Magnetic fields are invisible so what can we do to determine where they are and how strong they are? If you don’t have a ferrofluid bottle or magnetic field viewer film on hand, you can create your own magnetic field sensor with this tutorial:


  • Magnetic ink (MICR code printer ink refill)
  • Small clear plastic bottle
  • Water
  • 2 - 10 neodymium disc magnets
  • Straws
  • Tape


  1. First, find a small clear plastic bottle. For example, you can use a travel-sized mouthwash bottle at a convenience store. Remove all labels from the bottle and rinse it thoroughly.
  2. Next, put two or three drops of magnetic ink in your bottle.
  3. Fill the rest of the bottle with water and put the cap on securely. You can make sure it won’t leak by gluing it on with superglue. Shake the bottle vigorously so that the ink doesn’t clump together.
  4. Now, you want to make a magnetic wand. You can cut a small slit (about ¼ to ½ inch) vertically along the tip of one end of a straw.
  5. Squeeze one or two of your magnets into the end you cut and tape the magnets in place at the end of the straw. You can make as many of these magnetic wands as you want.

Now that you have your magnetic wands and your magnetic sensor bottle you can start experimenting with magnetic fields! When you hold the wands near the bottle, the ink will attract to the magnets and follow the magnetic fields. It will resemble a lava lamp with a little, black blob squirming around inside your bottle.

So, now you can determine whenever a magnetic field is nearby because the ink inside the bottle will come alive and separate from the water when manipulated by a magnetic field. You can experiment with different sizes of magnets and even different types. For example, how does a fridge magnet influence the magnetic ink compared to a rare-earth magnet? What happens to the ink when you put two or more magnets near the magnetic sensor? What else can you learn from a magnetic field sensor bottle?

Leave a comment and share the results of your experiments with us!