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is it possible to enhance (or redirect) the earth's magnetic field in a room or house so that one can write a small program that makes smartphones with hall-effect sensors detect more reliably in which direction they are pointing?

I presume a fridge magnet won't do the job...

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closed as too broad by Chuck May 31 '16 at 21:05

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • $\begingroup$ Welcome to Robotics ivoWelch, but I'm afraid that questions like this really aren't a good fit for a stack exchange site. We prefer practical, answerable questions based on actual problems that you face. Take a look at How to Ask and tour for more information on how stack exchange works. Also, the Robotics question checklist has good advice on how to write a good question. If you edit your question to fit our community guidelines we can reopen it for you. $\endgroup$ – Chuck May 31 '16 at 21:05
  • $\begingroup$ As it stands, the answer to your question is, "Yes, it's probably possible to make a stronger magnetic field in a room." Whether or not it's practical, the cost, how you would do it - all the things I think you're looking for - are all part of an unbounded, speculative design question. If that's the case, that's fine, but please ask it in chat. $\endgroup$ – Chuck May 31 '16 at 21:17
  • $\begingroup$ ok. makes sense. for me, this question seemed halfway between robotics (there are hall effect sensors for arduinos, for example) and physics. $\endgroup$ – ivo Welch Jun 1 '16 at 2:38
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This question might be better suited to the Physics StackExchange, but the back-of-the-envelope calculation goes like this:

The Earth's magnetic field is 31.869 µT, and a refrigerator magnet is 5000µT. So the refrigerator magnet will very readily affect the magnetometer (not usually called a hall-effect sensor) on a smartphone.

However, you will run into two very serious problems

  1. The strength of a magnet follows the inverse square law -- it drops off rapidly with distance. You can verify this with a handheld compass: see how close your magnet needs to be in order for it to pull the needle away from magnetic north.

  2. The magnetic field curves around the magnet. For the earth, this is not a major concern because you are (more or less) standing on the surface where the lines run (more or less) straight from north to south.

For your room-sized magnetic field to work, you would need to establish a magnet far enough away that its field lines would have minimal curvature through the area that you want to measure, and strong enough that those field lines would have enough strength to dominate the 31.869 µT provided by Earth's magnetic field.

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