Treatise on natural philosophy : Vol 1. Part 2 / by Sir William Thomson and Peter Guthrie Tait.

Date:
1883
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Licence: Public Domain Mark

Credit: Treatise on natural philosophy : Vol 1. Part 2 / by Sir William Thomson and Peter Guthrie Tait. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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    APPENDIX F. Contact electricity of metals. Energy of electric attraction between plates of different metals in metallic contact. [V. other one thousand bricks : thus two contiguous cubes of 20,000 centimetres breadth may be considered as sensibly similar. But two adjacent lengths of forty centimetres each might contain one of them, one brick, and two half bricks, and the other two whole bricks; and contiguous cubes of forty centimetres would be very sensibly dissimilar. In short, optical dynamics leaves no alternative but to admit that the diameter of a molecule, or the distance from the centre of a molecule to the centre of a contiguous molecule in glass, water, or any other of our transparent liquids and solids, exceeds a ten-thousandth of the wave-length, or a twoJiundred-millionth of a centimetre. By experiments on the contact electricity of metals made in the year' 1862, and described in a letter to Dr Joule*, which was published in the proceedings of the Literary and Philosophical Society of Manchester [Jan. 1862], I found that plates of zinc and copper connected with one another by a fine wire attract one another, as would similar pieces of one metal connected with « the two plates of a galvanic element, having about three-quarters of the electro-motive force of a Daniel’s element. Measurements published in the Proceedings of the Royal Society for 1860 showed that the attraction between parallel plates of one metal held at a distance apart small in comparison with their diameters, and kept connected with such a galvanic element, would experience an attraction amounting to two ten- thousand-millionths of a gramme weight per area of the opposed siirfaces equal to the square of the distance between them. Let a plate of zinc and a plate of copper, each a centimetre square and a hundred-thousandth of a centimetre thick, be placed with a corner of each touching a metal globe of a hundred-thousandth of a centimetre diameter. Let the plates, kept thus in metallic communication with one another be at first wide apart, except at the corners touching the little globe, and let them then be gradually turned round till they are parallel and at a distance of a hundred-thousandth of a centimetre asunder. In this position they will attract one another with a force equal in all to two grammes weight. By abstract dynamics and the theory of energy, it is readily proved that the work done by the changing force of attraction during the motion bv which we have supposed * [Now published as Art. xxii. in a “Reprint of Papers on Electrostatics and Magnetism ” by Sir William Thomson. New edition, 1883.J