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ArrowTorsion Balance

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Early instruments used in investigations of electricity could do little more than detect the presence of a charge. Then, in the mid-1780s, Charles-Augustin de Coulomb introduced a device known as the torsion balance that could measure it. The torsion balance had been invented previously by English geologist John Michell, who had intended to use it not to study electricity but to measure the gravity and the Earth's mass. The device did not become widely known, however, until it was independently reinvented by Coulomb.

A replica of Coulomb's torsion balance is illustrated in the applet below.


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The instrument consists of a mounted glass case that holds a needle suspended from a silk thread. The thread hangs from a piece of metal located at the top of a long glass tube that extends out of the top of the case. Located at one end of the needle is a small metal sphere. A similar sphere, this one stationary, protrudes into the glass case, attached to a vertical rod that extends outside the case, terminating in yet another sphere.

When a charged object approaches the sphere outside the case, it transfers a charge to that sphere. This charge then travels to the opposite end of the rod, then from there to the sphere-end of the abutting needle. Because the two internal spheres then hold the same charge, they repel each other. This brings about the displacement of the needle and subsequent twisting (torsion) of the thread. Coulomb was able to measure the distance traveled by the revolving sphere by consulting a scale that circumscribed the glass case, and to gauge the amount of torsion in the thread with a second, smaller scale divided into degrees near the top of the glass tube.

Experiment with the torsion balance in a manner similar to Coulomb. Before you start, notice that the two internal rods are touching: This indicates they possess opposite (or neutral) charges. See what happens when you change that, first by giving the rod a charge using the charge level slider, then by moving the charged rod closer to the outer metal sphere of the instrument by adjusting the rod position slider. Notice that the needle inside the instrument is displaced when the rod is placed near the torsion balance. The amount of torsion experienced by the thread (and the distance traveled by the needle) depend on the amount of charge to which the device is exposed. Observe that as the charge increases, the distance traveled by the needle increases.

Play with the charge slider to see how the balance behaves under different scenarios. After charging the rod once, you can recharge it, using the same, opposite or neutral charge, by repositioning the charge level slider and hitting the charge button again. To neutralize both the rod and the balance, hit the discharge button.

By making such observations and carefully recording the measurements he obtained with variously charged objects placed at different distances from his torsion balance, Coulomb was able to establish the law that bears his name. Coloumb's law states that the attraction or repulsion force between two electrical charges is proportional to their product and inversely proportional to the square of their distance apart. Coulomb further demonstrated the similar law for magnetic forces by experimenting with magnetic rather than charged objects. Others had arrived at similar conclusions regarding magnetic and electrical forces first, but Coloumb's torsion balance experiments definitively demonstrated their accuracy.

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