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 science of electromagnetism, or, as he called it, electrodynamics, that Ampère’s fame mainly rests. On the 11th of September 1820 he heard of H. C. Oersted’s discovery that a magnetic needle is acted on by a voltaic current. On the 18th of the same month he presented a paper to the Academy, containing a far more complete exposition of that and kindred phenomena. (See .) The whole field thus opened up he explored with characteristic industry and care, and developed a mathematical theory which not only explained the electromagnetic phenomena already observed but also predicted many new ones. His original memoirs on this subject may be found in the ''Ann. Chim. Phys.'' between 1820 and 1828. Late in life he prepared a remarkable Essai sur la philosophie des sciences. In addition, he wrote a number of scientific memoirs and papers, including two on the integration of partial differential equations (Jour. École Polytechn. x., xi.). He died at Marseilles on the 10th of June 1836. The great amiability and childlike simplicity of Ampère’s character are well brought out in his Journal et correspondance (Paris, 1872). AMPÈRE, JEAN JACQUES (1800–1864), French philologist and man of letters, only son of André Marie Ampère, was born at Lyons on the 12th of August 1800. He studied the folk-songs and popular poetry of the Scandinavian countries in an extended tour in northern Europe. Returning to France, he delivered in 1830 a series of lectures on Scandinavian and early German poetry at the Athenaeum in Marseilles. The first of these was printed as De l’Histoire de la poésie (1830), and was practically the first introduction of the French public to the Scandinavian and German epics. In Paris he taught at the Sorbonne, and became professor of the history of French literature at the Collège de France. A journey in northern Africa (1841) was followed by a tour in Greece and Italy, in company with Prosper Mérimée and others. This bore fruit in his Voyage dantesque (printed in his Grèce, Rome et Dante, 1848), which did much to popularize the study of Dante in France. In 1848 he became a member of the French Academy, and in 1851 he visited America. From this time he was occupied with his chief work, L’Histoire romaine à Rome (4 vols., 1861–1864), until his death at Pau on the 27th of March 1864.

AMPEREMETER, or, an instrument for the measurement of electric currents in terms of the unit called the ampere. (See ; ; and .) Since electric currents may be either continuous, i.e. unidirectional, or alternating, and the latter of high or of low frequency, amperemeters may first be divided into those (1) for continuous or direct currents, (2) for low frequency alternating currents, and (3) for high frequency alternating currents. A continuous electric current of one ampere is defined to be one which deposits electrolytically 0·001118 of a gramme of silver per second from a neutral solution of silver nitrate. An alternating current of one ampere is defined to be one which produces the same heat in a second in a wire as the unit continuous current defined as above to be one ampere. These definitions provide a basis on which the calibration of amperemeters can be conducted. Amperemeters may then be classified according to the physical principle on which they are constructed. An electric current in a conductor is recognized by its ability (a) to create heat in a wire through which it passes, (b) to produce a magnetic field round the conductor or wire. The heat makes itself evident by raising the temperature and therefore elongating the wire, whilst the magnetic field creates mechanical forces which act on pieces of iron or other conductors conveying electric currents when placed in proximity to the conductor in question. Hence we may classify ammeters into (1) Thermal; (2) Electromagnetic, and (3) Electrodynamic instruments.

1. Thermal Ammeters.—These instruments are also called hot-wire ammeters. In their simplest form they consist of a wire through which passes the current to be measured, some arrangement being provided for measuring the small expansion produced by the heat generated in the wire. This may consist simply in attaching one end of the wire to an index lever and the other to a fixed support, or the elongation of the wire may cause a rotation in a mirror from which a ray of light is reflected, and the movement of this ray over a scale will then provide the necessary means of indication. It is found most convenient to make use of the sag of the wire produced when it is stretched between two fixed points (K1K2, fig. 1) and then heated. To render the elongation evident, another wire is attached to its centre, S2, this last having a thread fixed to its middle of which the other end is twisted round the shaft of an index needle or in some way connected to it through a multiplying gear. The expansion of the working wire when it is heated will then increase or create a sag in it owing to its increase in length, and this is multiplied and rendered evident by the movement of the index needle. In order that this may take place, the heated wire must be flexible and must therefore be a single fine wire or a bundle of fine wires. In ammeters for small currents it is customary to pass the whole current through the heating wire. In instruments for larger currents the main current passes through a metallic strip acting as a bye-pass or shunt, and to the ends of this shunt are attached the ends of the working wire. A known fraction of the current is then indicated and measured. This shunt is generally a strip of platinoid or constantin, and the working wire itself is of the same metal. There is therefore a certain ratio in which any current passing through the ammeter is divided between the shunt and the working wire.

Thermal ammeters recommend themselves for the following reasons:—(1) the same instrument can be used for continuous currents and for alternating currents of low frequency; (2) there is no temperature correction; (3) if used with alternating currents no correction is necessary for frequency, unless that frequency is very high. It is, however, requisite to make provision for the effect of changes in atmospheric temperature. This is done by mounting the working wire on a metal plate made of the same metal as the working wire itself; thus if the working wire is of platinoid it must be mounted on a platinoid bar, the supports which carry the ends of the working wire being insulated from this bar by being bushed with ivory or porcelain. Then no changes of external temperature can affect the sag of the wire, and the only thing which can alter its length relatively to the supporting bar is the passage of a current through it. Hot-wire ammeters are, however, liable to a shift of zero, and means are always provided by some adjusting screw for slightly altering the sag of the wire and so adjusting the index needle to the zero of the scale. Hot-wire ammeters are open to the following objections:—The scale divisions for equal increments of current are not equal in length, being generally much closer together in the lower parts of the scale. The reason is that the heat produced in a given time in a wire is proportional to the square of the strength of the current passing through it, and hence the rate at which the heat is produced in the wire, and therefore its temperature, increases much faster than the current itself increases. From this it follows that hot-wire ammeters are generally not capable of giving visible indications below a certain minimum current for each instrument. The instrument therefore does not begin to read from zero current, but from some higher limit which, generally speaking, is about one-tenth of the maximum, so that an ammeter reading up to 10 amperes will not give much visible