Page:The New International Encyclopædia 1st ed. v. 01.djvu/562

AMPELOPSIS. the dying leaves of ampelopsis turn a most bril- liant red and yellow. The vine called Japanese ivr or Boston ivy belongs to this same genus of plants, and is probably the favorite of all the hardy vines grown in cities in the eastern United States. It is especially effective for a considerable area from Massachusetts to Florida and on the Pacific coast, but north of the Ohio and west of the Mississippi River it is tender. Fossil forms. — A single fossil species of this genus, Ampelopsis tertiaria, has been recognized in the White River beds of the Tertiary in Wyoming.

AMPERE, .am-par' (Derived from the name of Ampere). The practical unit employed in measuring tlic intensity of an electric current, and technically defined as one-tenth of the C.G.S. electro-magnetic unit (see Electrical Units) of current. By intensity of current is meant the quantity of electricity which passes any cross section of the wire or conductor in the course of one second of time. The current de- pends upon the resistance of, and the difference of potential at the ends of, the conductor, vary- ing inversely as the former and directly as the -p latter. From Ohm's law that C^ -, when C is ix the current, E the difference of potential, and R the resistance, we have amperes^-^j. A cur- ohms rent of electricity can" do work in decomposing certain chemical substances into their respective elements, consequently by measuring the amount of a substance so decomposed in unit time we can ascertain the strength of the current. The ampere, accordingly, has been legally defined as the amount of a constant current which, when passed through a solution of nitrate of silver, in accordance with standard specifications, de- posits silver at the rate of 0.001118 of a gram per second. The detailed specifications prepared by the National Academy of Sciences of the United States provide that in measuring currents of about one ampere in strength the silver vol- tameter (q.v. ) emplo.ved should consist of a pla- tinum bowl as the cathode, containing a neutral solution of pure silver nitrate in the proportion of 15 parts by weight of the nitrate to 85 parts of water, and an anode consisting of a disk or plate of pure silver wrapped with pure filter paper. Precautions are to be observed to insure cleanliness and accuracy of measurement before, during, and after the experiment. The silver deposited in the platinum bowl is then washed and weighed, and the gain in weight expressed in grams is divided by the number of seconds during which the current passed and by 0.001118. Within the past few years it has been proved that the quantity of silver deposited in a vol- tameter depends upon many conditions previous- ly unsuspected, such as the age of the solution, tile construction of the voltameter, etc. P'or full details as to our present knowledge of the subject, the reader should consult a paper by A. Leduc on the electro-chemical equivalent of silver, copper, and water, in the Reports of the International ('ovfjrcss of I'hysics at Paris, Vol- ume II. (1900), and the original report of the National Academy of Sciences on Standards for Elect rirnl Measure.

AMPERE ax'par', (1775- 1836). A distinguished French physicist, math- ematician, and naturalist, born at Lyons. The death of his father under the guillotine in 1793 made a deep and melancholy impression on th& mind of the young man, and he sought solace in the study of nature and the Latin poets. In 1,801, after he had been engaged for some time as private mathematical tutor at Lyons, he be- came professor of physics in the Central School of the department of Ain at Bourg. He was afterwards professor of mathematics at Lyons. He was called to Paris, where he distinguished himself as an able teacher in the Polytechnic School. He began his career as an author by the essay on the mathematical theory of chances, Sur la thcorie muthematique dii jeu (Lyons, 1802). In 1814 he was elected a member of the Academy of Sciences, and in 1824 was appointed professor of experimental physics in the College de France. Science is largely indebted to Am- pere, especially for his electro-dynamic theory and his original views of the identity of electri- city and magnetism, as given in his Recueil d'ob- servatioiis electro-dynainiques (Paris, 1822), and his Thcorie des phcnomenes clectro-dyna- miques (Paris, 1826). Amp6re was the inventor of the astatic needle (q.v.), which made possible the modern astatic galvanometer (q.v.). He was the first to sliow that two parallel conductors carrying currents traveling in the same direction attract each other, while if traveling in opposite directions they I'epel each other. Amp6re also formulated the theory that there were currents, of electricity circulating in the earth in the direc- tion of its diurnal revolution which attracted the magnetic needle. The ampere (q.v.), or unit of the strength of an electrical current, is named after him. Ampere's scientific papers are largely contained in the Annales de Physique et de Chimic. A eulogy by Arago, delivered shortly after his death, which contains an account of his life, will be found translated into English in the annual report of the Smithsonian Institu- tion for 1872 (Washington, 1872).

AMPERE, iiN'par', (1800-1804). A French academician, essayist, literniv liistorian and professor in the College de France. He was born at Lyons, the son of Andrg Marie Amp&re. His essays, collected as Littcrature et voyuycs (2 volumes, 1834), attest his knowledge of foreign countries and their lit- eratures. Better known are the essays on the- formation of the French language, Histoire de la formation de la lanyiie fran^uise (1841), and La Grece, Pome et Dante (1850). Ampere was a judicious critic, a profound scholar, and master of a precise style. Consult Potton, Etudes snr la vie et les travaux de Jean Jacques Ampere (Paris, 1867).

AMPERE TURNS. In problems involving the magnetic field produced b3' a current flowing in a coil of wire, two of the factors necessary are the strength of current in amperes, and the num- ber of turns or revolutions that the conductor through which the current passes makes in forming the coil or solenoid. Their product is known as the ampere turns.

AM'PHIARA'US (Gk. 'Afiipidpaoc, Amphiaraos). A Greek chthonic divinity. At Oropus he had a celebrated oracle, healed the sick, was honored with games, and was worshiped elsewhere as a hero and propliet. In legend Amphiaraiis is prominent in the war of the Seven against Thebes, into which he was forced by the treachery of his wife, who was bribed by Poly-