Levitating Pencil Lead – Demonstration of Diamagnetism

Experiment number : 2121

  • Goal of experiment

    The experiment demonstrates a diamagnetic levitation in an untraditional manner: a pencil lead levitates in a “trough” of magnets.

  • Theory

    We can divide matter in three basic types according to its magnetic properties: diamagnetic, paramagnetic and ferromagnetic. Magnetic properties depend on the electron shell structure of the atom and the arrangement of atoms in the structure of the matter. The properties and behaviour of individual types of magnetic materials are described in detail in the experiment Demonstration of Properties of Diamagnetic and Paramagnetic Substances.

    Diamagnetic substances weaken the magnetic field slightly, which means that diamagnetic matter is slightly repelled from the magnet. In a suitable arrangement, the repulsive diamagnetic forces can compensate for the gravitational force float in the air above the magnets.

    A suitable material for this diamagnetic levitation is graphite (usually called pyrolytic graphite), which is strongly diamagnetic as well as light. The arrangement described in this experiment uses commonly available refills of mechanical pencils.

  • Tools

    • L-shaped iron profile with dimensions 1 × 1 cm and a length of at least 15 cm
    • neodymium magnets with a diameter of 8 mm and thickness of about 4 mm (at least 30 pieces)
    • mechanical pencil graphite leads

    Fig. 1. Tools

    Note: Some graphite leads contain traces of iron. Because the repulsive diamagnetic force is quite small compared to the force with which iron is attracted to magnets, it is necessary to choose leads that do not contain trace amounts of iron. According to our experience, most of Faber-Castell leads are suitable for the experiment. Leads provided by other manufacturers did not work.

  • Making a track for lead

    Put the magnets in the iron bars so that their poles alternate and at the same time one magnet is inserted and the opposite magnet lays on the inserted one (see Figure 2, north poles are marked with a cross).

    Fig. 2. Placing the magnets

    At the end of the track you can place two magnets on top of each other so that the lead does not run out (see Figure 3).

    Fig. 3. End of the track
  • Sample result

    If we put a pencil lead into the “trough”, it will float freely about 0.5 mm above the magnets. This way, we can also partially demonstrate a motion without friction, as seen in the video.

    In the side view, we can see that the lead really hovers:

    Fig. 4. Side view
  • Technical notes

    • When moving, the lead “skips” the gap between the magnets, so the gaps must not be too large. In practice, magnets with a diameter of up to 8 mm have proven useful.
    • A suitable iron profile is for example an L-profile rod from the Czech Merkur toy kit.
  • Learn more

    More information can be found in the article: Koudelkova V.: How to simply demonstrate diamagnetic levitation with pencil lead; Phys. Ed. 51(1), (2016)

Type of experiment: Qualitative
Difficulty level: From Upper secondary level
Necessary tools: Specific tools and equipment required
Preparation time: Under 3 minutes
Duration of experiment: Under 3 minutes
Experiment is video recorded
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