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Page 243 - Returning to the phenomena in question, the first thought that arises in the mind is, that the electricity circulates with something like momentum or inertia in the wire, and that thus a long wire produces effects at the instant the current is stopped, which a short wire cannot produce. Such an explanation is, however, at once set aside by the fact, that the same length of wire produces the effects in very different degrees, according as it is simply extended, or made into a helix, or forms the circuit...
Page 64 - Such a needle always arranges itself nearly due north and south, and always in the same position. The compass sold by opticians is simply such a needle mounted above a dial, marked with the " points of the compass." The end of the needle or magnet that points to the north is called the north pole, and the other the south pole. If a magnet be broken at any point between the two poles, each of the pieces becomes a magnet with two poles; it is therefore impossible to make a magnet with but one pole....
Page 74 - The repulsion between two magnetic poles is in the straight line joining them, and is numerically equal to the product of the strengths of the poles divided by the square of the distance between them. 374.] This law, of course, assumes that the strength of each pole is measured in terms of a certain unit, the magnitude of which may be deduced from the terms of the law. The unit^pole is a pole which points north, and is such that, when placed at unit distance...