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4.   Performing One of Gilbert's Experiments:
Induced Magnetism


1. De Magnete
    Review #1

2. De Magnete
Review #2

3. More on "De Magnete"

4. A Gilbert Expt.

5. Before Gilbert

6. London in 1600

7. 1600-1820

8. Oersted & Ampére

9. The Lodestone

10. Gauss

11. The Magnetic Sun

12. Fluid Dynamos

13. Dynamo in the
    Earth's Core

14. Magnetometers and
    Tobacco Smoking

15. Magnetic Reversals
    & Moving Continents

16. The Magnetosphere

17. Magnetic Planets

       Given two magnets, Gilbert knew that magnetic poles can attract or repel, depending on polarity. In addition, however, ordinary iron is always attracted to a magnet--even soft iron, which loses all its magnetism again when it is removed again.

        Gilbert guessed, correctly, that near a permanent magnet such iron became a temporary magnet, of a polarity suitable for attraction. That is, the end of an iron bar stuck to an S pole of a magnet (south-seeking pole) temporarily becomes an N-pole. Because magnetic poles always come in matched pairs, the other end of the bar temporarily becomes an S-pole, and can in its turn attract more iron.

        You see this if you dip (say) a horseshoe magnet into a cup with iron pins. As expected. many pins will stick to the poles, but in addition, some more pins will stick to those pins. Yet when the pins are pulled loose, they all are seen to be non-magnetic again.

       Gilbert confirmed his guess of temporary ("induced") magnetism by an original experiment (see drawing). Using strings, he hung two parallel iron bars above the pole of a terrella, and noted that they repelled each other. Under the influence of the terrella, each became a temporary magnet with the same polarities, and the temporary poles of each bar repelled those of the other one.

(By the way--can you find the same experiment portrayed on the front cover of "De Magnete"?)

  •    A demonstration of this experiment by Gilbert was constructed and is described here. However, it takes some preparation, and its results are nowhere as dramatic as Gilbert's figure might suggest.

  •     We recommend a simpler and more striking tabletop version. For teachers it has the great advantage that it can be performed on the surface of an opaque projector ("vu graph"), allowing an entire class to watch it.

        You will need (1) a small button magnet (drawing), of the type made of steel and used to pin messages to metal partitions in offices (not a flat refrigerator magnet, too weak), and (2) two thin finishing nails, the long kind with small heads, about 5 cm (2") long. If these are not available, find two thin long nails and cut or saw off their heads, leaving equal lengths.

    Place the magnet on a flat table or the surface of a projector, as drawn, with the attracting pole-ends downwards (the figure is drawn in the plane of the table or projector). Stick the point of one nail to the side of one pole of the magnet, perpendicular to it. Restrain the nail with your fingers from moving off-perpendicular, as it tends to do when it tries to increase its surface contact with the magnet.

    Put the point of the second nail next to it, and use thumb and forefinger to hold the two nails together, parallel. Now separate the fingers slightly, as shown, and note that the ends of the nails repel each other, as in Gilbert's drawing. By closing and opening the spacing between thumb and forefinger, you can easily demonstrate the repulsion.

    ...and by the way:

        If you are familiar with electricity, you may realize that what you have produced is essentially a magnetic analog of the electroscope, a sensitive device for detecting electric charge. In the electroscope, two thin leaves of gold foil hang next to each other from a metal bar on an insulated stand (aluminum foil also works). If an electric charge is transmitted to the bar, e.g. by touching it with an electrically charged object, part of the charge will end up on the leaves. Since the leaves carry charges of the same kind (both + or both -), they will repel each other, and they will form an angle the same way as the nails do here.

    Next Stop:   7. Magnetism from Gilbert to 1820

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  • Author and Curator:   Dr. David P. Stern
         Mail to Dr.Stern:   earthmag("at" symbol)

    Last updated 25 November 2001
    Re-formatted 18 March 2006