610 WATER BATTERY. [299. he obtained results of great interest. This battery was composed of 3520 pairs of copper and zinc plates, arranged in separate glass vessels, covered with a coating of lac varnish; the glass cells were supported on slips of glass thickly coated on both sides with shelllac, and these glass plates were insulated on varnished oaken boards, each board being further insulated by resting on thick plates of glass similarly varnished. All these precautions were found by experience to be necessary in order to preserve the insulation. When the conducting wires of this battery were brought within of an inch (omm.5) of each other, sparks were obtained, and when the wires were made to terminate in brass disks which were brought very near each other, a rapid succession of sparks was maintained, which on one occasion continued without interruption for five weeks. A permanent deflection of the galvanometer was obtained when this instrument was included in the circuit whilst the sparks were passing; under similar circumstances, paper moistened with potassic iodide and included in the circuit, speedily gave indications of the chemical decomposition of the iodide. The chemical effects produced by the water battery are, however, always feeble, but they are similar in kind and in direction to those which are obtained when acids are employed as the exciting liquid in the cells; and the principal effect that would be obtained if dilute acid were substituted for water in such a combination would be an increase in the quantity of electricity, by increasing the consumption of zinc and the chemical action in each cell in a given time. The tension of the charge would be increased by the change of the exciting liquid, in proportion as the electro-motive force of each cell was augmented when compared with the resistance offered by the liquid employed in charging the battery. Neither in the water battery nor in any other form of battery is the tension, as measured by its power of overcoming resistance to conduction, increased by increasing the size of the plates. Messrs. Warren, De La Rue, and Hugo Müller (Journ. Chem. Soc. 1868 [2], vi. 488) have constructed a battery in which the elements consist of rods of zinc and chloride of silver in a solution of sodic chloride placed in test tubes closed by stoppers of paraffin, and carefully insulated. Experiments with 2000 of these cells were made in 1875 (Proc. Roy. Soc. 1875, xxiii. 356), with 5640 in 1876 (Proc. Roy. Soc. 1876, xxiv. 167), and the number is now increased to 8040 (Proc. Roy. Soc. 1876, xxv. 322). This enormous battery has sufficient tension to produce sparks 8.5" (348 inch) long in air at the ordinary pressure when the positive 300.] ELECTRO-MAGNETISM. 611 electrode is a copper point, and the negative a copper disk. The authors find (Proc. Roy. Soc. 1876, xxiv. 167) that the length of the spark is proportional to the square of the number of cells, 600 cells having a striking distance of oc33 inch; 1200 of 0130 inch; 1800 of 0345 inch; and 2400 of 0535 inch. Calculating from the effect produced by 600 cells the theoretical lengths for 1200, 1800, and 2400 would be 0132, 0297, and 0528 inch respectively, which very closely approximate to the experimental results. The 8040 cells should give theoretically sparks 5925 inch long; but the difference between this and the experimental number is due to the fact that the cells are not quite uniform in construction, and some have been longer in use than others. 100,000 of these cells would give a spark 91.66 inches long, and a million, 9166 inches or 764 feet. It thus appears, 1. That by voltaic arrangements electricity may be obtained, exactly similar to that developed by the common machine, in its effects of tension and in induction towards surrounding objects, in the polar character of its action, and in the opposite nature of the electricities accumulated at the extremities of the apparatus. 2. That the quantity of electricity obtained by voltaic action is almost immeasurably greater than that procured by friction; but that unless its tension be exalted by using a very numerous series, it does not pass so readily through nonconductors in the form of sparks as the electricity of the common machine. 3. That, on the other hand, by allowing the electricity of the machine to discharge itself gradually through very small masses of imperfect liquid conductors which are susceptible of electrolysis, true electrolytic action may be produced. The identity of the two forces under these different degrees of tension no longer admits of question in the voltaic action the quantity is great, but the tension is feeble; whilst in the elec-. tricity of the machine the reverse is the case, the tension is very high, whilst the quantity is extremely small. § IV. ELECTRO-MAGNETISM. (300) Law of Electro-Magnetic Action.-The influence of an electric current upon a freely suspended magnetic needle has been already pointed out, but it will be needful to examine the nature of the connexion between magnetism and electricity somewhat more closely. Electricity in a state of rest has no influence upon a magnetized bar. It is only when the electricity is in motion that this magnetic action is excited. It has already been 612 TANGENT GALVANOMETER. [300. explained (253) that the direction in which the magnetic needle is deflected depends upon the direction of the current; and it has been stated that when the needle points north and south, and a wire is placed parallel to the needle, if the current flow from south to north above the needle, the north end of the needle will move westward. The power which the wire exerts upon the needle varies directly as the quantity of electricity which traverses the wire: and when the current passes through a straight wire of considerable length (so that it may be regarded as infinite in relation to the needle) the effect upon the needle varies inversely as the distance of the wire from such needle. (301) Tangent Galvanometer.-For measuring the strength of the current, galvanometers of various forms have been employed. When the strength is extremely feeble, the astatic galvanometer (fig. 207), is well adapted to the purpose, but in this form the value of the angular deviation requires to be experimentally determined for each instrument. When the current has a greater strength than can be conveniently estimated by the astatic combination, the tangent galvanometer is frequently employed. This W FIG. 238. instrument is simple, both in construction and in principle. The conductor, w, fig. 238, which is used for conveying the current round the needle, consists of a single coil of thick copper wire, bent into a circle of about 30 centimetres, or one foot in diameter. It is supported vertically in a small table t; the extremities of the wire being connected by means of binding-screws with the wires from the bat tery. Within the circle, w, a magnetic needle about an inch, or 25mm. long, is suspended by fibres of unspun silk, c, horizontally over a copper plate graduated to degrees In order to enable the movements of the needle to admit of more accurate measurement, its apparent length is increased by fastening a piece of fine copper wire to each end. This arrangement is protected from currents of air by means of a glass shade. The point of suspension of the needle is made to coincide accurately with the centre of the circle formed by the 302.] MAGNETISM OF CONDUCTING WIRE. 613 conducting wire at a is a screw for raising or lowering the needle. When the coil of the instrument is placed with its plane exactly in that of the magnetic meridian, the needle, under the influence of the directive action of the earth's magnetism, assumes a position parallel to the plane of the circle. On transmitting the current through the wire, the needle receives an impulse which, if it were free from the inductive action of the earth, would place it exactly at right angles to the coil: owing, however, to the influence of the earth, the needle is unable ever really to assume this position; but it takes one which represents the resultant of the two forces, and as the action of the earth may be assumed to be uniform, the measurement of the angle enables the strength of the current which produces the deviation to be calculated. It may be demonstrated that the strength of the current is proportioned to the tangent of the angle of deviation. The instrument cannot be relied on for angular deviations which much exceed 70°, owing to the rapidly diminishing angular deviation produced by equal increments in the strength of the current when the deflection has reached this extent; but for all currents which produce a deviation of smaller amount, it affords a convenient measure. Other forms of galvanometer have been contrived, which it will not be necessary to describe in this work. (302) Influence of a Conducting Wire in exciting Magnetism.The action of the conducting wire upon the magnetic needle is not interfered with by interposing a sheet of glass or other insulator of electricity; and the magnetic influence is cqually transmitted, although a sheet of copper, of lead, or of any other nonmagnetic metallic conductor of electricity be introduced between the needle and the wire. The electric current, however, produces no divergence of the leaves of an electroscope which is brought into its vicinity. Not only does a wire which is conveying electricity affect a needle which has been already magnetized, but the conducting wire itself, so long as it is transmitting the electric current, displays magnetic properties. If a thin wire of copper, or of any other non-magnetic metal, be employed to complete the voltaic circuit, such a wire will, for the time, attract iron filings: and the filings will be arranged in a layer of uniform thickness around the whole circumference of the wire, and along its whole length. The moment that the connexion of the battery is broken, the magnetism ceases, and the filings fall off; but the attraction may be again instantly renewed on completing the circuit. The iron filings in this case become magnets, the poles of which are 614 FIG. 239. FORMATION OF ELECTRO-MAGNETS. S [302. arranged alternately north and south around the wire. This arrangement may be better understood by reference to fig. 239, in which if w be supposed to represent a section of the wire which is transmitting a current from + to, the north end of each fragment of iron would be arranged as represented by the points, n, n, of the arrows. If short wires of soft iron be placed in the direction of the arrows around the wire, they become temporary magnets, the north and south ends of which are indicated by the letters n and s. If pieces of steel be substituted for soft iron, they become permanently magnetic; all those which are above the wire, if the current be passing in the direction shown in the figure, will have their north ends to the left, whilst in all those below, the north ends will be to the right. W n (303) Formation of Electro-Magnets.-We see, then, that every part of the wire along which a current is passing is magnetic. By coiling the conducting wire into a ring, a larger number of particles is brought to act upon a piece of soft iron, which is passed through the axis of the ring at right angles to the plane in which it lies; and by coiling up the wire into a helical form, without allowing the contiguous turns to touch A FIG. 240. W B each other, and supporting them upon a glass tube, the action of a very considerable length of wire may be concentrated in a very effective manner upon the same piece of soft iron, placed as at c, d, fig. 240. Very powerful temporary magnets may thus be obtained. If the wire be covered with cotton, or, still better with silk, to insulate the coils from each other, the effects may be greatly augmented by winding a second series of coils upon the first, and a third upon the second, and so on, till six or seven layers of wire are coiled around the bar which is to be magnetized. A row of coils which follows the direction of a left-handed screw would neutralize the effect produced by the right-handed helix, unless the current were reversed in its direction as it passes through such a coil, as a glance at fig. 241 will show, where a represents a right-handed helix, в a left-handed helix: in the straight portions of the wire, the current, as indicated by the arrows, flows in the same direction in both: but it is reversed in |