A brilliant arc of about 900 candle-power requires a current of 15 amperes and a potential difference of 50 volts at the carbonsthat is to say, 750 watts, or almost exactly one horse-power. The brightness of the illumination increases much more rapidly than the energy expended; and therefore large lamps are relatively more economical than small ones.

429. Use of Alternating Currents.—With alternating currents the light is as steady as with continuous ones, although it gives rise to a peculiar humming sound, the pitch of which depends on the number of reversals of the current per second. The carbons wear away at equal rates, and both become pointed. By means of a kind of phenakistoscope their condition may be examined, and even photographed, at various phases of the period. The arc disappears when the current is zero, but it is spontaneously renewed in the incandescent gas. The brightness of the two carbons varies periodically, and passes through maxima and minima; further, each of the two carbons becomes in turn the most luminous when it is positive. These variations can be easily observed, even when they are reproduced a hundred times in a second.

If the difference of potential is continuously observed, it is found that it does not vary harmonically in the same way as the strength of the current. Its absolute value remains almost constant, but the sign changes very rapidly the moment the current is reversed, a result agreeing well with the existence of an inverse electromotive force.

430. Carbons.-Davy used wood charcoal. Foucault replaced this by the carbon deposited in the interior of gas retorts and known as gas-graphite; it is harder, is a better conductor, and does not waste so rapidly. Carbons prepared artificially are used now; they are purer, more homogeneous, and more regular in shape. They are prepared from a paste of powdered coke, lampblack, and a very thick syrup of gum and sugar. The whole is well mixed, pressed, passed through a draw-plate, dried, and hardened at a high temperature. The carbons are baked repeatedly, being each time dipped in boiling syrup. What are called cored carbons are now frequently used; the centre of these carbons is of a different composition from the rest. They take a better shape when in use, and give a more regular arc, like that in Fig. 356. It is very important that the carbons should be pure; in particular, they ought to be free from silica.

Carbons are often coated with copper or nickel; by this means their conductivity is increased, and they last rather better.

431. Regulators of the Electric Light.-The prime requirement of any system of lighting is the constancy of the light. In order to obtain this constancy with the electric arc, notwithstanding the wasting of the carbons and accidental variations in the current, regulators are used. We shall not attempt more than to give the principle of the chief arrangements which are used.

Two mechanisms are obviously necessary; one, which brings the carbons near each other when the current diminishes, and makes them touch each other when it stops, since they do not relight unless they are in contact; and another which, as soon as the current is re-established, pulls the carbon apart to the proper distance, so as to strike the arc, and separates them still further when the current tends to increase. To keep the light steady, the two carbons must move at the same rate, if the current is an alternating one; while the positive carbon must move at twice the rate of the negative one, if the current is continuous. Otherwise it is sufficient if one of the carbons is movable. In most cases, the upper carbon, which should be the positive one, tends to sink from its own weight. The function of the mechanism is to regulate its descent and to raise it by a suitable amount if it comes into contact with the negative carbon. The mechanism, of whatever nature, is controlled by a coil through which the whole or part of the current passes. Two plans are in use; in one the coil is traversed by the whole of the current, and the regulation depends upon and determines the strength of the current; in the other the coil is wound with a very fine wire, forming a shunt in respect of the two carbons. The regulation in this case depends upon the strength of the current in the coil, but as this in its turn depends on the difference of potential at the ends of the shunt, the regulation causes a fixed difference of potential in the arc. As the former system affects the whole current, it can only be applied where there is a single lamp in a circuit; when there are several lamps in series in the same circuit, one regulator must not be affected by the action of another regulator in its vicinity; the second plan is adapted for this case.

432. Cost of Electric Lighting.-Experiment shows that one horse-power supplied in the form of electrical energy at the

terminals of a lamp gives about 900 candle-power with a continuous arc, 450 with an alternating arc, and 180 with incandescent lamps.

Taking 90 per cent. as the efficiency of the dynamo, and 10 per cent. for the loss in the leads, one horse-power at the lamp represents 1.25 horse-power in the engine. With the very best engines this represents a consumption of between two and three pounds of coal in the hour.

Of 1000 units of thermal energy due to the combustion of the coal, the steam-engine utilises 100; of this the lamp receives 80, and converts 8 into light in the case of an arc, and 4 in that of an incandescent lamp.

These numbers are far smaller in the case of gas. A simple calculation shows that the quantity of light produced by burning a given volume of gas directly is far less than if the same volume is used in a gas-engine employed to drive a dynamo.

433. Electrical Furnace.-Another application of the heating effects produced by electricity is to the fusion of minerals and metals by the voltaic arc. The temperatures which can be obtained by combustion are necessarily limited by dissociation, but this does not apply to the voltaic arc. The question of temperature, however, is not the only one that comes into account ; in many cases the reducing properties of the negative electrode appear to play an important part.

Sir William Siemens showed that electric energy might be economically employed to melt iron and steel. A crucible of graphite about 20 centimetres in diameter is used, surrounded by charcoal powder. A carbon rod, like those used for electric lighting, passes through the bottom of the crucible and forms the positive electrode. The negative electrode is kept at a proper distance above it by a kind of regulator. With a current of 36 amperes and a motive power of 4 horse-power, a kilogramme of steel is melted in fifteen minutes. It may be taken that the heating and fusion of a kilogramme of steel requires 450 kilogramme-degrees of heat, or 450 × 4180 = 1,881,000 joules of work. Now, 4 horsepower in fifteen minutes give 4 × 745 × 15 × 60 2,682,000 joules; so that the efficiency is about 70 per cent.

=

434. Electrical Welding. Of all known modes of heating, the electric arc is that by which the greatest quantity of heat can be concentrated in a given point. This fact is made use of industrially for fusing together two pieces of the same metal-iron, for

example-without using a second metal as solder. In order to join two pieces of sheet-iron, they are placed one over the other with their edges together; they are then connected with the positive and negative pole of a battery of 80 to 90 volts, and the carbon, held in a suitable handle, is moved along the edge; the arc which passes between the carbon and the iron melts the iron and solders the two pieces firmly together.

Another entirely different method, for joining two rods end to end, consists in pressing the ends, suitably prepared, against each other, and then passing a current which fuses the parts in contact. The current, which only needs to act a short time, should have great strength with low electromotive force. It is obtained by means of a transformer. The apparatus is arranged, for instance, so that a mean current of 20 amperes and 600 volts in the primary gives I volt and 12,000 amperes in the secondary circuit.

CHAPTER XXXVI.

GALVANIC DEPOSITION.

435. Applications of Electrolysis.-This chapter gives a brief account of the methods in which the electric current is utilised to liberate the metal contained in the solution of a salt; either in order to deposit a thin coating of the metal on any object, or to take a cast in the metal, or for the extraction or purification of the metal in the solution.

The only metals which meet with important industrial applications in one or the other of these respects are nickel, silver, gold, and copper. The methods must obviously be different for different metals. We have to consider-(1.) the nature and composition of the bath; (2.) the electromotive force required for the operation, and the density of the current, that is, the strength of the current per unit surface of the electrode; (3.) the treatment of the object both before and after its immersion in the bath.

436. Electrodes.—The object to be coated forms the negative electrode or kathode; it must have a conducting surface, and if the material is not itself a conductor, a conducting surface is produced by a coating of graphite or of silver sulphide, which is obtained by dipping the object in a solution of silver nitrate, and then exposing it to the action of sulphuretted hydrogen. The preparation of the surface is one of the most important conditions for the success of the process.

The positive electrode or the anode is ordinarily a plate of the same metal as that in solution; it loses as much metal as is deposited on the kathode, and the average strength of the solution remains constant. The strength of the solution is kept uniform throughout by stirring it.

When the metal of the two electrodes is the same as that of the bath, the fall of potential is the same at both electrodes, and the corresponding consumption of energy at the two electrodes is the same but of opposite signs. It follows that the work of