Circuits of iron, with gold or silver, have like wise, as well as those which it forms with copper, a minimum of effect, by a certain elevated temper ature, and in a still higher one their current changes its direction. In circuits of platina with gold, silver, lead, zinc, copper, and palladium, the differences of the intensities form an increasing arithmetical series.
Mr. Becquerel found that two pieces of platina form an active thermoelectrical current, when they are not of a perfectly equal nature. He cut through a piece of platina wire, and had one of the pieces drawn thinner; these two formed a thermo electrical circuit. He maintains that the circuit is not efficacious unless a piece of some other metal is soldered to the one end of the wire, upon which statement we cannot but entertain some doubt, though Mr. Becquerel's authority is of no little weight. As Mr. Becquerel had found that the in crements of the magnetic effect preserve the more their proportion to the increments of temperature, the more difficult the metal is in being melted, he considers a circuit of two unequal pieces of platina as a pyrometer. By means of this, he has tried the temperature of the different parts of a spirit flame, and estimated the temperature of the blue flame bordering the white, at 1350° Centigr., or 2162° Fahr.; in the white part he estimated it to be 1080° Cent, or 1976° Fahr., and in the darker part of the flame to be 780° Cent. or 1426° Fahr. The last he considers as too high, because the other parts of the flame contributed to heat the junction.
7'crrestrial 'e cannot pass by this subject entirely, though we must treat it very briefly. Mr. Ampere, who thinks that magnetism consists only in transverse electrical currents, must, in consequence of his hy pothesis, suppose an electrical current round the earth, from east to west. He thinks that the nu merous strata, of which our globe is composed, may form considerable galvanic arrangements; still he supposes that the rotation of the earth can not but have an effect on the electric currents around it. Mr. Ampere, in consequence of his system, admits no other magnetism of the earth than these currents. The opinion, that the earth is surrounded by electrical currents, though not strictly proved, is very probable. As for the gal vanic arrangements which the earth is supposed to contain, there can be no doubt that the strata of the earth may form such combinations; but it is not at all proved that they produce a current from cast to west. As far as the different currents formed by the strata, do not destroy the effect of each other, it is probable that their resultant effect lies nearly in the perpendicular; for the most general situation of the strata, is that one is placed above the other, generally with some inclination; but as this inclina tion may have all possible directions, the effects of the galvanic arrangements, (in so far as their ac tion should have a horizontal direction, and thus be founded upon the inclinations,) must destroy each other, even if the inclination towards one side should be somewhat predominant; for galvanic ar rangements combined in variable directions of their currents, produce a total effect much feebler than the difference of their positive and negative effects. The most efficacious excitation of electri city upon the earth appears to be produced by the sun. Its light passes round the globe from sunrise
to sunrise, and produces evaporation, deoxidation and heat. Evaporation in contact with oxidable matters, produces electricity, as has already been asserted, but first exactly elucidated by the ingeni ous experiments of Mr. Pouillet. That the deoxi dation which the sun produces during the day not only of the surface of plants, but also upon the sur face of many other bodies, particularly when mois tened, excites electric currents, is a well known galvanic fact. That the heat produced by the sun beams, and also circulating from east to west, must produce an electrical current can scarcely be doubt ed; for though the surface of the earth be not com posed of perfect conductors, and this resistance should make a common current insensible, the cele rity of the circulation may, on the other hand, aug ment the effect to a degree sufficient for producing some effect upon the magnetic needle. Now, if it be admitted that the sun produces an electric cur tent round the earth, this current must form a zone of considerable breadth, whose most intense part is situated in the plane of the circle, in which the sun seems to make its daily motion. Thus the situation of the most intense part of the zone varies with every day of the year. If we suppose that the earth had no other magnetism than that of this zone, a steel needle made magnetic by an artificial current, and then freely suspended, should take a direction towards the north and the south. Even a steel needle laid across the great natural current should be made magnetic, and suspended, take its direction accordingly. liut the great current must also produce magnetism in the body of the earth it self; and as the magnetic effects of the inferior side of the current are opposite to those of the superior one, the magnetic poles of the earth become the opposite to those of the needle directed by the cur rent, and should, therefore, if we for a moment sup pose the eleetric zone destroyed, still give it the same direction. Thus the earth seems to have a constant magnetic polarity, produced, in the course of time, by the electrical currents which surround it, and a variable magnetism produced immediately by the same current. As the sun does not produce an equal effect upon water as upon solid bodies, the intensity of the current cannot be equal in all parts of a parallel circle, and therefore the direc tion of the needle cannot be perpendicular to the equator, nor can it form everywhere the same angle with the equator, for the lines of equal electromag netic intensity must he twice bent by the influence of the two great masses of continent. The yearly and daily change of the electromagnetic zone, must occasion yearly and daily variations. As to the va riations comprehended in greater periods, we might perhaps attribute them to a motion of the coolest points in each continent, which it appears cannot remain the same for ever, because the currents of warmer air must principally be directed towards such points; but we shall leave this research to future times, which may discover causes concealed from us, for explaining the great and secret revolu tion, which is continually performing in our globe.