THE ELECTRIC CURRENT

First Class.—Metals, &c.

116. The first class includes all the metals, whether in the solid or liquid state, together with some other substances not regarded by chemists as met- als. In these the smallest external electromotive force is capable of producing an electric current, and this current continues to flow as long as the electro- motive force continues to act, without producing any change in the chemical properties of the substance. The strength of the permanent current is pro- portional to the electromotive force. The ratio of the numerical value of the electromotive force to that of the current is called the Resistance of the conductor.

The same thing may be otherwise stated by saying that the flow of the current is opposed by an internal electromotive force, proportional to the strength of the current, and to a quantity called the Resistance of the conduc- tor, depending on its form and nature. When the strength of the current is such that this internal electromotive force balances the external electromotive force the current neither increases nor diminishes in strength. It is then said to be a steady current.

These relations were first established by Dr. G. S. Ohm, in a work pub- lished in 1827. They are expressed by the formula, Electromotive force = Current × Resistance, which is called Ohm’s Law. Generation of Heat by the current. 117.] During the flow of a steady current through a conductor of uniform material of the first class heat is generated in the conductor, but the substance of the conductor will not be affected in any way, for if the heat is allowed to escape as fast as it is generated, the temperature and every other physical condition of the conductor remains the same.JOULE’S LAW.

The whole work done by the external electromotive force in urging elec- tricity through the body is therefore spent in generating heat. The dynamical equivalent of the heat generated is therefore equal to the electrical work spent, that is, to the product of the electromotive force into the quantity of electricity transmitted by the current.

Now, the electromotive force is, by Ohm’s law the product of the strength of the current into the resistance, and the quantity of electricity is, by the definition of a current, the product of the current into the time during which it flows, so that we find, Heat generated measured in dynamical units = Square of Current × Resistance × Time. This relation was first established by Dr. Joule, and is therefore called Joule’s law. It was also established independently by Lenz.

Second Class.—Electrolytes.

118.] The second class of substances consists of compound bodies, gener- ally in the liquid form, called Electrolytes. When an electric current passes through fused chloride of silver, which is an electrolyte, chlorine appears at the anode where the current enters, and sil- ver at the cathode where the current leaves the electrolyte. The quantities of these two substances are such that if combined they would form chloride of silver. The composition of those portions of the electrolyte which lie between the electrodes remains unaltered. Hence, if we fix our attention upon a por- tion of the electrolyte between two fixed planes perpendicular to the direction of the current, the quantity of silver or of chlorine which enters the portion through one plane must be equal to the quantity which leaves it through the other plane. It follows from this that in every part of the electrolyte the sil- ver is moving in the direction of the current, and the chlorine in the opposite direction.

This operation, in which a compound body is decomposed by an electric current, is called Electrolysis, and the mode in which the current is trans- mitted is called Electrolytic Conduction. The compound body is called an

FARADAY’S LAWS OF ELECTROLYSIS.

Electrolyte, and the components into which it is separated are called Ions. That which appears at the anode is called the Anion, and that which appears at the cathode is called the Cation.

The quantity of the substance which is decomposed is proportional to the total quantity of electricity which passes through it, and is independent of the time during which the electricity is passing. The quantity corresponding to the passage of one unit of electricity is called the Electrochemical Equiva- lent of the substance. Thus, when one unit of electricity is passed through fused chloride of silver, one electrochemical equivalent of silver appears at the cathode and one electrochemical equivalent of chlorine at the anode, and one electrochemical equivalent of chloride of silver disappears.

119.] The electrochemical equivalents of the same substance, as deduced from experiments on different electrolytes which contain it, are consistent with each other. Thus the electrochemical equivalent of chlorine is the same, whether we deduce it from experiments on chloride of silver, or from ex- periments on hydrochloric acid, and that of silver is the same, whether we deduce it from experiments on chloride of silver, or from experiments on ni- trate of silver. These laws of electrolysis were established by Faraday∗ . If they are strictly true the conduction of electricity through an electrolyte is always electrolytic conduction, that is to say, the electric current is always associated with a flow of the components of the electrolyte in opposite direc- tions.

Such a flow of the components necessarily involves their appearance in a separate form at the anode and the cathode. To effect this separation a certain electromotive force is required depending on the energy of combination of the electrolyte. Thus the electromotive force of one of Daniell’s cells is not sufficient to decompose dilute sulphuric acid. If, therefore, an electrolytic cell, consisting of a vessel of acidulated water, in which two platinum plates are placed as electrodes, is inserted in the circuit of a single Daniell’s cell, along with a galvanometer to measure the current, it will be found that though there is a transient current at the instant the circuit is closed, this current rapidly diminishes in intensity, so as to become in a very ∗ Exp. Res., series vii and viii.POLARIZATION. 115 short time too weak to be measured except by a very sensitive galvanometer. Neither oxygen nor hydrogen, the chemical components of water, appear in a gaseous form at the electrodes, but the electrodes themselves acquire new properties, shewing that a chemical action has taken place at the surface of the platinum plates.