Page:Text-book of Electrochemistry.djvu/21

4 INTRODUCTION. CHAP. partially reacts "secondarily" with the water present to form oxygen and hydrochloric acid.

Ions. — During the course of the electrolysis certain substances, whose nature is determined by chemical analysis, are removed from the solution to each pole. Those substances which "migrate" to the positive electrode (P1), or anode, are called anions, those which migrate to the negative electrode (P2), or cathode, are called cations (see Chap. VIII.), this being the nomenclature introduced by Faraday.

Coulomb. — According to the law discovered by Faraday in 1834, the quantity of gas and the quantity of silver which separate at P2 in the examples given above are exactly proportional to the quantity of electricity which passes through the solution during the electrolysis. The mass of the deposited material, therefore, is a convenient measure of the quantity of electricity which passed through the electrolyte. The coulomb, our unit for the quantity of electricity, is that quantity required for the ^separation of 1.118 milligrams of silver (the equivalent quantity of copper, 0.3284 mgms., or of hydrogen, 0.0104 mgms., according to Faraday's law).

Ampere. — The current strength is determined by the quantity of electricity which passes through a circuit in a specified time. As unit, we use the ampere (amp.), which is obtained when 1 coulomb passes through the circuit in one second.

Ohm. — The unit of electrical resistance is that resistance offered at 0° by a column of pure mercury 106.30 cm. long with a section of one square millimetre. This is the international ohm, and is equal to 1.063 Siemens' units.

Unit for Conductivity. — The electrical conductivity of a substance is the reciprocal of the value of its resistance. As unit, the conductivity of a substance is used, a column of which 1 cm. long and of 1 sq. cm. section possesses the resistance of 1 ohm. The best conducting solutions of acids have nearly this conductivity at about 40°.

The Absolute Systems. — In scientific work it is frequently necessary to calculate the above "practical"