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 14 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 (0 – 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 + N 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 fired 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 (ie. 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. 198. Corkscrew Rule. - More convenient is the following rule suggested by Maxwell. The direction of the current and that of the resulting magnetic force are related to one another, as are the rotation and the forward travel of an ordinary (righthanded) corkscrew. In Fig. 108, if the circle represents the circulation of current, the arrow gives the direction of the resulting magnetic force. One advantage of this rule is, that it is equally applicable in the other case. If the arrow represents the direction of the current along a straight wire, the circle will represent the direction of the resulting magnetic force around it. Fig. 108. 199. Galvanoscope. 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. 109, 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 b the S-seeking, pole of the suspended needle, then the tendency of the current in the lower Fig. 109. part of the wire will be to turn the needle so that a comes towards the observer, while b 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. 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; (ii.) decrease the opposing directive force of the earth's magnetism by some compensating contrivance. 200. 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. 110, the needle is suspended, the frame being placed so that the wires lie in the magnetic meridian. The greater the number of turns the more powerful will be the magnetic |