already great, as in telegraphing through a long line, it is little use to diminish the internal resistance if this is already much smaller than the resistance of the line wire. It is, on the contrary, advantageous to increase the number of cells in series, though every cell adds a little to the total resistance. If Example. If the line has a resistance of 1000 ohms, and five The E.M.F. of the single-fluid cells of Volta and Smee is marked in the table as doubtful, for the opposing E.M.F. of polarization sets in almost before the true E.M.F. of the cell can be measured. The different values assigned to other cells are accounted for by the different degrees of concentration of the liquids. Thus in the Daniell's cells used in telegraphy, water only is supplied at first in the cells containing the zincs; and the E.M.F. of these is less than if acid or sulphate of zinc were added to the water. 193. Other Batteries. Numerous other forms of battery have been suggested by different electricians. There are three, of theoretical interest only, in which, instead of using two metals in one liquid which attacks them unequally, two liquids are used having unequal chemical action on the metal. In these there is no contact of dissimilar metals. The first of these was invented! by the Emperor Napoleon III. Both plates were of copper dipping respectively into solutions of dilute sulphuric acid and of cyanide of potassium, separated by a porous cell. The second of these combinations, due to Wöhler, employs plates of aluminium only, dipping respectively into strong nitric acid and a solution of caustic soda. In the third, invented by Dr. Fleming, the two liquids do not even touch one another, being joined together by a second metal. In this case the liquids chosen are sodium persulphide and nitric acid, and the two metals copper and lead. A similar battery might be made with copper and zinc, using solutions of ordinary sodium sulphide, and dilute sulphuric acid in alternate cells, a bent zinc plate dipping into the first and second cells, a bent copper plate dipping into second and third, and so on; for the electromotive-force of a copper-sodium-sulphide-zinc combination is in the reverse direction to that of a copper-sulphuric acid-zinc combination. Upward proposed a chlorine battery, having slabs of zinc immersed in chloride of zinc and kathodes of carbon surrounded by crushed carbon in a porous pot, gaseous chlorine being pumped into the cells, and dissolving into the liquids to act as a depolarizer. It has an E.M.F. of 2 volts. Bennett described a cheap and most efficient battery, in which old meat-canisters packed with iron filings answer for the positive element, and serve to contain the exciting liquid, a strong solution of caustic soda. Scrap zinc thrown into mercury in a shallow inner cup of porcelain forms the anode. Marié Davy employed a cell in which the zinc dipped into sulphate of zinc, while a carbon plate dipped into a pasty solution of mercurous sulphate. When the cell is in action mercury is deposited on the surface of the carbon, so that the cell is virtually a zinc-mercury cell. It was largely used for telegraphy in France before the introduction of the Leclanché cell. Obach's dry cell has an outer cylinder of zinc which serves as a case, lined with plaster of Paris soaked in salammoniac; with a central carbon kathode surrounded with binoxide of manganese mixed with graphite. The Fitch cell, used in the United States, is a zinccarbon cell with an excitant composed of salammoniac solution to which the chlorates of potash and soda have been added. Papst used an iron-carbon cell with ferric chloride solution as excitant. The iron dissolves and chlorine is at first evolved, but without polarization; the liquid regenerating itself by absorbing moisture from the air. It is very constant but of low E.M.F. Jablochkoff described a battery in which plates of carbon and iron are placed in fused nitre; the carbon is here the electropositive element, being rapidly consumed in the liquid. Planté's and Faure's Secondary Batteries, and Grove's Gas Battery, are described in Arts. 492, 493. The so-called Dry Pile of Zamboni deserves notice. It consists of a number of paper disks, coated with zincfoil on one side and with binoxide of manganese on the other, piled upon one another, to the number of some thousands, in a glass tube. Its internal resistance is enormous, as the internal conductor is the moisture of the paper, and this is slight; but its electromotive-force is very great, and a good dry pile will yield sparks. Many years may elapse before the zinc is completely oxidized or the manganese exhausted. In the Clarendon Laboratory at Oxford there is a dry pile, the poles of which are two metal bells: between them is hung a small brass ball, which, by oscillating to and fro, slowly discharges the electrification. It has now been continuously ringing the bells for fifty years. 194. Effect of Heat on Cells. If a cell be warmed it yields a stronger current than when cold. This is chiefly due to the fact that the liquids conduct better when warm, the internal resistance being thereby reduced. A slight change is also observed in the E.M.F. on heating; thus the E.M.F. of a Daniell's cell is about 11 per cent higher when warmed to the temperature of boiling water, while that of a bichromate battery falls off nearly 2 per cent under similar circumstances. In the Clark standard cell the E.M.F. decreases slightly with temperature, the coefficient being 0-00077 per degrees centigrade. Its E.M.F. at any temperature may be calculated by the formula, E.M.F. = 1·434 [1 − 0·00077 (@ – 15) ] volt. LESSON XVI.- Magnetic Actions of the Current 195. Oersted's Discovery. - A connexion of some kind between magnetism and electricity had long been suspected. Lightning had been known to magnetize knives and other objects of steel; but almost all attempts to imitate these effects by powerful charges of electricity, or by sending currents of electricity through steel bars, had failed.* About 1802 Romagnosi, of Trente, vaguely observed that a voltaic pile affects a compass-needle. The true connexion between magnetism and electricity remained, however, to be discovered. In 1819, Oersted, of Copenhagen, showed that a magnet tends to set itself at right angles to a wire carrying an electric current. He also found that the way in which the needle turns, whether to the right or the left of its usual position, depends upon the position of the wire that carries the current - whether it is above or below the needle, and on the direction in which the current flows through the wire. 196. Oersted's Experiment. Very simple apparatus suffices to repeat the fundamental experiment. Let a magnetic needle be suspended on a pointed pivot, as in Fig. 107. Above it, and parallel to it, is held a stout * Down to this point in these lessons there has been no connexion between magnetism and electricity, though something has been said about each. The student who cannot remember whether a charge of electricity does or does not affect a magnet, should turn back to what was said in Art. 99. copper wire, one end of which is joined to one pole of a battery of one or two cells. The other end of the wire is then brought into contact with the other pole of the battery. As soon as the circuit is completed the current flows through the wire and the needle turns briskly aside. If the current be flowing along the wire above the needle in the direction from north to south, it will cause the N-seeking end of the needle to turn eastwards; if the current flows from south to north in the wire the N-seek + Fig. 107. ing end of the needle will be deflected westwards. If the wire is, however, below the needle, the motions will be reversed, and a current flowing from north to south will cause the N-seeking pole to turn westwards. 197. Ampère's Rule. — To keep these movements in memory, Ampère suggested the following fanciful but useful rule. Suppose a man swimming in the wire with the current, and that he turns so as to face the needle, then the N-seeking pole of the needle will be deflected towards his left hand. In other words, the deflexion of the N-seeking pole of a magnetic needle, as viewed from the conductor, is towards the left of the current. For certain particular cases in which a fixed magnet pole acts on a movable circuit, the following converse to |