The jar is half filled with sulphuric acid (I), after which, in order to charge the jar, the knob (G) is brought into contact with the knob (M) of the friction generator (Fig. 17). VOLTAIC OR GALVANIC ELECTRICITY.--The second method of generating electricity is by chemical means, so called, because a liquid is used as one of the agents. [Illustration: _Fig. 18._ LEYDEN JAR] Galvani, in 1790, made the experiments which led to the generation of electricity by means of liquids and metals. The first battery was called the "crown of cups," shown in Fig. 19, and consisting of a row of glass cups (A), containing salt water. These cups were electrically connected by means of bent metal strips (B), each strip having at one end a copper plate (C), and at the other end a zinc plate (D). The first plate in the cup at one end is connected with the last plate in the cup at the other end by a conductor (E) to make a complete circuit. [Illustration: _Fig. 19._ GALVANIC ELECTRICITY. CROWN OF CUPS] THE CELL AND BATTERY.--From the foregoing it will be seen that within each cup the current flows from the zinc to the copper plates, and exteriorly from the copper to the zinc plates through the conductors (B and E). A few years afterwards Volta devised what is known as the voltaic pile (Fig. 20). VOLTAIC PILE--HOW MADE.--This is made of alternate discs of copper and zinc with a piece of cardboard of corresponding size between each zinc and copper plate. The cardboard discs are moistened with acidulated water. The bottom disc of copper has a strip which connects with a cup of acid, and one wire terminal (A) runs therefrom. The upper disc, which is of zinc, is also connected, by a strip, with a cup of acid from which extends the other terminal wire (B). [Illustration: _Fig. 20._ VOLTAIC ELECTRICITY] _Plus and Minus Signs._--It will be noted that the positive or copper disc has the plus sign (+) while the zinc disc has the minus (-) sign. These signs denote the positive and the negative sides of the current. The liquid in the cells, or in the moistened paper, is called the _electrolyte_ and the plates or discs are called _electrodes_. To define them more clearly, the positive plate is the _anode_, and the negative plate the _cathode_. The current, upon entering the zinc plate, decomposes the water in the electrolyte, thereby forming oxygen. The hydrogen in the water, which has also been formed by the decomposition, is carried to the copper plate, so that the plate finally is so coated with hydrogen that it is difficult for the current to pass through. This condition is called "polarization," and to prevent it has been the aim of all inventors. To it also we may attribute the great variety of primary batteries, each having some distinctive claim of merit.