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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by, that the motions of these bodies are produced by a precisely similar mechanical action ; for their material particles, although electrified, do not acquire any real influence over each other; what lakes place is effected by the vitreous and resinous electricities which cover them, and whose reciprocal action is confined to aug- menting or diminishing, upon certain parts of their surfaces, the pressure exerted there by the electricity against the surrounding air which retains it, or in general against the obstacles which oppose its change of place. After what is now laid down, if we continue to employ the words attraction and repulsion to denote the mo- tions of electrified bodies, the terms are to be considered as expressing simply the circumstances of these m-otions, and not as indicating the real cause on which they depend. The attraction and repulsion under consideration take ])lace not only through the air; they are exerted also through other non-conducting bodies, as glass and resin. If we suspend within a glass phial a stick of sealing wax rubbed and electrified, it at- tracts light bodies situated without the phial, just as if there were nothing, interposed. This transmission manifests itself also through conducting bodies; but it is disguised under another phenomenon, of which we shall speak hereafter. To discover whether a given substance, on being rubbed in a certain manner, acquires the vitreous or resinous electricity, we must observe the effect it produces upon the electrical pendu- lum previously charged Avith a known electricity. For example, we touch this pendulum with a glass tube rubbed with woollen cloth ; and it receives the vitreous electricity. We rub with the same substance the body whose electricity is to be tried, and bring it toward the ball of the pendulum. If it repels the ball, its electricity is vitreous, if the ball is attracted, the electricity is resinous. We may vary the experiment if we choose, by first giving to the pendulous body the resinous electricity. As the signs of electricity in certain cases are very feeble, it be- comes desirable to increase the sensibility of the apparatus. This is effected by reducing the size of the pith ball, and suspending it by a fine silk thread. If we use, for example, one of the orig- inal fibres, as they proceed from the silk worm, and not less than 10 or 12 inches in length, a very weak electricity will be suffi- cient to put it in motion. There are still more sensible instru- E, <^ M, 2

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