_=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 negative 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. [Illustration: Fig. 84.] _=269. Poles or Electrodes.=_ If the wire were cut, the electricity coming from the + plate would be stopped at the end of the wire marked +, Fig. 84, after passing through the acid and up Cu. This end of the wire is called the + _pole or electrode_ (positive). The end of the wire joined to Zn is called the - _pole_ or _electrode_; that is, the + electrode is the end of the wire attached to the - plate. The tops of Cu and Zn may be considered electrodes. The top of Cu is the + _pole_ of the cell, while Cu is the - _plate_. =270. Chemical Action in the Simple Galvanic Cell.= The chemical formula of sulphuric acid is H_{2}SO_{4} (read H, 2, S, O, 4). This means that it is a compound of hydrogen (H), sulphur (S), and oxygen (O). The H_{2}SO_{4} stands for _molecule_ of acid, and the small figures show that the molecule is made up of 2 _atoms_ of hydrogen (H_{2}), one of sulphur (S), and 4 of oxygen (O_{4}). The atoms are held together by _chemical attraction_ or _affinity_. There is a stronger chemical affinity between zinc (Zn) and SO_{4} than between H_{2} and SO_{4}; so, as soon as the Zn gets a chance, as it does in the cell, it drives out the H_{2}, and it takes its place in the molecule. This chemical _reaction_ may be shown by the following chemical _equation_: Zn + H_{2}SO_4 = ZnSO_4 + H_2. Zinc and sulphuric acid produce zinc sulphate and hydrogen.