When it is made to oscillate too much it may be brought to rest by lowering the piece nip a mo ment. lf the two needles of the index have exact ly the same power, it will have the highest mobi lity; but if this is not obtained, the bent magnet tie is to be so placed upon the pillar tt that the two nearest poles of the index are repelled. By ap proaching or retiring the. pillar, the magnet may be brought into such a position that the directive power of the index is scarcely sensible. When the instrument is in this state it can make sensible the difference between two pieces of metal, of which one differs only from the other by alloys, when a powerful liquid is applied. When a more con siderable effect is to be tried, the bent magnet is put in such a position that it attracts the nearest poles of the index. When the magnet is near the index, and the current makes the index deviate very little, the deviation increases as the magnet is removed. The distance of the magnet being mea sured by the scale, this arrangement may contri bute much to the determination of the powers. As the needles submitted to the effect of the current can never rest at an angle greater than 90°, the needle is prevented from going farther by means of two small pins here marked with the Greek let ter c.
The use of the electromagnetical multiplier is very extensive. Before the invention of this in strument, a prepared frog was considered as the nicest test for galvanism; the multiplier surpasses it by far. Mr. Po,ggendorg has made a very ex tensive trial upon the galvanic series of metals and other concLuctors, by means of this instrument. Professor Oersted has made use of it, for confirm ing the discovery earlier made by Zamboni, upon electrical currents which two pieces of one metal makes with a liquid. He has also discovered, by means of this instrument, that two equal pieces of metal give galvanical effects, when one of the pieces is earlier introduced in the fluid than the other, a fact which Sir Humphry Davy has confirm ed, as it appears, without knowing Oersted's ex periments. Professor Oersted has also made use of this instrument for trying silver. With a pow erful liquid conductor, solution of potash and muri atic acid for instance, silver pieces, whose alloy differs less than a hundredth, give a deviation of several degrees. As silver containing brass gives more effect than silver containing an equal quanti ty of copper, when muriatic acid is employed, but less when solution of potash is the liquid conduc tor, the presence of brass in silver is easily dis covered by this instrument. It need scarcely he mentioned that gold and other metals may be tried in the same manner. Dr. Seebeck, at Berlin, has investigated, with much care, all the circumstances belonging to the construction of the multiplier. These researches are given in an excellent paper, read at the Royal Academy of Berlin, on the t4th December 1820, and the 8th February 1821, con taining a valuable detail of experiments upon se veral points of electromagnetism. Dr. Seebeck has proved, by experiment, what might be presum ed in theory, that the increase of the effects of the multiplier, with the number of the turns, is limited by the resistance against the transmission increas ing with the length. The effects of the multiplier increase also with the breadth of the conductor, which he made of a long and thin lamina, in the place of a wire; still the advantage of broad con ductors is only confined to experiments with consi derable powers: in feeble currents the effects of broad and narrow conductors are equal.
Several philosophers have given themselves much trouble to produce upon the needle, by means of common electricity, the same effects as those pro duced by galvanism. A simple electric spark trans mitted through a conductor passes too speedily to move the needle. A current produced by the elec
trical machine does not seem to contain a sufficient quantity of electricity for acting upon the needle without the aid of the multiplier. Even by this in strument it was tried often, without decided suc cess, until of late Mr. Colladon, at Geneva, repeat ed the experiment with a multiplier, in which the wire was covered with three folds of silk, and thus well isolated. When he approached the two ends of the wire of this instrument to the two conductors of an electric battery of 4000 square inches, so as to make the discharge go a little distance through the air, before it enters in the wire. In this man ner a current strong, and of some dura tion, is produced, whereby a considerable deviation is effected. The current produced by an electric machine caused also a deviation of several degrees in this instrument.
Professor Oersted proposed, in a paper printed in Schweigger's Chemical make use of magnetical needles, suspended in various direc tions for investigating the electrical currents in the atmosphere; but he has published nothing since that time. Mr. Colladon has, with full success, employed the multiplier, to prove the presence of electromagnetism in a thunder storm.
The idea of magnetical revolutions around the uniting wire experienced much opposition at its first publication. Professor Schweigger objected to it, that when such revolutions did exist, it would be possible to make a magnet circulate round the uniting wire. Dr. Wollaston drew the same con clusion, but with the contrary meaning; finding this result probable, he invented an instrument to prove it. The experiment having been stopped by an accident, Mr. Faraday took it up, and made an extensive series of experiments on the subject, con ducted with the same skill which he has displayed in so many other investigations. He found that not only the magnet may be made to turn round the conductor, but that likewise a movable conductor, may be made to turn round the magnet. We shall have an opportunity to return to this subject; here we can only give an account of the experiments by which the motion was communicated to the mag net. Plate DXXII. Fig. 12, represents an appara tus proper for the experiment; CCCC represents a cup of glass, or some other non-conductor, through the bottom of which passes the conductor EFG. The cup is filled with mercury, in which a small magnet All floats, being kept in a vertical position by a piece of platinum, fixed at its inferior extrem ity. It can also be kept in this position by fixing the inferior extremity to the bottom by means of a short thread of silk. D is a conductor whose low er end dips in the mercury. When a strong elec trical current is now caused to pass through this arrangement, the magnet revolves about the con ductor D. The directions of the rotations are in all cases such as the fundamental law of electromag netism indicates that they should be. A magnet can also he made to turn round its own axis by an electrical current. Let CCCC, Fig. 13, be a cup of glass or wood, nearly filled with mercury; AB a magnet, having at its lower extremity a steel point, introduced into the agate II. JK is a slip of brass or ivory, having a hole through which the magnet passes freely, and by means of which it is kept per pendicular at the superior extremity; A, is a cavity for receiving mercury; EF is a wire, at whose ex tremity is also a cup for mercury; and at I) is placed a similar one, from which proceeds a wire amalgamated on its lower extremity, in order to favour the electrical communication. When the electrical current is established by conductors plung ed in the mercury at D and F, the magnet will turn with great rapidity.