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.

  • Farrar, John, 1779-1853.
Date:
1826
Catalogue details

Licence: Public Domain Mark

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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    The summit only of the stem appears above the glass, and this admits of being turned so that the slips shall diverge parallel to one of the faces upon which is traced a small graduated arc, to measure the amount of the divergence. It is evident that a greater or less divergence will indicate a greater or less degree of electricity ; but as the tendency of gravity to bring the slips back to a vertical position, augments in proportion as they be- come more oblique, it is manifest that the repulsive force which supports them is not simply proportional to their divergence, but follows a law less simple, depending on the weight of the slips and their figure; and consequently the parts of the graduated arc, supposed equal among themselves, do not represent equal degrees of electricity. Therefore, when it is proposed to meas- ure equal degrees, it is necessary to have recourse to the balance of Coulomb or to his electroscope, which alone possesses the double advantage of indicating the smallest electrical forces and of measuring them at the same time. 66. We can communicate to electroscopes of whatever descrip- tion either the vitreous or resinous electricity, by touching the ex- terior knob of the stem with an insulated conductor charged with this kind of electricity. But there is another method equally suited to this purpose, which it may be well to explain, since it requires only a tube of glass or sealing wax, or other electric, which, on being rubbed with a proper substance, produces a known kind of electricity. p. y Let us suppose, for example, that a stick of sealing wax is used, and that the electroscope is that of Coulomb. The circle of tinsel C being in contact with the fixed ball £, we rub the sealing wax with a cat skin, and present it to the exterior knob B of the metallic stem .4B at some distance; the needle SC is immediately repelled. The repulsion continues as long as the sealing wax is presented. If it be brought nearer to the knob, the needle is driven to a greater distance ; if it be removed fur- ther off, the needle approaches the fixed ball; if it be entirely withdrawn, the needle returns and touches the ball, and remains in contact with it at its point of rest. All these phenomena are to be referred to the case of influ- ence exerted at a distance. The electricity of the stick of seal- ing wax is resinous. It decomposes the combined electricities