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. 182.-Other Batteries.-Numerous other forms of battery have been suggested by different electricians. There are three, of theoretical interest only, in which the electromotive-force is due, not to differences of potential at the contact of dissimilar metals, but to differences of potential at the contact of a metal or metals with liquids. 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 caustic soda, 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. Bennett has lately described a cheap and most efficient battery, in which the metals are iron and zinc, and the exciting liquid a strong solution of caustic soda. Old meat-canisters packed with iron filings answer well for the positive element, and serve to contain the solution. Scrap zinc thrown into mercury in a shallow inner cup of porcelain forms the negative pole. Skrivanoff has modified the zinc-carbon cell of Latimer Clark, by employing a stiff paste made of ammonio-mercuric chloride and common salt, thereby rendering the cells dry and portable. Jablochkoff has described a battery in which plates of carbon and iron are placed in fused nitre; the carbon is here the electro-positive element, being rapidly consumed in the liquid. Plante's and Faure's Secondary Batteries, and Grove's Gas Battery, are described in Arts. 415, 416. The so-called Dry Pile of Zamboni deserves notice. It consists of a number of paper discs, coated with zinc foil 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 oxidised 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 electricity. It has now been continuously ringing the bells for over forty years. 183. Effect of Heat on Batteries.-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 1 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. LESSON XVI.—Magnetic Actions of the Current. 184. About the year 1802 Romagnosi, of Trente, discovered that a voltaic pile affects a magnetised needle, and causes it to turn aside from its usual position. The discovery, however, dropped into oblivion, having never been published. A connection of some kind between magnetism and electricity had long been suspected. Lightning had been known to magnetise 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.1 The true connection between magnetism and electricity remained to be discovered. In 1819, Oerstedt, 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. 185. Oerstedt's Experiment.-Very simple apparatus suffices to repeat the fundamental experiment. Let a magnetic needle be suspended on a pointed pivot, as in Fig. 78. Above it, and parallel to it, is held a stout + N Fig. 78. 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 1 Down to this point in these lessons there has been no connection 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. 91. 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.-seeking 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. 186. 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 deflection 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 Ampère's Rule will be found convenient. Suppose a man swimming in the wire with the current, and that he turns so as to look along the direction of the lines of force of the pole (i.e. as the lines of force run, from the pole if it be N.-seeking, towards the pole if it be S.-seeking), then he and the conducting wire with him will be urged toward his left. 187. A little consideration will show that if a current be carried below a needle in one direction, and then back in the opposite direction above the needle, by bending the wire round, as in Fig. 79, the forces exerted on the needle by both portions of the current will be in the same direction. For let a be the N.-seeking, and 6 the S.-seeking, pole of the suspended needle, then the tendency of the current in the lower part of the wire will be to turn the needle so that a comes towards the observer, while b Fig. 79. retreats; while the current flowing above, which also deflects the N.-seeking pole to its left, will equally urge a towards the observer, and b from him. The needle will not stand out completely at right-angles to the direction of the wire conductor, but will take an oblique position. The directive forces of the earth's magnetism are tending to make the needle point north-and-south. The electric current is acting on the needle, tending to make it set itself west-and-east. The resultant force will be in an oblique direction between these, and will depend upon the relative strength of the two conflicting forces. If the current is very strong the needle will turn widely round; but could only turn completely to a right-angle if the current were infinitely strong. If, however, the current is feeble in comparison with the directive magnetic force, the needle will turn very little. 188. This arrangement will, therefore, serve roughly as a Galvanoscope or indicator of currents; for the movement of the needle shows the direction of the current, and indicates whether it is a strong or a weak one. This apparatus is too rough to detect very delicate currents. To obtain a more sensitive instrument there are two possible courses: (i) Increase the effective action of the current by carrying the wire more than once round the needle: (i) Decrease the opposing directive force of the earth's magnetism by some compensating contrivance. 189. Schweigger's Multiplier.—The first of the above suggestions was carried out by Schweigger, who constructed a multiplier of many turns of wire. A suitable frame of wood, brass, or ebonite, is prepared to receive the wire, which must be "insulated," or covered with silk, or cotton, or guttapercha, to prevent the separate turns of the coil from coming into contact with each other. Within this frame, which may be circular, elliptical, or more usually rectangular, as in Fig. 80, the needle is suspended, the frame being placed so that the |