The primary use of the balance was in measuring very small charges and estimating the attractive and repulsive forces between bodies of known surface area. Bodies were introduced into the receiver next to the ball and their charge measured by the degree of deflection of the indicator needle or, to be more precise, by the torsion under which the thread must be placed in order to bring the needle back to its original position. However, exactly what these precautions are is not stated, being left, presumably, to the judgement of the experimenter, based on his skill and practical knowledge.
In attempting to replicate the experiment, Heering encountered several unexpected problems. Protruding through the top of the glass case is also a metal rod with metal spheres at both ends one inside the case, one outside. To use the torsion balance, Coulomb would hold an object near the metal sphere at the upper end of the metal rod.
Any charge held by the object being studied would transfer to the metal sphere, then travel along the rod to the sphere at the other end. There the charge could affect the needle suspended in the case, which in its resting state touched the rod's lower sphere. The repulsion caused the needle to move and the thread holding it to twist. The twisting action is called torsion, hence the name of the instrument.
An angular scale is fixed to the outside of the larger cylinder. Clamped to the torsion micrometer is a 76cm long metallic wire with a diameter of 0. The rod itself is a mounting stage for a sealing wax coated silk filament which in turn holds a paper disc on one end and on the other an elder pith ball.
The weight of the copper rod is used to tighten the metallic wire without disrupting it. In the glass plate towards its rim there is a hole through which another rod with a elder pith ball on its end is held into the chamber.
The second pith ball is placed into the position of the first pith ball's resting state. If the balls came too close to one another, they would jump together and stick, ending the experiment. With difficulty, he did measure the relation between force and separation in this case, but he decided to use a completely independent method to confirm the result [ 3 ].
He suspended a needle with a small plate on one end, and the plate was then charged. The opposite charge was placed on the surface of a hollow sphere of copper or metal-coated cardboard, about a foot in diameter.
Coulomb assumed that the large sphere would behave as if all its charge were concentrated in a point at its center. The needle was made to oscillate in a narrow arc in the horizontal plane. The period of oscillation depended on the force between the charged sphere and the charged plate on the needle, just as the period of the ordinary simple pendulum depends on the force exerted by gravity.
Coulomb then measured the period of oscillation at various distances from the large sphere and, using an equation similar to that for the pendulum, related the period to the force between the charges. Ernest Rutherford, studying the scattering of alpha particles, used this equation to show that the diameter of the atomic nucleus is orders of magnitude less than that of the atom — i.
Later, Niels Bohr used this result as the starting point of his theory of the line spectrum of the hydrogen atom. His contributions to the weights and measures committee and the supervision of the water supply ceased during the revolution, but in later years he was able to resume some of this work.
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