currents by the deflection of a magnetic needle is known as a galvanometer.

603. Galvanometers Measure Current.-Faraday showed that a galvanometer measures the quantity of charge transmitted per second, or what is called the current strength. For he found that when a Leyden jar was discharged through a sensitive

galvanometer there was an instantaneous swing of the needle to one side, the amount of which depended only on the quantity of the charge; that is, the swing produced by forty turns of his electrical machine was the same whether the charge was held in a small jar at high potential or in a large Leyden battery at low potential, and whether the FIG. 343.—Galvanometer. wet string through which the discharge was sent was long or short.

It was also established by Faraday that when a constant current flowed through a galvanometer producing a steady deflection of the needle, the magnetic force on the needle due to the current was proportional to the quantity of charge transmitted per second.

604. Unit Current.-The unit of current used in practice is called the ampère and the quantity of charge which it transmits per second is called the coulomb. One coulomb is equal to 3000,000,000 electrostatic unit charges as

defined in $528.

The reason for the choice of this particular unit of current will appear later (8734).

부

FIG. 344.

605. Resistance.-Let a circuit be made up of two battery cells A and B joined in series with some other conductors and a galvanometer, the two cells being so connected that their electromotive forces act in the same direction. After observing the current strength as shown by the galvanometer, let the circuit be rearranged, taking the same components in any other order whatever. If the two electromotive forces still act together the current will be found the same as before, but if one cell has been turned around so that its electromotive force opposes that of the other cell, the

current will be to that in the previous case as the difference of the two electromotive forces is to their sum.

That is, the current strength is proportional to the effective electromotive force; or, in other words, the ratio of the electromotive force to the current strength in a given circuit is a constant, which depends only on the make-up and physical condition (temperature, stress, etc.) of the circuit. This constant is called the resistance of the circuit and is not affected by the order in which the various conductors, cells, etc., are connected, nor by the direction in which the current flows through them.

This relation, established by the German physicist G. S. Ohm, is known by his name and may be stated as follows:

Ohm's Law: The ratio of electromotive force to current in a given circuit is a constant which may be called the resistance of the circuit.

where E represents the electromotive force, I the current, and R the resistance of the circuit.

It is also established by experiment that each battery cell and piece of wire or other conductor has a definite resistance which belongs to it individually and depends only on its temperature and state of stress (provided that the same two points on the conductor are always used in making connection with the rest of the circuit); and when the several parts of a circuit are joined together one after another, in series as it is called, the resistance of the whole is the sum of the resistances of the several parts.

606. Unit of Resistance.-In honor of the discoverer of this law the unit of resistance in the practical system is called the ohm; it is the resistance of a circuit in which an electromotive force of one volt will produce a current of one ampère.

Ohm's law may then be expressed in units of the practical system, thus:

Current in ampères =

Electromotive force in volts

Resistance in ohms

The electrical resistance of a conductor is analogous to the frictional resistance which a pipe offers to the flow of liquid through it. In both

cases work done against the resistance appears as heat, and in neither case does the resistance have any tendency to produce a back current.

607. Exception to Ohm's Law.-In gaseous conductors the ratio of the electromotive force to the current is not constant as in other conductors, but depends on the strength of the current.

H

CHEMICAL EFFECTS OF CURRENT.

608. Decomposition of Water.—When a current of electricity is passed through dilute sulphuric acid (1 part acid to 20 of water), using platinum electrodes immersed in the acid, gas is given off at each electrode. The gases may be separately collected in tubes filled with the dilute acid and inverted over the electrodes as shown in figure 345. The gas liberated at the positive electrode is found to be oxygen while that at the negative electrode is hydrogen, and the volume of hydrogen is just twice the volume of the oxygen. These volumes are exactly in the ratio in which the gases combine to form water, and on this account it was at first supposed that the current directly decomposed

FIG. 345.-Electrolysis

of water.

water.

The decomposition of water in this way by the electric current was first accomplished in 1800 by Carlisle and Nicholson.

609. Discovery of Potassium and Sodium.-Sir Humphrey Davy, in 1807 by the use of a powerful battery of 250 cells, decomposed caustic potash, obtaining metallic potassium at the negative electrode. A fragment of caustic potash slightly moistened was laid on a platinum plate which was connected to the positive pole of the battery; on touching the potash with a platinum wire connected with the negative pole, minute globules appeared at the negative electrode which rapidly oxidized in air or took fire; these he recognized as a new metal which he named potassium. In a similar manner metallic sodium was obtained from caustic soda.

610. Faraday's Researches.-About the year 1833 Faraday' began the systematic investigation of the chemical effects of the electric current.

Substances which are decomposed by the passage of the electric current he called electrolytes; the electrode connected with the positive pole of the battery or that through which (according to ordinary convention) current enters the electrolyte was named the anode (Greek, inward path), while the electrode through which the current leaves the electrolyte was named the kathode (Greek, outward path). The two constituents into which a molecule of the electrolyte is broken up were called ions (Greek, wanderers), that which is set free at the kathode being the kation, while that which appears at the anode was named the anion.

611. Faraday's Laws.-The following are some of the most important results of Faraday's investigations:

1. By introducing a number of electrolytic cells in different. parts of a circuit it was shown that the amount of substance decomposed is the same in each cell through which the whole current passes, and in case of a divided circuit the sum of the amounts decomposed in the branches is equal to the amount in the undivided parts of the circuit.

2. The quantity of a given substance electrolyzed in a cell is proportional to the amount of charge or quantity of electricity which passes.

3. If several electrolytic cells containing different substances are connected in series in the same circuit, the quantities of the ions set free at the electrodes are proportional to their chemical combining equivalents.

612. Electrochemical Equivalents. The electrochemical equivalent of a substance is the quantity that is set free per second by a current of one ampère or by the passage of one coulomb of electricity. The following table gives the electrochemical equivalents of some well-known substances. It will be noticed that they are proportional to the combining equivalents.

96550 coulombs are transmitted when the number of grams liberated equals the combining equivalent of the substance.

613. Primary and Secondary Actions. It is important to distinguish between the direct or primary effect of the current in electrolysis and the secondary chemical reactions that take place when the ions are set free. In illustration of this difference

H

H&SO

H

Na OH

H2SOL

Na2SO

FIG. 346.-Electrolytic cells in series.

take the electrolytic apparatus containing dilute sulphuric acid, as described in paragraph 608, and connect it in series with a precisely similar apparatus containing a solution of sodium sulphate in water, colored by an infusion of purple cabbage. On sending a current through, both hydrogen and oxygen gases are set free in one cell exactly as in the other, and at the same time the coloring matter in the sodium sulphate solution turns red around the positive electrode, or anode, and green around the negative electrode, or kathode, showing that the originally neutral salt has become acid at the anode and alkaline at the kathode. Analysis shows that sodium hydroxide (NaOH) has appeared at the one electrode and sulphuric acid (H2SO,) at the other.

It might seem at first that more decomposition was effected by the current in one cell than in the other, in violation of Faraday's law, for equal amounts of gas are set free in both cells,