=78. Chemical Meters= also measure the quantity of current that is used; for example, one may be placed in the cellar to measure the quantity of current used to light the house. [Illustration: Fig. 75.] Fig. 76 shows a chemical meter, a part of the current passing through a jar containing zinc plates and a solution of zinc sulphate. Metallic zinc is dissolved from one plate and deposited upon the other. The increase in weight shows the amount of chemical action which is proportional to the ampere hours. Knowing the relation between the quantity of current that can pass through the solution to that which can pass through the meter by another conductor, a calculation can be made which will give the current used. A lamp is so arranged that it automatically lights before the meter gets to the freezing-point; this warms it up to the proper temperature, at which point the light goes out again. [Illustration: Fig. 76.] CHAPTER VII. CHEMICAL EFFECTS OF THE ELECTRIC CURRENT. =79. Electrolysis.= It has been seen that in the voltaic cell electricity is generated by chemical action. Sulphuric acid acts upon zinc and dissolves it in the cell, hydrogen is produced, etc. When this process is reversed, that is, when the electric current is passed through some solutions, they are decomposed, or broken up into their constituents. This process is called _electrolysis_, and the compound decomposed is the _electrolyte_. (See "Study," § 369, etc., with experiments.) [Illustration: Fig. 77.] Fig. 77 shows how water can be decomposed into its two constituents, hydrogen and oxygen, there being twice as much hydrogen formed as oxygen. Fig. 78 shows a glass jar in which are placed two metal strips, A and C, these being connected with two cells. In this jar may be placed various conducting solutions to be tested. If, for example, we use a solution of copper sulphate, its chemical formula being CuSO_{4}, the current will break it up into Cu (copper) and SO_{4}. The Cu will be deposited upon C as the current passes from A to C through the solution. A is called the _anode_, and C the _cathode_. [Illustration: Fig. 78.] Fig. 79 shows another form of jar used to study the decomposition of solutions by the electric current. [Illustration: Fig 79.] =80. Ions.= When a solution is decomposed into parts by a current, the parts are called the _Ions_. When copper sulphate (Cu SO_{4}) is used, the ions are Cu, which is a metal, and SO_{4}, called an acid radical. When silver nitrate (Ag NO_{3}) is used, Ag and NO_{3} are the ions. The metal part of the compound goes to the cathode. CHAPTER VIII. HOW ELECTROPLATING AND ELECTROTYPING ARE DONE. =81. Electricity and Chemical Action.= We have just seen, Chapter VII., that the electric current has the power to decompose certain compounds when they are in solution. By choosing the right solutions, then, we shall be able to get copper, silver, and other metals set free by electrolysis.