Elements of electricity, magnetism, and electro-magnetism : embracing the late discoveries and improvements : digested into the form of a treatise, being the second part of a course of natural philosophy : compiled for the use of the students of the University at Cambridge, New England / by John Farrar.

John Farrar

Date:: 1826

Credit: Elements of electricity, magnetism, and electro-magnetism : embracing the late discoveries and improvements : digested into the form of a treatise, being the second part of a course of natural philosophy : compiled for the use of the students of the University at Cambridge, New England / by John Farrar. Source: Wellcome Collection.

Provider: This material has been provided by the Francis A. Countway Library of Medicine, through the Medical Heritage Library. The original may be consulted at the Francis A. Countway Library of Medicine, Harvard Medical School.

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are supposed to be known, we shall obtain the ratios of their densi- ties. Cavendish, who made this fine experitneot, found in the way we uAv^e stated, the mean density of the earth, that of water being unity, to be equal to 5,5. Coulomb applied the torsion balance to the purpose of measuring the intensities of electric and magnetic forces. He even used it to ascertain the adhesive force of liquids considered with respect to themselves and to other bodies. For this purpose, he immersed in the liquids, plane discs, suspended by their centres in a horizontal position by means of wires of a known force, and he compared to- gether the velocities of the oscillations performed by these discs in the liquids and in the air. IL Instructions respecting the best Form i^c. of Lightning Rods, extract- ed fro7n a Memoir of M. Gay-Lussac. [Annales de Chimie.] The most advantageous form that can be given to lightning-rods appears evidently to be that of a very sharp cone ; and the higher it is elevated in the air, other circumstances being the same, the more its efficacy will be increased, as is clearly proved by the ex- periments with electrical kites, made by MM. de Romas and Charles. It has not been accurately ascertained how far the sphere of ac- tion of a lightning-rod extends, but, in several instances, the more remote parts of large buildings on which they have been erected, have been struck by lightning at the distance of three or four times the length of tlie conductor from the rod. It is calculated by M. Charles, that a lightning-rod will eifectually protect a circular space, whose radius is twice the height of the conductor; and they are now attached to buildings according to this principle. A current of electric matter whether luminous or not, is always accompanied by heat, the inteasity of which depends on the velocity of the current. This heat is sufficient to make a wire red-hot, or to fuse or disperse it, if sufficiently slender ; but it scarcely raises the temperature of a bar of metal, on account of its large mass. It is by the heat of the electric current, as well as by that disengaged from the air, cTnd'^nsed by the passage of the lightning through it, when not conveyed by a good conductor, that buildings struck by it are frequently set on fire.

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