zero field strength the output voltage hovers exactly half-way between the (+) and (-) supply voltage. For a 5.000v supply that is 2.50v. A south-pole placed against the top surface will make the output voltage increase by 10mV (0.010v) per mT (10 gauss or oersteds). The full-scale range is about 2v, either up or down, so the sensor can measure up to about +/-2000 Gauss full scale. It's sensitivity depends on the zero-drift you happen to get, but it tends to be about 1 gauss. There are other models more sensitive, if you want that. One is 10 times more sensitive (100mv/mT). Of course it only goes to +/- 200 Gauss.

You can omit the 78L05 regulator, but then you must not use 9v. Use perhaps 3 fresh alkaline AA cells for 4.5v. The output voltage changes are proportional to the supply voltage, so your sensor will be about 10% wrong, and change a little hour by hour as the battery gets tired. But some people won't mind that, to get started.

The Digikey catalog, section N for sensors, is fascinating to me. (, 1.8 megabytes to download) Many things there. Hall-effect magnetic sensors, GMR magnetic sensors, pressure sensors. There are 3-axis chips or small modules which go for $100-$300. You can buy the Melexis sensor there, or just get oriented as to what devices should be available.

A given IC chip always has a fixed maximum magnetic field that it can measure. If that is 20 gauss and you want to map out a 0-300 gauss area, you are out of luck. On the other hand, if you want to see distortions of the earth's 0.5 gauss magnetic field around iron objects, you probably want the most sensitive sensor chips, probably 5- to 20-gauss full scale. Getting about 4 cheap chips with different full-scale ranges can help you with this.

Strangely, it is more difficult to find linear sensors for very large fields, such as 1 Tesla (10,000 Gauss). Linear Hall sensors are intrinsically easier to make if they are less sensitive. For years hobbyists and instrument-makers have wanted more sensitivity, to be able to make an electronic compass. Finally they have it. Maybe now someone will make a complete range of Hall sensor IC's.

There must be five other ways of making your own magnetometer, too. But most of them would probably be somewhat bigger projects of their own.

Hope that is accessible to you-

Jim Swenson

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