It was learned that if two different metals, such as copper and zinc amalgam, are placed in a weak acid solution (such as one part H_{2}SO_{4} to four parts H_{2}O), and connected by a wire fastened securely to the metals, a current of electricity (about two volts) will pass through the wire. Carbon (a non-metal) and a metal upon which the solution acts chemically may be used instead of two metals. There must be chemical action between the liquid and one metal, or there will be no current. Such a combination constitutes a _cell_, and two or more cells make a _battery_. The current starts with the zinc, is conducted by the solution to the copper, and thence by wire back to the zinc, completing a _circuit_. The zinc constitutes the negative pole (or electrode), the copper or carbon the positive pole (or electrode). A cell frequently employed, where a weak (about 1.1 volts) but constant electromotive force (“E. M. F.”) is required, is one devised by the English physicist, John D. Daniell (1790-1845). In this cell a copper sulphate solution containing a copper electrode is placed in contact (by means of a porous wall or partition—usually an unglazed porcelain cup) with a zinc sulphate solution containing a zinc electrode. The zinc electrode is negative to the copper. At each electrode there exists a potential difference between solution and electrode.[16] The two electrodes being connected externally by a wire, a current of electricity will flow through the wire from the copper to the zinc, and zinc will dissolve at the anode (positive pole) and copper deposited on the cathode (negative pole). The current in this case, as in the preceding, is said to be produced by _voltaic action_ and the cell is a primary battery. Voltaic action and _electrolysis_—the process of chemical decomposition (or dissociation of compounds or molecules)—by the action of an electric current produced externally (as by a dynamo) and forced through the cell, are essentially identical phenomena, and obey the same laws.[17] The familiar _dry cell_ contains no liquid which might be spilled, and is very useful for certain purposes, as in automobiles, and in operating door-bells. It is merely a voltaic cell whose chemical contents are made practically solid (or paste-like) by the use of some absorbent, as gelatine, sawdust, etc. In cells of the Leclanché type, a mixture of plaster of Paris, flour, and sal ammoniac takes the place of the solution which acts chemically upon one of the contained metals. When used up, a dry cell must be replaced by an entirely new cell. Two or more dry cells constitute a _dry battery_.