This form of the voltaic, or, as it is sometimes called, galvanic battery, has given place to the "cell" shown in figure II, where the two plates Z C are immersed in acidulated water within the vessel, and connected outside by the wire W. The zinc plate has a positive and the copper a negative charge. The positive current flows from the zinc to the copper inside the cell and from the copper to the zinc outside the cell, as shown by the arrows. It thus makes a complete round, which is called the voltaic "circuit," and if the circuit is broken anywhere it will not flow at all. The positive electricity of the zinc appears to traverse the liquid to the copper, from which it flows through the wire to the zinc. The effect is that the end of the wire attached to the copper is positive (+), and called the positive "pole" or electrode, while the end attached to the zinc is negative (-), and called the negative pole or electrode. "A simple and easy way to avoid confusion as to the direction of the current, is to remember that the POSITIVE current flows FROM the COPPER TO the ZINC at the point of METALLIC contact." The generation of this current is accompanied by chemical action in the cell. Experiment shows that the mere CONTACT of dissimilar materials, such as copper and zinc, electrifies them--zinc being positive and copper negative; but contact alone does not yield a continuous current of electricity. When we plunge the two metals, still in contact, either directly or through a wire, into water preferably acidulated, a chemical action is set up, the water is decomposed, and the zinc is consumed. Water, as is well known, consists of oxygen and hydrogen. The oxygen combines with the zinc to form oxide of zinc, and the hydrogen is set free as gas at the surface of the copper plate. So long as this process goes on, that is to say, as long as there is zinc and water left, we get an electric current in the circuit. The existence of such a current may be proved by a very simple experiment. Place a penny above and a dime below the tip of the tongue, then bring their edges into contact, and you will feel an acid taste in the mouth.