Pith balls

Coulomb charged the pith balls in the apparatus with electrostatic charges. The first charged ball was fixed in place the second was attached to a horizontal bar suspended by a fiber or wire. When the two balls had like charges, they repelled one another. The force of that repulsion was measured by the distance between the two balls, which was the point where the tension in the twisting fiber equaled the force of repulsion. Using this difficult, sensitive instrument, Coulomb came up with the law now named for him. 

Figure 4.1 In this setup, two charged pith balls repel one another. Coulomb used this apparatus to come up with a law that determines the force of repulsion between two charged objects. In this diagram g1 and g2 represent the magnitude of the electrical charge on each pith ball, and r represents the distance between the two pith balls.

Pith balls Pith is the spongy material found in the center of the stem of most plants. Pith balls are bits of pith attached to a string for use in scientific demonstrations to show how like electrical charges can attract and repel the balls. These days, pith is usually replaced by lightweight plastics in these demonstrations. 

The cause of this electrical adhesion has not been explained completely but appears to be related to the electrical conductivity of the powder. Only for particles with resistivities between 10 and 10, ohm-metre is the adhesion apparent. Below this, the particles discharge on contact with the electrode, acquiring a charge of opposite sign and jumping off the electrode, the so-called pith-ball effect. For larger resistivities, locahzed electric breakdown occurs and no adhesion develops. 

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