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Out of the various discoveries and applications of electricity almost a new language has sprung. This is especially so of terms expressive of the measurements of electric energy, and of the laws governing the application of electric power. For a time, various nations measured and applied by means of terms chosen by themselves. This led to a jargon very confusing to writers and investigators. It became needful to have a language more in common, as in pharmacy, so that all nations could understand one another, could compute alike, and especially impart their meaning to those whose duty it became to apply discovered laws and actual calculations to practical electric operations. This was a difficult undertaking, owing to the tenacity with which nations clung to their own nomenclatures and terminologies. But the drift, though slow, finally ended at the Electrical Congress in Paris in 1881, in the adoption of a uniform system of measurements of electric force, and an agreement upon terms for laws and their application, which all could understand.

Three fundamental units of measurement were first agreed upon,—the Centimetre (.394 in.) as a unit of length; the Gramme (15.43 troy grains) as a unit of mass; the Second (1/60 of a minute) as a unit of time. These three units became, when referred to together by their initial letters, the basis of the C.G.S. system of units. Now by these units of measurement something must be measured, as, for instance, the electric force; and when so measured, an absolute unit of force must be the result.

Dyne:—This is but a contraction of dynam, force. It was adopted as the name of the “Absolute Unit of Force,” or the C.G.S. unit of force, and is that force which, if it act for a second on one gramme of matter, gives to it a velocity of one centimetre per second.

Ampere:—Electrical force produces electrical current. Current must be measured and an absolute unit of current strength agreed upon. The “Absolute Unit of Current” was settled as one of such strength as that when one centimetre length of its circuit is bent into an arc of one centimetre radius, the current in it exerts a force of one dyne on a unit magnet-pole placed at the centre. But the absolute unit of current as thus obtained was decided to be ten times too great for practical purposes. So a practical unit of current was fixed upon, which is just one tenth part of the above absolute unit of current. This practical unit of current was called the ampere, in honor of the celebrated French electrician, Ampère. It may be ascertained in other ways, as when a current is of sufficient strength to deposit in a copper electrolytic cell 1.174 grammes (18.116 grains) of copper in an hour, such current is said to be of one ampere strength; or a current of one ampere strength is such a one as would be given by an electro-motive force of one volt through a wire offering one ohm of resistance.

Volt:—This was named from Volta, the celebrated Italian electrician, and was agreed upon as the unit of electro-motive force. It is that electro-motive force which would be generated by a conductor cutting across 100,000,000 C.G.S. lines in a field of force per second; or it is that electro-motive force which would carry one ampere of current against one ohm of resistance.

Ohm:—So called from Ohm, a German electrician. It is the unit of resistance offered by a conductor to the passage of an electrical current. As an absolute unit of resistance, it is equal to 1,000,000,000 C.G.S. units of resistance. As a practical unit, and as agreed upon at the International Congress of Electricians (Chicago, 1893), it represents the resistance offered to an electric current at the temperature of melting ice by a column of mercury 14.451 grammes in mass, of a constant cross-sectional area, and 106.3 centimetres in length. This is called the international ohm. The resistance offered by 400 feet of ordinary telegraph wire is about an ohm.

These three units—ampere, volt, and ohm—are the factors in Ohm’s famous law that the current is directly proportional to the electro-motive force exerted in a circuit, and inversely proportional to the resistance of the circuit; that is,—

Current = Electro-motive force / Resistance

or,

Electro-motive force = Current × Resistance

or

Resistance = Electro-motive force / Current.

Erg:—From the Greek ergon, work, is the unit of work required to move a force of one dyne one centimetre. One foot-pound equals 13,560 ergs.

Calorie:—Latin calor, heat, is the unit of heat; being the amount of heat required to raise the temperature of one kilogram of water one degree centigrade.

Coulomb:—In honor of C.A. de Coulomb, of France. It is the practical unit of quantity in measuring electricity, and is the amount conveyed by one ampere in one second.

Farad:—From Faraday, the physicist. It is the unit of electric capacity, and is the capacity of a condenser that retains one coulomb of charge with one volt difference of potential.

Gauss:—From Carl F. Gauss (1785–1855). The C.G.S. unit of flux-density, or the unit by which the intensity of magnetic fields are measured. It equals one weber per normal square centimetre.

Gilbert:—The unit for measuring magneto-motive force, being produced by .7958 ampere-turn approximately.

Henry:—From Joseph Henry, of the Smithsonian Institution, Washington, D.C. The practical unit for measuring the induction in a circuit when the electro-motive force induced is one international volt, while the inducing current varies at the rate of one ampere per second.

Joule:—The C.G.S. unit of practical energy, being equivalent to the work done in keeping up for one second a current of one ampere against a resistance of one ohm. Named from J.P. Joule, of England.

Oersted:—From Oersted, the electrician. It is the practical unit for measuring electrical reluctance.

Watt:—The practical electrical unit of the rate of working in a circuit, when the electro-motive force is one volt, and the intensity of current is one ampere. It is equal to 107 ergs per second, or .00134 horse-power per second. Named from James Watt, of Scotland.

Weber:—The practical unit for measuring magnetic flux. Named from W. Weber, of Germany.