In the early part of the nineteenth century methods were found for producing a steady _electric current_ in metal wires. In 1820, the Danish physicist, H. C. Ørsted, discovered that an electric current influences a magnet in a characteristic way, and that, conversely, the current is affected by the forces emanating from the magnet, by a magnetic field in other words. The French scientist, Ampère, soon afterwards formulated exact laws for the _electromagnetic_ forces between magnets and currents. In 1831, the English physicist, Faraday, discovered that an electric current is _induced_ in a wire when currents or magnets in its neighbourhood are moved or change strength. Faraday’s views on electric and magnetic fields of force around currents and magnets were further of fundamental importance to the _electromagnetic wave theory_ as developed by Maxwell. The branch of physics dealing with all these phenomena is now generally known as _electrodynamics_. [Illustration: FIG. 14.—Picture of electrolysis of hydrogen chloride. _A_, anode; _K_, cathode; _H_, hydrogen atoms; _Cl_, chlorine atoms.] Electrolysis. Faraday also studied the chemical effects which an electric current produces upon being conducted between two metal plates, called _electrodes_, which are immersed in a solution of salts or acids. The current separates the salt or acid into two parts which are carried by the electric forces in two opposite directions. This separation is called electrolysis. If the liquid is dilute hydrochloric acid (HCl), the hydrogen goes with the current to the negative electrode, the _cathode_, and takes the positive electricity with it, while the chlorine goes against the current and takes the negative electricity to the positive electrode, the _anode_. We must then assume with the Swedish scientist, Arrhenius, that, under the influence of the water, the molecules of hydrogen chloride always are separated into positive hydrogen atoms and negative chlorine atoms, and that the electric forces from the anode and the cathode carry these atoms respectively with and against the current. The electrically charged wandering atoms are called _ions_, _i.e._ wanderers. The positive electricity taken by the hydrogen atoms to the cathode goes into the metal conductor, while the anode must receive from the metal conductor an equal amount of positive electricity to be given to the chlorine atoms to neutralize them. The negative charge of a chlorine atom must then be as large as the positive charge of a hydrogen atom. These assumptions imply that equal numbers of the two kinds of atoms are present in the whole quantity of atoms transferred in any period of time.