263. Directions. (A) Holding the amalgamated zinc strip in one hand and the copper strip in the other ([Fig. 83]), dip them into the acid, but do not let them touch each other. Note any chemical action.
(B) Touch the copper and zinc together, below the surface of the acid. Watch the copper.
(C) Separate the lower ends of the strips, then touch them together above the acid. Anything still happen to the copper?
(D) Slip one spring connector with the attached wire upon the zinc strip, then stand the strips in the tumbler, so that they can not touch each other. Now touch the copper strip with the free end of the wire, at the same time watching the copper.
(E) Raise the wire from Cu, touch it to Cu again, and repeat several times until you are sure that something takes place every time the wire touches Cu.
264. The Electric Current. Something must happen in or through the wire, and it can only happen[105] when the two metals are joined in some way. This peculiar, invisible action in the wire is called the electric current, and such an arrangement is called a Galvanic cell.
265. Source of the Electrification. When two different metals are placed in acid they are electrified unequally by chemical action. Each has a higher potential than the acid, but their potentials are not the same. This electrification tends to pass from the place of higher to the place of lower potential, and the conducting wire allows this transfer to take place. As the difference of potential is kept up by the continued chemical action, the current is continuous, and not simply instantaneous, as in discharges of frictional electricity. As heat is produced by the burning of coal, so electrification is produced by the chemical burning of zinc. Chemical energy is the source of electrification in the Galvanic cell, just as muscular energy was the source of the electrification in the experiments with frictional electricity.
266. The Electric Circuit; Open and Closed Circuits. The simple Galvanic cell just used, together with the wire which joined the metal strips, is called an electric circuit. If the current should pass through a telegraph instrument, for example, on its way from one strip to the other, the telegraph instrument would also be in the circuit. When the wire is cut or removed from one metal strip, the circuit is said to be open—that is, we have an open circuit. When the current passes, the circuit is closed. We also say make and break the circuit, and that the circuit has been broken.
267. Plates or Elements. The copper and zinc strips are called the plates or elements of the cell. The zinc, Zn, [Fig. 84], is dissolved by the acid, and is called the positive plate (+ plate). The copper, Cu, is the neg[106]ative plate (- plate). The copper is also called the cathode, and the zinc the anode.
268. Direction of Current. It has been agreed, for convenience, that in the cell the current passes from the zinc through the liquid to the copper, where the hydrogen bubbles are deposited. It then passes through the wire, or other conductor furnished, back to the zinc, through the liquid to Cu again, and so on around and around thousands of times per second. The current really starts at the surface of the zinc, where the chemical action is. When carbon and zinc are used, the action and direction of the current are the same, carbon being the - plate.