AMPERE, 5n-par' (Derived from the name of Ampere). The practical unit, employed in measuring the intensity of an electric current, and technically defined as one-tenth of the C.G.S. electromagnetic 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 latter. From Ohm's law when C is the current, E the difference of potential, and R the resistance, we have 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.) employed 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, the construction of the voltameter, etc. For full details as to our present knowledge of the subject, the reader should consult a paper by A. Leduc on the electrochemical equivalent of silver, copper. and water, in the Reports of the International Congress of Physics at Paris, Vol tune II. (1900), and the original report of the National Academy of Sciences on Standards for Electriral Measure,