Homebrew Super Simple Magnetic Field Meter (Gaussmeter) John Mills c. 1993 John Mills Words 722 New information presented at the 1992 International E.M.F. (Electro Magnetic Field) Research Conference in San Diego has given us some new reasons to be more careful around electricity. This study by Maria Feychting and Anders Ahlbom examined the medical records of about 500,000 people who lived near large power lines. They found that children continuously exposed to levels of ac magnetic fields of 1 milliGauss had twice the risk of developing leukemia compared with children whose exposure was much less. The risk for children continuously exposed to 2 milliGauss was three times higher and those exposed to 3 milliGauss levels was about four times higher than normal. A milliGauss is a measurement of magnetic field. These magnetic fields are around just about everything electrical that is energized, such as: power lines, house wiring, tools, lights, and appliances. This project is a super simple ac Gaussmeter that you can build and use to learn about the magnetic fields in your environment. How It Works The reed relay coil acts as an antenna. The steel nail inside boosts its sensitivity. When the coil is near an ac magnetic field, it produces a small signal and the mini amplifier is used to make this stronger. The meter tells us exactly how strong the magnetic field is numerically. It is not necessary to calibrate your Gaussmeter. You can use it with the volume on the mini amp at maximum. At this setting the magnetic field, in milliGauss, is approximately equal to volts on the meter times 28.6. For example: CHART You can listen in to magnetic fields around you by simply unplugging the meter. Fields from inverters may sound different from those produced by the regular power lines. If you put the coil near speaker wires or headphones, you might be able to hear some unique sounds also. If you would like to make accurate readings of electrical things around you in milliGauss, you can calibrate your meter. To get a calibrated magnetic field, we need a loop of wire with a controlled current flowing in it. Remove one end of an empty salt container (3.75 inches in diameter). Attach a 110 volt plug and socket to the ends of a 2-3 foot length of #18 lamp cord. Pull the wires apart in the center of the cord for about 5.5 inches. Now slip this over the open end of the salt container. Plug a 100 watt incandescent lamp into the socket and the other end into a 110 volt ac source. The light bulb limits the current to just the right amount so that the center of the coil (at the black dot on the diagram) is about 100 milliGauss. Next, place the Gaussmeter probe coil into the center of the loop and adjust the amplifier volume so that the voltmeter reads: 1.000 volts ac. Your Gaussmeter is now calibrated to this scale: volts on meter times 100 equals magnetic field in milligauss. 2CHART Mark this place on the volume control as a reference point. Now you are equipped to measure and listen to magnetic fields. You will discover that the position of the coil is important. When making measurements, always orient the coil for the highest reading on the meter. Because of limitations inherent in the digital multimeter, you will only be able to measure magnetic fields up to about 1000 Hertz. Computers, televisions, and electronically-ballasted fluorescent lamps produce some magnetic fields above 1000 Hz so you may not be able to measure them accurately with this meter. It will work well, however, for power lines, house wiring, and just about everything else. If you wish to measure strong fields above 200 milliGauss, you will need to calibrate at a lower volume to ensure that your meter is not overloaded. If you set the volume so that 100 milligauss is equal to 0.100 volts then: volts times 1000 equals magnetic field in milliGauss. 3Chart Be sure to remember to turn the mini amplifier off after making measurements to keep the battery fresh. Stay tuned Ä next issue we will work on a tin can gizmo that will turn your Gaussmeter into a microwave meter for checking microwave oven leakage. Access John Mills, 10475 Vineland Rd, Ben Lomond, CA 95005