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.
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.
Faraday found that the quantity of hydrogen which in the above experiment is transferred to the cathode in a given time is proportional to the quantity of electricity transferred in the same time. A gram of hydrogen always takes the same amount of electricity with it. By experiment this amount of electricity can be determined, and, since the weight in grams of the hydrogen atom is known, it is possible to calculate the amount of one atom. In electrostatic units it is 4·77 × 10⁻¹⁰, i.e., 477 billionth[1] parts. A chlorine atom then carries with it 4·77 × 10⁻¹⁰ electrostatic units of negative electricity. Since its atomic weight is 35·5, then 35·5 grams of chlorine will take as much electricity as 1 gram of hydrogen. The ratio e/m between the charge e and the mass m is then 35·5 times as great for hydrogen as for chlorine.
[1] Billion used here to mean one million million, and trillion to mean one million billion.
We have temporarily restricted ourselves to the electrolysis of hydrogen chloride. Let us now assume that we have chloride of zinc (ZnCl₂), which, by electrolysis, is separated into chlorine and zinc. Each atom of chlorine will, as before, carry 4·77 × 10⁻¹⁰ units of negative electricity to the anode; but since zinc is divalent ([cf. p. 17]) and one atom of zinc is joined to two of chlorine, therefore one atom of zinc must carry a charge of 2 × 4·77 × 10⁻¹⁰ units of positive electricity to the cathode. An atom or a group of atoms, with valence of three, in electrolysis carries 3 × 4·77 × 10⁻¹⁰ units, etc.
We see then, that the quantity of electricity which accompanies the atoms in electrolysis is always 4·77 × 10⁻¹⁰ electrostatic units or an integral multiple thereof. This suggests the thought that electricity is atomic and that the quantity 4·77 × 10⁻¹⁰ units is the smallest amount of electricity which can exist independently, i.e., the elementary quantum of electricity or the “atom of electricity.” The atom of a monovalent element, when charged or ionized, should have one atom of electricity; a divalent, two, etc. On the two-fluid theory it was most reasonable to assume that there were two kinds of atoms of electricity representing, respectively, positive and negative electricity. In [Fig. 15] there is given, in accordance with the two-fluid theory, a rough picture of a chlorine ion and a hydrogen ion and their union into a molecule.