Page:The American Cyclopædia (1879) Volume VII.djvu/604

 592 GALVANISM FIG. Becquerel's Oxygen Circuit. potash, and having a cork in the top through which passes a wire. The bottom of the tube is stopped by a piece of linen cloth, which is covered with clay, and this with cotton wool, to prevent the clay from mixing with the liquid. The wire con- nects two plates of platinum, a and JE>, and the connection may be made through the coil of a galvanometer if it is desired to measure the strength of the current. The two liquids meet each other in the clay, and a current of considerable strength is gen- erated, which passes through the wire from the acid to the potash solution, and through the clay from the potash solu- tion to the acid ; the latter answering to the copper, and the potash solution to the zinc plate of an ordinary couple. The water in the potash solu- tion is decomposed, its oxygen escaping in bub- bles, and its hydrogen going to the nitric acid, which it reduces to nitrous acid. The current which is generated is of constant strength, and the plates do not become polarized. The power is increased by making the' plate in the pot- ash solution of amalgamated zinc, which being attacked by the nascent oxygen produces po- larization in the direction of the current. A simple couple of this kind is sufficient to effect the electrolysis of water, and several couples form a powerful battery. Dry Piles. A modi- fication of the voltaic pile was made by Deluc in 1809, and improved by Zamboni in 1812, which is remarkable for generating electricity of very high tension, approaching in character that of the frictional machine. The dry pile of Zamboni is made by covering a sheet of porous paper on one side with tin foil, and on the other with a paste made of powdered per- oxide of manganese mixed with moistened gela- tine, starch, or British gum. When the paste is dry the paper is cut into circular disks about an inch in diameter. These are placed one above the other in a glass tube, the tin-foil faces all turned one way, and the peroxide of manganese ones the other, so that one end terminates in tin foil and the other in manga- nese. Several hundred or thousand of these couples may be formed into a pile. They should be sufficiently pressed together to effect good contact, each end being fastened by a metallic cap, and one end provided with a knob, the other standing upon a metallic plate. The peroxide of manganese end shows positive, and the tin end negative electricity. Within the pile, therefore, tin corresponds to zinc, and peroxide of manganese to copper, in the ordinary pile or battery. It must not be sup- posed that the pile will act if it is perfectly dry; it requires, and under ordinary circum- FlG. 11. Bohnenberger's Electroscope. stances possesses, a certain degree of moisture. It does not produce a continuous current, but after being discharged requires a certain time for the electricity to acquire sufficient tension to pass through the paper from the tin to the peroxide of manganese. A pile of several thousand disks causes strong divergence of the leaves of the electroscope and will decompose water. Its principal use is in the construction of Bohnenberger's electroscope, represented in fig. 11. Two piles stand with their opposite poles upon a metal plate, and from the top of a bell glass which covers the piles a strip of gold leaf is suspended from a conductor which passes through the top and terminates in a knob. The gold leaf hangs between the two knobs of the piles, and the instrument is so deli- cate that whenever a body only slightly electrified is brought within a few feet of it, the gold leaf will move to- ward one or the other of the piles. A pith ball suspended by a silk thread between them will oscillate as long as the chemical action of the pile con- tinues, which may be for two or three years. Galvanometers. It has been stated that if a magnetic needle is brought near a wire through which a galvanic current is passing, it will be deflected; the direction depending upon the relative position of the wire to the needle, and upon the direction of the current. Upon this peculiar action depends the construction of an instrument which is used for measuring the strength of a galvanic 3T current, called a gal- > ^^^ vanometer. If the wire is held above the mag- netic needle, and paral- lel to it, and a current is passed in the direc- tion of its north enc that end will be deflt ed to the left, as repi sented in fig. 12, when the observer is lool ing downward and to the north. If the wii is held under the needle, and the currt passed in the same di- rection, the north end will be deflected to the right ; but if the cur- rent is passed from north to south, the nee- dle will be deflected in the same direction as when the current passed above it from south to north. If, therefore, the wire is turned upon itself, as represented in fig. 13, two forces will act upon the needle, tending to deflect it in the same direction ; and if the wire is formed into a flat coil, the deflect- FIG. 12. FIG. 13.