Page:The American Cyclopædia (1879) Volume VI.djvu/511

 between the excited globe and the cylinder be- ing disregarded ; but according to the researches of Faraday, the molecules of this intervening medium, which is called a dielectric because the electric action is propagated through it, become alternately positively and negative- ly electrified. This condition he called polar- ization of the medium, and to exhibit it ex- perimentally he placed small fragments of silk in a vessel of turpentine, in the opposite sides of which two conductors were placed. Upon charging one of these, and connecting the other with the ground, the particles of silk immediately placed themselves end to end, forming a continuous chain from one conductor to the other. The influence of the dielectric is also shown by placing a plate of shellac be- tween the electrified globe and the cylinder, fig. 2, when the latter will manifest more ex- citement than before, the divergence of the pith balls increasing. By using different sub- stances as dielectrics, it will be found that each exerts a specific influence, the relative amount of which Faraday has termed specific inductive capacity. An experiment of Matteucci supports Faraday's theory. Several thin plates of mica were placed closely together, the ter- minal ones having metallic coatings. While in a state of excitation by induction the plates were removed one by one, and found charged, on one side with positive and on the other with negative electricity. If a pith ball be suspend- ed by a linen or cotton thread, it will be more strongly attracted by an excited body than if suspended by a silk thread, because the linen or cotton thread will allow the electricity which is of the same kind as that of the excited body to pass away from the ball, while it be- comes fully charged with the opposite kind. The silk thread, however, prevents this, and only allows the fluid which is of the same kind as that of the excited body to pass to the fur- ther side of the ball. Electroscopes. These are instruments used for the purpose of detect- ing the presence of free electricity, and also its nature. The pith balls represented in fig. 2 are electroscopes, but they possess little delicacy. A much more efficient apparatus is formed by two strips of gold leaf placed together and sus- pended by a conductor, as in Bennet's gold-leaf electroscope, fig. 3. It is protected by a glass cover, within which, at either side, project from the bottom two conductors for the purpose of preventing the rupture of the leaves if they should diverge rapidly from a strong charge. A body having only a small amount of elec- tricity, upon being held near the knob of the conductor, causes induction and separation of the leaves. In order to ascertain the kind of electricity any excited body may have, it is first brought near the knob; this is then touched with the finger to bring it in commu- nication with the earth, and thus permit the escape of the electricity which is of the same kind as that of the body to be tested. Then, while the excited body is near, the finger is ELECTRICITY 503 removed from the knob, the gold leaves and the conductor being thereby again insulated. The instrument is now charged with electricity of an opposite kind to that of the body, the presence of which is manifested by a diver- gence of the leaves. The body to be tested is FIG. 3. Bennet's Gold-leaf Electroscope. now removed, and a glass rod that has been rubbed by a piece of silk is brought near the conductor. . If the divergence of the leaves in- creases, it will indicate the presence of posi- tive electricity, because the positive electri- city of the glass repels that of the same kind from the knob to the leaves. If, on the con- trary, the approach of the excited glass rods causes a diminution of divergence, it will show the presence of negative electricity, be- cause it will be attracted from the leaves to the knob. Electrometers. For the purpose of ascertaining the intensity with which a body is charged, and also the laws of electrical attrac- tion, an instrument was invented by Coulomb, called Coulomb's torsion balance electrometer, represented in fig. 4. Its essential parts con- sist of a very fine metallic wire, weighing about one tenth of a grain to the foot, which is held in a cap at the top of the apparatus, and supports a delicate needle of shellac in a horizontal position at its lower end. This shellac needle has a small gilt ball, <?, at one ex- tremity, which being insulated may be made to receive, and for a short time to retain, a quantity of electricity. Another rod of shellac, j is so adjusted in the cover of the large glass cylinder that the small gilt ball g' which it carries, and which is of the same size as the ball g, may be passed down and held at a level with the latter. If now the horizontal needle f is so placed that the ball g may be touched by the ball #', and the latter is charged with electricity and introduced, it will deliver to the former half of its charge, and the two will be mutually repelled, the repulsion being measured by the amount of torsion of the sus- pending wire, which may be ascertained by