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', André Marie (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', Jean Jacqites Antoine
(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-
