ELECTRICAL UNITS. Final and official recommendation of the Chamber of Delegates of the International Electrical Congress, held at Chicago, 1893. Resolved, That the several governments represented by the delegates of this International Congress of Electricians be, and they are hereby, recommended to formally adopt as legal units of electrical measure the following: As a unit of resistance, the international ohm, which is based upon the ohm equal to 109 units of resistance of the C. G. S. system of electro-magnetic units, and is represented by the resistance offered to an unvarying electric current by a column of mercury at the temperature of melting ice 14.452 1 grams in mass, of a constant cross-sectional area and of the length of 106.3 centimeters. As a unit of current, the international ampere, which is one-tenth of the unit of current of the C. G. S. system of electro-magnetic units, and which is represented sufficiently well for practical use by the unvarying current which, when passed through a solution of nitrate of silver in water, and in accordance with accompanying specifications,1 deposits silver at the rate of 0.001 118 of a gram per second. 1. In the following specification the term silver voltameter means the arrangement of apparatus by means of which an electric current is passed through a solution of nitrate of silver in water. The silver voltameter measures the total electrical quantity which has passed during the time of the experiment, and by noting this time the time average of the current, or, if the current has been kept constant, the current itself can be deduced. In employing the silver voltameter to measure currents of about one ampere, the following arrangements should be adopted: The kathode on which the silver is to be deposited should take the form of a platinum bowl not less than 10 centimeters in diameter and from 4 to 5 centimeters in depth. The anode should be a plate of pure silver, some 30 square centimeters in area and 2 or 3 millimeters in thickness. This is supported horizontally in the liquid near the top of the solution by a platinum wire passed through holes in the plate at opposite corners. To prevent the disintegrated silver which is formed on the anode from falling onto the kathode, the anode should be wrapped around with pure filter paper, secured at the back with sealing wax. The liquid should consist of a neutral solution of pure silver nitrate, containing about 15 parts by weight of the nitrate to 85 parts of water. The resistance of the voltameter changes somewhat as the current As a unit of electro-motive force, the international volt, which is the electro-motive force that, steadily applied to a conductor whose resistance is one international ohm, will produce a current of one international ampere, and which is represented sufficiently well for practical use by 19 of the electro-motive force between the poles or electrodes of the voltaic cell known as Clark's cell, at a temperature of 15° C., and prepared in the manner described in the accompanying specification.2 As a unit of quantity, the international coulomb, which is the quantity of electricity transferred by a current of one international ampere in one second. As a unit of capacity, the international farad, which is the capacity of a condenser charged to a potential of one international volt by one international coulomb of electricity. As a unit of work, the joule, which is equal to 107 units of work in the C. G. S. system, and which is represented sufficiently well for practical use by the energy expended in one second by an international ampere in an international ohm. As a unit of power, the watt, which is equal to 107 units of power in the C. G. S. system, and which is represented sufficiently well for practical use by work done at the rate of one joule per second. As the unit of induction, the henry, which is the induction in a circuit when the electro-motive force induced in this circuit is one international volt, while the inducing current varies at the rate of one ampere per second. passes. To prevent these changes having too great an effect on the current, some resistance besides that of the voltameter should be inserted in the circuit. The total metallic resistance of the circuit should not be less than 10 ohms. 2. A committee, consisting of Messrs. Helmholtz, Ayrton and Carhart, was appointed to prepare specifications for the Clark's cell. Their report has not yet been received. IN COPPER WIRE. IN THE following tables of copper wire the value of the mil-foot is taken as the standard. The temperature coëfficient is interpolated for 60° F. and 75° F. from the values given in the second table. In the table for B. & S. G., the actual sizes to which wire is drawn, are used. In many cases the nearest whole number of pounds is taken when the variation is less than that found in actual weights of drawn wire. In computing the weights, the specific gravity of copper is taken at 8.89, water being at its greatest density 62.425 pounds per cubic foot. International ohms are used, unless the kind of unit is specifically stated. The following formulæ were used: Resistance per 1 000 feet at 60° F. Resistance per 1 000 feet at 75° F. 10 180.694 d2. 10 507.4 = d2. Weight per 1 000 feet = .003 027 X d2. Weight per mile = .015 983 X d2. The following data and formulæ may be useful: One B. A. unit = .988 9 legal ohms = .986 6 International ohms. Resistance per 1 000 feet at 60° F. = Resistance per 1 000 feet at 75° F. 30.815 weight per 1 000 feet. If a copper wire of length 1, diameter d, and weight w, has a resistance R at temperature t, then its conductivity by diameter is given by the first formula, and by weight Here, a is the resistance of a mil-foot in same units as R, k is the temperature coëfficient for t° Centigrade, and b is the resistance of one meter-gram at temperature to and in same units as R. When 1 is in meters and w in grams, c = 1. When 1 is in feet and w in pounds, c = .000 204 8. Mile-ohm = weight per mile X resistance per mile. The following tables are taken from the report of the Standard Wiring Table Committee, published in the report of the meeting of the American Institute of Electrical Engineers, held January 17, 1893: MATTHIESSEN'S STANDARD. |