Page:The American Cyclopædia (1879) Volume VI.djvu/523

 I conductor on the short perpendicular one. AJ. the former be bent into a horizontal circle, then it is evident, from the reasons we have before given, that the short conductor, moving perpetually round it parallel to itself or retain- ing its perpendicular position, will describe a ELECTRO-MAGNETISM 515 FIG. 1. Rotation of a Vertical Current. circle. This may be shown experimentally by bending a piece of wire into the form of a n, and supporting it vertically on the point of a perpendicular wire which fits lightly into a socket on the under side of the middle of the arch. (See fig. 1.) If the two ends of this bent wire dip into a circular basin of mercury through the middle of which, surrounded by a glass tube, the supporting pointed wire passes, and if a powerful current of galvanism be sent down through this wire, it will descend through the legs of the f] into the mercury; and if at the same time a powerful current be passed through a ring or hoop conductor placed hori- zontally around it, a rapid rotation of the bent wire will take place. Now since mag- netism, according to the theory which we have adopted, consists in currents of electricity re- volving at right angles to the magnet, if a magnetized bar be introduced within the branches of the bent conductor, a similar rotary motion will be produced. This fact was first shown experimentally by Prof. Faraday. It is, however, a logical consequence of the theory of Ampere, and might have been deduced from it. A beautiful illustration of the phenomena of terrestrial magnetism was first exhibited by Prof. Barlow of Woolwich, England. He prepared a wooden globe, into the surface of which a long conductor was buried in a spiral groove extending with many turns from pole to pole. This globe was after- ward covered with paper, on which were drawn the continents and oceans. When a small dipping needle was placed over this ap- paratus and a current of galvanism sent through the concealed conductor, the needle assumed a direction similar to that which would be due to an analogous position on the earth's sur- face ; and since, in all cases, the needle tends to arrange itself at right angles with the direc- tion of the current, by a proper adjustment of the conducting wire in the groove the varia- tion of the needle at every point of the earth's surface could be accurately represented. The explanation of all the phenomena of ordinary magnetism readily flows from the same prin- ciples. We have stated that if a magnet be broken in two, each half becomes a separate magnet, exhibiting north and south polarity. If the hypothetical magnet which we have il- lustrated by a pile of shillings be broken in the same way, each part will become a separate magnet, and the two ends of the two parts which were previously in contact will attract each other, because the currents will be revolving in the same di- rection ; but if we turn the other end of one magnet to the same end of the other, repulsion will ensue, because the currents are revolving in different directions. By a little reflection it will not be difficult to explain or to antici- pate the action of the two mag- nets on each other under any assumed con- dition. In adopting this hypothesis, it is not necessary to contend for the actual existence of electrical currents in the magnet or even in the earth. It is sufficient to assert that all the peculiarities of he known phenomena of mag- netism are precisely such as would result from an assemblage of currents such as Ampere has supposed to exist. It is probable that in the phenomena of magnetism a molecular distribu- tion of the fluid takes place which is analogous to that in a wire transmitting a current. In- deed, we know that at the moment of magnet- izing a bar of iron, a molecular change is pro- duced in the metal of sufficient intensity to cause a sensible sound ; a fact which was first noticed by Prof. Charles G. Page of Washing- ton. It is an interesting fact in the history of science, that discoveries in one branch serve to throw light on other branches, and in many cases to furnish instruments by which actions too delicate to be appreciated by ordinary means may be exhibited and measured. Soon after the discovery of Oersted, Prof. Schweig- ger of Germany covered a long wire with silk and coiled it into the form of a rectangle, within which he suspended by means of a fibre of silk a magnetic needle. When a very feeble current of electricity was sent through this con- ductor, each turn of the wire acted on the needle to turn it at right angles to its own di- rection ; and in this way an instrument called the galvanometer was produced, by which the most feeble galvanic action in the form of a current is exhibited. It has been before stated that Arago and Davy discovered that the con- ducting, wire through which a galvanic current is flowing is capable of inducing magnetism in iron filings. They also showed that a discharge of ordinary electricity, when made above or below a sewing needle, gave it definite polar- ity ; and in this way the reason of the failure of Franklin and others, who had attempted to magnetize steel wire by ordinary electricity, was explained. In these attempts the elec- tricity was sent through the length of the