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
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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![348 by calculation the attractive force at different distances of the electrified globe, from the number of oscillations of the needle which take place in a given time, just as we determine the force of terrestrial gravity from the number of oscillations of the MecT], common pendulum. The results thus obtained confirm the law of the inverse duplicate ratio of the distance, before discovered by means of the torsion balance. 12. The same method would serve also to determine the law of repulsive forces; for by communicating to the globe and to the disc similar electricities, the disc will be repelled, the direc- tion of the needle will be inverted, and it will oscillate in virtue of this repulsion in a direction diametrically opposite to the for- mer ; but with the exception of this turning, which will affect the distance of the disc from the globe, the observations and the calculations will be the same as in the preceding case. By means of the results which we have^ obtained, we can calculate, for all possible distances, the force of attraction or re- pulsion of two electrified balls, when we have determined this force for a single known distance. But this gives us only the measure of the total effect; we do not know what proportion each ball contributes. Still, unless they are perfectly equal and equally electrified, it is manifest that they must contribute unequally. It is proposed, therefore, to discover this proportion; which is readily done, if we could give to one of the balls, or take from it, a portion of electricity having a known ratio to that which it had before. For, by measuring the new torsion which produces an equilibrium in this new state, and comparing it with that which took place before at the same distance, we should discover what influence the prop- er electricity of each ball has upon the total effect. Now it is very easy to take from each ball half its electricity. To this end, we have only to touch it for an instant with another ball of the same substance, of the same diameter, and equally insu- lated ; for it is manifest that the two balls being perfectly similar, the electricity will be equally divided between them, so that after the contact, the proper action of the ball touched will be less by one half. Now by proceeding in this way, we find that the total force of attraction or repulsion, which was at first exerted be- tween this ball and the fixed ball of the balance, is^ after the contact, reduced exactly one half.](https://iiif.wellcomecollection.org/image/b21051483_0034.jp2/full/800%2C/0/default.jpg)
