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magnetic needle is placed in its neighbourhood and parallel to it, the needle changes its direction, even when the electricities which are combining in the wire have but a very feeble tension, provided only their quantity be sufficiently great. When the positive electricity enters at the north end of the wire and the negative electricity at its south end, and the magnetic needle is placed above the wire, the north pole of the needle turns to the west; if the needle is below the wire, its north pole turns to the east. If the needle is placed on the same level with the wire and on its eastern side, the north pole moves upwards; but the same pole acquires a downward motion, when the needle is placed on the western side of the conductor. We may imagine that the two electricities move through the wire towards one another in spiral lines, the positive electricity moving to the right and disturbing the north pole of the magnetic needle, the negative electricity turning to the left and disturbing the south pole. (Oerstedt.)—One pole of a moveable magnetic needle revolves constantly in one direction round a wire conveying a current of electricity, —and with a different arrangement, the other pole revolves in the contrary direction. (Faraday.) A metallic wire twisted in the form of a helix behaves, while an electric current is running through it, exactly like a magnet, showing a north pole at one end of the helix and a south pole at the other. (Ampère & Arago.)

These effects are best exhibited by voltaic electricity, on account of its greater quantity; but common electricity likewise acts on the magnet, when it is conveyed from the conductor to the wire, not in sparks, but by fine points, or damp threads, or rarefied air. (Colladon, Pogg. 8, 336; Nörrenberg, Zeilsch. Ph. math. 3; Faraday.) Equal quantities of electricity combining together in the wire in the same time produce equal effects upon the magnet, whatever may be their tension. Hence the deflection of the needle shows the absolute quantity of electricity which is passing through the conductor. (Faraday.)

The Electrical Multiplier or Galvanometer invented by Schweigger consists of a magnetic needle (or of several needles fastened together with their opposite poles placed over one another) suspended by a delicate thread and surrounded by 100 or several hundred coils of a copper wire covered with silk. By this arrangement, the electric current is made to circulate many times round the needle, and the effects of its several parts become added together, so that a very small quantity suffices to produce deviation. (Schweigger, Schw. 31, 1; 32, 320; Oerstedt, Schw. 52, 14; Norrenberg, Zeitschr. Ph. Math. 3; Becquerel, Pogg. 2, 206; Nobili, 8, 338; 20, 213; Nervander, Ann. Chim. Phys. 55, 156.)

The electric current, besides its deviating or deflecting action on the magnetic needle, likewise exhibits a magnetizing action: If the wire through which the current passes be twisted in a spiral form round a mass of iron, the latter will become strongly magnetic, as long as the current passes through the wire. Steel likewise retains a part of the magnetism thus developed in it, after the current has ceased. A steel wire also becomes magnetized when placed outside the helix and parallel to it, but the magnetism thus excited is weaker and the direction of the poles is the reverse of that in the former position of the needle. The electro-magnetic action is not prevented by surrounding the needle by a glass tube. (Ampère & Arago.)

2. Decomposition of Latent Electricity into its two opposite kinds :-De

velopment of Electricity.

A. Electricity by Induction. When a conductor is charged with one kind of electricity-positive for example—is separated from another containing only latent electricity, by a non-conductor such as air, glass, varnish, &c. the tendency of the positive electricity of the first conductor to combine with the negative electricity of the second causes the latent electricity of the latter to be resolved into its two parts, the negative electricity going towards that part of the second conductor which is nearest to the first, whilst the positive electricity appears in the free state at the other end.

Since, as appears from this, one kind of electricity developes the opposite kind in a body containing only latent electricity, and endeavours to unite with it, it follows that electrified bodies will attract even those in which the electricity is wholly latent. Hence two easily moveable bodies suspended in air appear to repel one another*, because they are attracted by those particles of air which have not yet received any electricity from the bodies, and are therefore in a condition to supply the opposite kind of electricity. This takes place to the greatest extent with those particles of air which are situated beyond the two electrified bodies, and least with those between them.

When the tension of the positive electricity in the first conductor, and of the negative electricity thereby developed in the second becomes sufficiently great, the two electricities make their way through the interposed non-conductor in the form of a spark (the so-called simple electric spark), or of a luminous brush, and combine,--and both conductors subsequently contain an excess of positive electricity of less tension than that which previously existed in the first conductor.

If

, before this combination takes place, the positive end of the second conductor be connected with the ground by means of another conductor, the positive electricity passes away to the earth, while the negative remains in the second conductor. This is the principle of the Electrophorus, the Condenser, and the Leyden Jar.

B. Magneto-Electricity. If either pole of a magnet be thrust within a metallic spiral connected with a galvanometer, a momentary deviation of the needle will be produced, the north and south poles of the magnet producing opposite effects. If a horseshoe magnet, or a horseshoe-formed keeper belonging to it, be surrounded with silk-covered wire, and the two sharpened ends of this wire amalgamated and made to touch each other loosely,—then, on suddenly pulling the keeper off the magnet, an electric spark will pass between the ends of the wire as they are separated from one another by the jerk. (Faraday.) In the Magneto-electric Machine of Pixii, Saxtorph, &c., the horseshoe-formed keeper is surrounded with wire, and the horseshoe magnet is made to rotate rapidlyt, so that its two poles alternately approach the two ends of the keeper. The direction of the current in the spiral is reversed with each half-revolution; but by means of the

* The bodies are of course supposed to be similarly electrified. [W.]

+ This very clumsy arrangement soon fell into disuse: in all magnecto-electric machines at present used, the magnet is fixed and the keeper revolves. [W.]

Commutator, matters may be so arranged, that, as the electricity issues from the extremities of the spiral, all the positive electricity may be communicated to one conductor, and all the negative electricity to another.

Induction.-Two helices of wire covered with silk, gumlac, or caoutchouc, and wound in the same direction, are placed parallel to each other or one within the other. If now an electric current be passed through one of these helices, the primary helix,—theu, at the first instant of its passage, a current in the opposite direction will be developed in the other, the secondary helix: and at the moment when the current in the primary helix ceases, a current will pass through the secondary helix in the same direction as that in the primary helix. If an iron cylinder, or still better, a bundle of thin iron rods covered with silk, be placed within the secondary helix, the current in that helix will be of considerable strength. (Faraday.)

C. Electricity of Capillarity ? If one end of the galvanometer wire be connected with a platinum spoon containing a liquid, the other with a pair of forceps holding a porous body, an electric current is set up on dipping the body into the liquid, and continues till the body is completely saturated with the liquid. With hydrochloric or nitric acid and spongy platinum, positive electricity goes from the platinum through the galvanometer to the acid; with nitric or dilute sulphuric acid and charcoal, the current lasts about two hours and travels in the opposite direction. (Becquerel.) In the case of charcoal, chemical action may be supposed to come into play, and likewise in that of the platinum when it has not been very strongly ignited. (Gm.)

D. Electricity of the Solar Rays ? When the sun shines on a perfectly dry glass plate, the latter becomes electrical; a second plate on which the light falls after passing through the first does not become so: neither does heating by fire produce electricity. (Matteucci.)

E. Electricity of Crystals. Many crystals, while they are being heated, exhibit opposite electricities at their opposite ends. Those extremities which are positive while the temperature is rising, become negative as the crystal cools; and conversely.

Crystal-electricity is exhibited by: Tourmalin (Æpinus), topaz (Canton), axipite (Beard), boracite, prehnite (both the radiant and fibrous varieties according to Von Kobelį, Kastn. Archiv. 13, 388), sphene, zincglas, and mesotype, (Hauy); (also, according to Brewster, Greenland mesotype, skolezite, and mesolite), rock-crystal, amethyst, tartaric acid, Rochelle salt, and common sugar (Brewster); rhodizite (borate of magnesia, G. Rose), neutral tartrate of potash (Hankel), and in a slight degree by milk-sugar (Böttger, Pogg. 43, 659). Besides these, the following are enumerated by Brewster as exhibiting crystal-electricity: Diamond, sulphur, sulphate of ammonia, carbonate of potash, chlorate of potash, heavy spar, coelestin, calcspar, fluor spar, sulphate of magnesia, sulphate of magnesia and soda, beryl, iolite, diopside, vesuvian, garnet, analcime, red orpiment, lead-spar, green vitriol, ferrocyanide of potassium, corrosive sublimate, oxalate of ammonia, citric acid, and acetate of lead. Since, however, Brewster merely examined whether these bodies after being heated in a flame, exhibited any signs of electric excitement-an effect which might proceed from various causes—the correctness of his statement is doubted by Hankel, who examined most of these substances and found no signs of crystal-electricity in them.

Most of the crystals above enumerated are unsymmetrical, i. e., they exhibit differently formed faces at their corresponding ends. Moreover, Hankel remarks that rock-crystal in the solid state, and neutral tartrate of potash, Rochelle salt, and common sugar in the state of solution, are said by Biot to exhibit circular polarization of light.

Tourmalines are generally more strongly electrical in proportion as they are darker in colour, and have fewer crevices in their interior. (G. Rose, Pogg. 39, 320.) Many of them become electrical only when suddenly heated and cooled. Small tourmalines are more easily excited than large ones; and fragments more readily than entire crystals. (Becquerel.) Even the most finely divided tourmaline powder is electrical, so that when warmed it adheres together and to the surface on which it rests. (Brewster.). When a tourmaline is heated, positive electricity appears at that end of the prism which bas either a single right terminal face, or merely the three faces of the primitive rhombohedron, or both kinds of faces together, -or in addition, the faces arising by truncation of the terminal edges; and the negative electricity at that extremity which, together with the right secondary plane, has also the three last named faces, or, together with the three faces of the primitive rhombohedron, likewise the three faces of a less obtuse one. (Köhler, Pogg. 17, 146.) It is only during heating and cooling that the tourmaline shows signs of electricity; as long as the temperature remains stationary after heating, it shows none; but as soon as the temperature begins to fall, the opposite electrical state is suddenly established. If only one end be heated, that end alone becomes electrical, the electricity diminishing gradually towards the cold end till it is reduced to nothing ;—when the heated end is cooled, it acquires the opposite electricity, which gradually diminishes towards the opposite end. If one end is heated while the other is cooled, both exhibit the same kind of electricity. (Bergman, Opusc. 5, 402; Becquerel.)

If the end of the tourmaline which exhibits positive electricity when heated, be rubbed with wool, it will become still more strongly electrical, because the friction likewise developes positive electricity; but the opposite end when rubbed with wool does not become electrical, because the positive electricity developed by friction and the negative electricity developed by heat neutralize each other. (P. Erman, Pogy. 26, 607.)

In boracite, the four summits of the cube in which the tetrahedral faces are absent become positively electric; the four other diagonally opposed summits, which are replaced by tetrahedral faces, negatively electric. (Köhler.) In the dodecahedrons of rhodizite, the four corners (rhomboledral summits) which have tetrahedral faces become positively electric when heated; the four opposite corvers which have no such faces, negatively electric. (G. Rose.)

The short prism of silicate of zinc (electric calamine) is modified at one end by the faces of a rectangular octohedron, at the other by those of a rhombic octohedron; the former becomes positively electric when heated, the latter negatively. (Köhler.)

Brazilian topaz becomes strongly electrical, Siberian slightly, Saxon not at all: when topaz is heated, negative electricity appears at both ends of the prism, and positive on all the lateral faces." (P. Erman.)

Skolezite and mesolite deprived by heat of their water of crystalliza

tion and reduced to powder, still show signs of crystal-electricity. (Brewster.)

Neutral tartrate of potash crystallizes in right rhombic prisms, but acuminated with two faces at the acute lateral edges, and perpendicularly truncated at the base. The first-mentioned extremity shows negative electricity even when gently warmed, and positive on cooling; the other end exhibits the opposite state. (Hankel.)

The supposition that crystal-electricity proceeds from a structure of the crystal similar to that of the voltaic pile, is contradicted by the fact that the chemical constitution of crystals is homogeneous-whereas, in the voltaic pile, a mechanical union of heterogeneous substances is essential.

F. Thermo-Electricity. When one part of a metallic circuit is more strongly heated than the rest, an electric current is excited in it under the following circumstances : (a.) When the circuit consists of a single metal, and the heating which takes place at a particular part diminishes more rapidly on one side than on the other; and (6.) When it consists of two metals, and one of the points of junction is heated.

a. With one Metal. A metallic wire connected with the two ends of a galvanometer gives no electric current when heated in the middle: but when each end of the galvanometer is connected with a wire, and the end of one of the wires is heated, and then quickly pressed on the cold end of the other, an electric current becomes manifest by the deflection of the needle of the galvanometer. The direction and strength of this current vary with the nature of the metal employed. In the so-called positively thermo-electric metals (bismuth, platinum, gold, silver, copper, &c.), positive electricity goes from the cold piece of metal through the galvanometer to the hot piece; in those which are negatively thermo-electric (zinc, iron, antimony) from the hot to the cold end. The more one end is heated, the stronger is the current. According to Yelin, bismuth produces the strongest current with a given degree of heating; then follows antimony, then zinc, silver, platinum, copper, brass, gold, tin, and lastly lead; but, according to Nobili, this order is correct for certain temperatures only.

A simple platinum wire connected with a galvanometer also produces a current when heated, if it be tied in a knot at one point and heated near the knot; because the more rapid cooling through the knot causes unequal distribution of heat on the two sides; the positive electricity proceeds from the knot through the galvanometer to the heated part. Two copper wires do not produce so strong a current when clean as they do when covered with oxide or with a thin film of silver or gold, because the covering binders the communication of heat to the cold end, and consequently interferes with its uniform distribution. (Becquerel.)—Mercury is not thermo-electric, according to Matteucci and De la Rive; very slightly, according to Peltier.

[Many metals, as antimony, iron, and zinc, conduct positive better than negative electricity; others, as platinum, copper, and silver, exhibit the contrary relation. When a metal is heated, its power of conducting electricity diminishes altogether; but the diminution is greatest with regard to that kind of electricity which it conducts least. When a piece of metal heated at the extremity touches a cold piece of the same metal, the heat diminishes gradually from the heated end to the part of

VOL. I.

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