# Magnetic Interference Explained

Every now and then you hear a story about someone who's autopilot does some strange things when someone else on board uses the head and hits the macerator button or the vacu-flush. Or maybe they are down below washing the dishes and using the pressure ...

28th July 2010.
By Ed Sherman

Every now and then you hear a story about someone who’s autopilot does some strange things when someone else on board uses the head and hits the macerator button or the vacu-flush. Or maybe they are down below washing the dishes and using the pressure water system on board when the autopilot starts steering a strange course. Or, maybe you’ve noticed that when you’re moving along in a thick fog and steering by your compass, and you suddenly turn on the windshield wipers the compass moves 20 degrees off your course.

All of these strange incidents are usually caused by electro-magnetic interference and it can be found all over your boat with a simple tool that costs about \$100 that I’ve added to Ed’s Tool Box. The tool is called a gauss meter and I’ve been using the one shown below for some time now to help me sort out these odd problems.

The tool is actually about the size of an iPod and is super sensitive, being able to read milligauss levels of magnetic field strength.

Before you use one you need to understand a basic electrical principle that I explain at length in my book entitled Advanced Marine Electrics and Electronics Troubleshooting. If you’re reading this, you may be interested in getting the book, which is available on this site through Amazon.com . Just click on the link on the left side of the home page.

Anyhow, back to the principle….Understand that any wire that has electrical current flowing through it has a magnetic field surrounding it. The strength of that magnetic field is proportional to the amount of current (amperes). Electrical appliances also have a magnetic field surrounding them. The strength will vary based on the design of the device and specific technology used within the device. The gauss meter can find these magnetic fields and help you to establish what I refer to as a “safe zone of separation” between either a wire or device. To learn more about this, click on the button at the top of the home page for this site labeled Ed’s Tool Box.

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