Chapter VI: Part 6

There are certain advantages in the use of direct current, and in this country it is more commonly employed than alternating current, but the substation plant is more costly and requires skilled attendance. If the circumstances are such that these advantages are not important, the lower initial cost and running expenses of an alternating-current distribution would lead to its adoption.

In the Thury system of power transmission high-voltage direct current is used throughout. Only one supply in this country is of this kind, but several are in operation on the Continent. Pressures up to 100,000 volts are used.

ELECTRIC TELEGRAPH. See _Telegraph_.

ELECTRIC TRACTION and ELECTRIC TRAMWAY. In electric traction the mechanical power required for the propulsion of the vehicle is obtained from electric motors. These motors are usually _series direct-current motors_, but for railway work single-phase and three-phase A.C. motors have also been successfully employed (see _Electric Motors_). Up to the present, electric traction on railways has only been employed for suburban traffic in this country. In one instance (L.B. & S.C.R. electrification) single-phase alternating current is used. In all the others the power supply is direct current (see _Railways, Electrification of_).

In electric tramways, except in some few instances where there are objections to the use of an overhead construction, the current is conveyed to the motors through a trolley pole carrying a wheel running on an overhead bare copper wire. A hand-operated drum controller, directly controlling the driving and electric braking of the motors, is used. A hand-brake, and commonly a separate electro-magnetic brake, are provided.

Except in very small tramway systems, the power is generated as high-tension alternating current, and transformed and converted at substations suitably placed in the area covered by the tramway (see _Electric Power Transmission and Distribution_). The low-tension D.C. power is distributed from the substations to the trolley wire. The car rails are earthed, and provide a return path for the current. In order to minimize the flow of current to other conductors in the vicinity of the car rails, copper cables returning directly to the substation are connected to the rails at suitable intervals. These earth-return cables are connected in series with special low-voltage dynamos (called _negative boosters_) at the substation. This arrangement automatically keeps the P.D. between the most distant point of the car rails and the substation within a prescribed maximum, and effectively prevents the corrosion of pipes laid near the car rails.

ELEC'TRODE (Gr. _hodos_, a way), a term introduced by Faraday to denote the wires or other terminals by which electricity either enters or leaves a body which is undergoing electrolytic decomposition. He called the electrode at which the current enters the _anode_ (_ana_, upwards), and the electrode at which the current leaves the electrolyte the _cathode_ (_kata_, downwards). (See _Electrolysis_; _Electro-metallurgy_.) The word is now commonly used in a wider sense to denote the conductor by which contact is made with a medium. In this way electrodes are spoken of in connection with electric furnaces, electric welding appliances, vacuum tubes, and mercury vapour lamps, although the actions are not electrolytic.

ELECTROL'YSIS (Gr. _lysis_, loosening) is the name give to the decomposition of fused salts or solutions of salts, &c., by means of the electric current, and is thus a branch of electro-chemistry. The substance through which the current is passed is termed the _electrolyte_, and must be either an acid, base, or salt in a fused state or in solution. The current enters the electrolyte by an electrode called the _anode_, or the positive terminal. The electrode by means of which the the current leaves the electrolyte is termed the _cathode_, or negative terminal.

During the passage of the current the electrolyte is decomposed, and the products of decomposition are released at the electrodes or terminals. According to the modern theory of electrolysis, all electrolytes contain a greater or smaller number of free _ions_. These ions are chemical radicles carrying a definite electric charge. The kind of charge, positive or negative, depends on the nature of the radicle. The ions exhibit none of the chemical properties of the uncharged radicle.

Thus, for example, in an aqueous solution of sulphuric acid, free ions of hydrogen H_2 carrying a positive charge, and free ions of SO_4 carrying a negative charge, exist. An _uncharged_ SO_4 radicle would react with the water present, and sulphuric acid would be formed and oxygen liberated. The _ion_ SO_4, however, is incapable of doing this. Owing to the nature of their charges, the hydrogen ions will move towards the negative electrode, and the SO_4 ions towards the positive electrode. On reaching the electrodes the ions give up their charges, and immediately exhibit their ordinary chemical properties. Hydrogen is given off at the negative electrode, while at the positive electrode the uncharged SO_4 radicle reacts with the water present, and oxygen is released.

This is an example of a secondary chemical reaction. This occurs in many cases, and where it occurs the final product is different from that first produced by the electrolytic action. Fresh ions are formed or _dissociated_ in the electrolyte as fast as the original ions give up their charges at the electrodes. If this were not so, the electrolytic action would soon cease, since there would be no ions left to move towards the electrodes. The stream of ions carrying their positive and negative charges constitutes the current flowing through the electrolyte. Since the ions carry definite charges, it follows that the amounts of the initial products of an electrolytic action are in the ratio of their chemical equivalents. Thus, if fused silver chloride be electrolysed, for every 108 grammes of silver deposited at one side of the vessel 35.5 grammes of chlorine are given off at the other side (see _Electrode_; _Electro-metallurgy_).

_The electrolytic action of the current is the same at all parts of the circuit._ If the current is made to traverse several vessels, each containing the same substance, all _in series_ (that is, the current that leaves the first entering the second, and so on), it will be found that in each of the cells precisely the same amount of decomposition goes on. There will be the same weight of silver deposited at one side, and a corresponding weight of chlorine set free at the other.

_The same quantity of electricity decomposes chemically equivalent quantities of different electrolytes._ If we pass the current through a series of cells containing different electrolytes, for example, dilute sulphuric acid, chloride of silver, sulphate of copper, and collect the products of decomposition, we find that the quantities of hydrogen, silver, and copper set free are strictly proportional to the chemical equivalents of these bodies.

_The quantity of the electrolyte decomposed in a given time is proportional to the strength of the current._ This fact is made use of in measuring electric currents for standardization purposes, and the practical unit of current (the ampere) is defined, "with sufficient accuracy for all practical purposes", as being "that steady and unvarying current which deposits silver from a specified solution of silver nitrate at the rate of 0.001118 grammes per second".

The practical applications of electrolysis include the refining of copper, the electro-deposition of metals, electroplating, electrotyping, and the production of metallic sodium and potassium (see _Electro-metallurgy_). Electrolytic action is also made use of in the storage of electric energy in secondary batteries (see _Secondary Cell_).

ELECTRO-MAGNETISM, that branch of science which deals with the mutual relations between electric and magnetic fields (see _Electricity_; _Magnetism_).

It may readily be shown that when an electric current flows in a conductor, a magnetic field is produced around that conductor, i.e. that a magnetic field is produced by the motion of an electric field. Similarly, if a magnetic field is moved at right angles to a conductor, a potential difference is established between the ends of the conductor, i.e. an electric field is produced by the motion of a magnetic field.

If a conductor is placed in a magnetic field so that its length is at right angles to the lines of magnetic force (see fig. 1), and a current is passed through the conductor, a mechanical force will act on the conductor, and this force will be at right angles both to the conductor and to the original magnetic field.

From the point of view of lines of magnetic force, the magnetic field produced by the current in the conductor (shown by the concentric circles in fig. 1) will react with the original magnetic field (shown by the horizontal straight lines in fig. 1), and the actual resultant magnetic field will have the form shown in fig. 2. The tendency of lines of force to shorten themselves and repel one another laterally results in a force tending to force the conductor vertically downwards. This force is, of course, mutual, and tends to move the original magnetic field in the opposite direction.

All the phenomena of electro-magnetic action have their basis in these three effects, viz. (1) the production of a magnetic field by an electric current; (2) the production of an E.M.F. or P.D. by the relative motion of a magnetic field and a conductor; and (3) the mutual mechanical action between a current-carrying conductor and a magnetic field system.

The strength of the magnetic field produced by the current may be increased by winding the conductor in the form of a helix or solenoid consisting of a number of turns. The effect can be very greatly increased by providing the solenoid with a soft-iron core. The iron is strongly magnetized as long as the current flows. Such an arrangement is called an _electromagnet_. Electromagnets specially designed to produce a very intense magnetic field are used commercially in handling scrap-iron, pig-iron, &c. The electromagnet takes the place of the crane-hook in an ordinary crane. When the current is switched on, the pieces of iron are attracted and held firmly until the current is switched off again.

Electromagnets are also used for extracting fragments of iron or steel from the eye, and for many laboratory purposes. The most important practical use is the production of the magnetic field required in dynamo-electric machinery (see _Generator_; _Electric Motors_). The magnetic field produced within a coil in which a current flows is made use of to give the deflecting couple in certain types of galvanometers and measuring instruments (see _Galvanometer_; _Electrical Measuring Instruments_).

The absolute C.G.S. unit of current is defined in terms of the magnetic field strength produced by it, viz. "when one absolute C.G.S. unit of current flows in a circular loop of one centimetre radius, the magnetic force produced at the centre of the loop is 2[pi] dynes".

The principle of the electromagnetic generation of an E.M.F. is dealt with under the article _Electro-motive Force_. The mechanical force produced when a current flows in a conductor placed in a magnetic field forms the basis of the action of electric motors, and certain types of galvanometers and measuring instruments. The mechanical force is a mutual one, and tends to move the conductor and the field system in opposite directions. The magnitude of the force varies as the magnetic field strength, the length of the conductor, and the intensity of the current, and also as the sine of the angle between the field and the conductor. Thus when the direction of the lines of magnetic force is parallel to the conductor, the force is zero; and when their direction is at right angles to the conductor, the force is a maximum. The direction of the force is always at right angles both to the conductor and the direction of the lines of magnetic force. In an electric motor the forces acting on the conductors produce the mechanical output of the machine. In a generator these mechanical forces come into existence as soon as current is taken from the machine. In this case they produce a torque which is opposite in direction to the mechanical torque which is applied to the shaft of the generator in order to drive it.

ELECTRO-MEDICAL APPARATUS. Electrical apparatus is now widely used in the treatment and diagnosis of disease. The action of the heart may be very accurately observed by means of the electric cardiograph. The cardiograph itself consists of a very sensitive 'string' galvanometer (see _Galvanometer_) and an arrangement whereby the spot of light is focused on a moving photographic plate. In this way a photographic record of the movements of the galvanometer mirror is obtained. The galvanometer terminals are connected to two different parts of the body of the patient (say to a hand and a foot placed in separate brine baths), and the variations of potential differences which occur during a heart-beat cause a movement of the galvanometer mirror.

The X-ray apparatus has recently been adapted for taking instantaneous photographs of the heart. A single powerful discharge from a static transformer takes place through the tube, and a photograph of the position of the heart at that instant is obtained. The X-ray apparatus is very well known from its use in locating fractures, foreign bodies, diseases of the bone, &c.

The X-ray discharge is used as a treatment for certain skin diseases (especially ring-worm), rodent ulcer, and cancer. Very high-frequency alternating currents may be passed through the body without producing the muscular contractions which are a feature of the passage of low-frequency currents through the tissues. Currents of considerable magnitude of very high frequency may thus be passed through the body without inconvenience to the patient. In this way general or local heating of the body may be obtained. This process is known as diathermy. The heating locally may be made sufficiently great to cause coagulation of the tissues, or even actual burning. This method is used in the treatment of tumours and other growths.

Another important electro-medical treatment consists of the local introduction of a drug, into the affected part, by electrolytic action. Thus in the treatment of rodent ulcer, a pad of lint saturated with a 5 per cent solution of zinc sulphate is placed over the ulcer. A zinc electrode is placed on the pad and connected to the positive pole of the supply. The negative pole is connected to a basin of brine in which the patient's hand is placed. The current is made as large as can conveniently be borne (say 30 to 60 milliamperes), and is maintained for about thirty minutes and then gradually reduced to zero. By this means zinc ions are carried into the ulcer. A number of diseases may be treated in this way, the ion used depending on the nature of the case.

Low-frequency intermittent currents from induction coils are frequently used where nerve stimulation or muscular contractions are required. Static electricity is also used for similar purposes. Large Wimshurst machines are used for the treatment of sciatica, and also for neurasthenia. In the latter case a brush discharge is used, and the patient experiences very little physical sensation. The high-frequency apparatus already referred to in connection with diathermy is valuable for the treatment of rheumatism in its earlier stages, and for the stimulation of the scalp in hair treatment. Suitable electrodes are passed backwards and forwards over the affected parts, a bluish brush discharge taking place between the patient and the electrode.

ELECTRO-METALLURGY is that branch of metallurgy which uses electrical energy, wholly or in part, for the extraction or treatment of metals. The energy may be converted into heat and used for processes in which high temperatures are necessary, or it may be used for the decomposition of a compound by electrolysis, which may proceed in a fused bath at a comparatively high temperature, or in a solution bath containing a compound of the metal dissolved in a suitable solvent.

The former method of utilizing the energy embraces electrothermal processes, and the latter method, electrolytic processes.

In electrothermal processes, the heat developed by the electric current has been used in a number of industries, including welding, annealing, heat treatment, smelting, refining, &c. Laboratory apparatus, such as tubes, muffles, and crucibles, are also frequently heated by means of an electric current.

For the electric welding of metals there are two systems in use: resistance welding, in which the portions to be welded are pressed together and heated by the resistance they offer to the passage of a current; and arc welding, in which portions of metal of the same composition as that to be welded are fused on by striking an arc from a suitable electrode. In the electrical annealing of metals, case-hardened steel plates are locally softened where rivet-holes, &c., are required by passing an electric current through copper poles placed 1 or 2 inches apart on the smooth surface. Metallic wire is frequently heated to the annealing temperature between drawing operations, and various types of annealing furnaces are also electrically heated.

In electric smelting, the high temperature of the arc (3600deg C.) may be used for the reduction of certain metallic oxides, which at the lower temperature of furnaces heated by coal, coke, gas, &c. (2000deg C.), will not give up their oxygen to carbon; other ores are also sometimes smelted by electrical means, especially in localities where current is cheap and fuels are dear. The production of refined steel, special alloy steels, and certain non-ferrous alloys is also carried out in electric furnaces of various types.

The electric arc was first applied to fusion by Siemens in 1879; he fitted, into the bottom of a crucible to receive the charge, a water-cooled copper casing to form the positive pole, and suspended a carbon rod centrally in the crucible to form the negative pole. The current crosses the air-gap between the metal and the negative pole, forms an arc, and rapidly fuses the metal. In 1885 the Cowles Brothers, of Cleveland, Ohio, began to produce aluminium-copper and aluminium-iron alloys by arc smelting, and later produced other metals, difficult to reduce, by the same means. More recently, the development of electric smelting has made rapid strides. Electric furnaces not only yield higher temperature, but have other advantages over furnaces heated by carbon. They develop the heat in a small space, just where it is required for the operation, so that the furnace can be smaller, and less heat is lost by radiation; the charge can be kept free from gaseous products of combustion; the temperature and the whole operation is under better control; and the expense of running the furnace is limited to the time the current is used for doing useful work.

Electric furnaces are now used in the production of pig-iron, steel, ferro-alloys, brass, zinc, &c., and in the heat treatment of various metals. Classifying them according to the manner in which the electrical energy is converted into heat, we have:--

1. Direct resistance furnaces, in which the heat effect is produced within the metal itself by the resistance offered to the passage of the current through it. This type is used in the refining of steel.

2. Indirect resistance furnaces, to which class belong the various tube and crucible furnaces used in laboratories. The vessels to be heated are wound with wire or ribbon of high resistance, such as platinum, nickel-chrome alloys, &c., and a suitable current passed. Heat-treatment furnaces on a fairly large scale also use this method, a nickel-chrome alloy ribbon being wound on a suitable framework; the heating element in these furnaces, however, generally consists of granular carbon confined in carborundum fire-sand troughs.

3. Induction furnaces, in which a primary coil of copper wire is used, the secondary being formed by the metal charge itself, contained in a suitable annular groove. In this furnace the current passes through the primary and induces a current in the charge, thus melting it. This type of furnace has been largely used in the refining of steel, and to some extent in the melting of non-ferrous metals and alloys.

4. Direct arc-heating furnaces, as exemplified in the Siemens crucible furnace mentioned above.

5. Indirect arc-heating is used in the Stassano furnace, in which the heat is obtained by radiation from the arc, and by reflection from the roof and sides of the furnace. This furnace has been used in the production of steel from scrap, and also direct from ore. There are three electrodes, which nearly meet in the centre of the furnace.

6. Combined resistance and arc furnaces are very largely used for the production of ferrous alloys, such as ferro-silicon, ferro-chrome, and ferro-manganese; for the production of steel from scrap, and for the final refining of steel produced by other processes. In these furnaces the heat is generated largely by the arc, and to a smaller extent by the resistance offered by the whole or a portion of the furnace charge to a powerful electric current. There are several well-known commercial furnaces working on this principle, the best known probably being the H['e]roult. This furnace is designed for tilting, and is lined with basic material, and large electrodes pass through the roof. An alternating current of 4000 amperes at 110 volts is used for a 3-ton furnace, and the intensity of the current passing through the bath is regulated by raising or lowering the electrodes.

The effect of the European War has been enormous on the development of the electric furnace in this country, for prior to the war in 1914, although the use of the electric furnace for steel-making was increasing, there were only 5 furnaces in operation in Sheffield, and two or three more in other parts of the country, producing in all about 15,000 tons per annum. Soon after the commencement of the war, it became necessary to deal with the rapidly accumulating quantity of shell turnings, to make substitutes for Swedish iron and steel, which could not be imported, and to make large quantities of special alloy steel for various war purposes. As a result of these demands, within four years the number of electric furnaces increased to over 100, the steel produced being over 200,000 tons per annum. Since 1918 the number of furnaces has further increased, and probably reached 150 of various sizes and makes in 1920. In America a similar development has taken place, the number of furnaces increasing from 7 in 1907 to 363 in 1920, the output of electric steel in 1918 amounting to over 500,000 tons. In France, also, great strides have been made, and owing to the shortage of pig-iron, synthetic processes for its production from iron and steel scrap and ore were developed in open-pit arc-resistance furnaces, yielding 220,000 tons in 1916-8.

_Electrolytic Processes._--The application of electrolysis for the production of metals from a fused electrolyte is most important in the case of aluminium. This metal cannot be produced by direct electrolysis in aqueous solution, but is deposited electrolytically from a fused bath of cryolite, containing alumina in solution. As the metallic aluminium is extracted from the molten bath, further quantities of purified oxide are added. The anodes consist of carbon blocks suspended in the molten bath, and the cathode consists of the carbon lining of the furnace. Calcium, cerium, lithium, magnesium, potassium, sodium, and strontium are obtained by the electrolysis of fused chlorides, sodium being also obtained from fused hydroxide and fused nitrate.

Metallic magnesium was obtained by the electrolysis of the fused chloride by Bunsen in 1852, but the application of electrolysis as a means of recovering metals from ores by means of aqueous solutions dates back to 1836, in which year Becquerel obtained copper from sulphide ores by first extracting the copper as sulphate or chloride, and then recovering the copper by the electrolysis of the solutions, using insoluble anodes. The method has since chiefly been applied to the treatment of copper ores and products, but has also been used for the recovery of nickel, gold, zinc, &c. The production of electrolytic zinc from solutions has been encouraged as a result of the shortage of pure zinc for war purposes, and several processes have been developed. In these processes the solution used consists either of zinc sulphate or of zinc chloride, the anodes consisting of metallic lead or of carbon, and the cathodes of pure zinc sheets.

It is in connection with the refining of metals that electrolytic processes become of prime importance. Elkington, in 1865, was the first to refine impure metallic copper electrolytically and recover the silver contained in it. Pure copper is now commonly obtained from impure copper anodes in an electrolyte of copper sulphate containing free sulphuric acid, a current density of 12 to 15 amperes per square foot being used at 0.34 to 0.44 volt. Gold is also refined by a similar process, the electrolyte used consisting of gold chloride solution containing free hydrochloric acid. In this case a current density of 100 amperes at 1 volt is used. Silver is likewise refined in a silver nitrate bath, iron by the electrolysis of sulphate or chloride solution, and lead in a solution of lead fluosilicate containing free hydrofluoric acid.

In all the above-mentioned processes the anode is cast from the impure metal to be refined, and the cathode consists of a sheet or plate on which the pure metal is deposited.

It will be seen that these refining processes are very similar to electroplating methods.

ELECTROMOTIVE FORCE, the name given to the force tending to produce a flow of electricity in an electric circuit. The electromotive force, or E.M.F., is measured in terms of the work done in carrying unit quantity of electricity once round the circuit.

Thus unit electromotive force (absolute) is said to exist in a circuit if 1 erg of work is done in carrying 1 coulomb of electricity once round the circuit. The potential difference, or P.D. (in electromagnetic units), between two points in an electric circuit is similarly defined in terms of the work done in carrying 1 coulomb of electricity from the one point to the other.

_Production of an Electromotive Force._--There are several sources of E.M.F., e.g. (_a_) chemical action, as in primary and secondary cells; (_b_) thermo-electric action, as in the thermopile; (_c_) electro-magnetic action, as in generators, motors, transformers, and induction coils.

The electromotive force due to chemical action depends on the material of the electrodes and the nature of the electrolyte, and also to a slight extent on the temperature. Thus, for any given pair of materials (say zinc and copper) immersed in a certain electrolyte of given strength (say dilute sulphuric acid), the E.M.F. produced at a given temperature has a definite value. For a discussion of the electromotive force produced by thermo-electric action, see _Thermo-electricity_.

The principle of the electromagnetic generation of an E.M.F. may be stated in its most general form as follows: If lines of magnetic force are interlinked with an electric circuit, and if by any means the number of interlinkages of the lines of magnetic force with the circuit is made to change, then an E.M.F. will be generated in the circuit, the magnitude of this E.M.F. being proportional to the time rate of change of the interlinkages. Thus, if the interlinkages are changing at the rate of one per second, one absolute unit of E.M.F. will be generated; or if the interlinkages are changing at the rate of a hundred million per second, an E.M.F. of 1 volt will be generated. It is immaterial in what manner the change of interlinkages is brought about.

A permanent magnet may be moved so as to vary the lines of magnetic force linked with an electric circuit, as in magneto-generators; or the circuit may be moved through a magnetic field (see _Generator_; _Electric Motors_); or the magnetic field produced by a current in one coil linked with a second coil may be varied by varying the current in the first coil, as in static transformers and induction coils.

The electromagnetic generation of an E.M.F. is the fundamental principle which has made possible the generation and utilization of electrical energy on a large scale.

ELECTRON, the atom of electricity, more especially of negative electricity. The first light on the question of the structure of electricity came from the laws of electrolysis (q.v.), established by Faraday. These laws are explained very naturally if we make the assumption that electricity, like matter, is atomic, the atom being the charge carried by the hydrogen ion. Clerk Maxwell even proposed to call this charge 'one molecule' of electricity, but added the remark that "it is extremely improbable that when we come to understand the true nature of electrolysis we shall retain in any form the theory of molecular charges, for then we shall have obtained a secure basis on which to form a true theory of electric currents, and so become independent of these provisional hypotheses". To-day, however, so far are we from discarding the hypothesis of the atomic nature of electricity that we find ourselves compelled by the pressure of experimental facts to interpret all electrical phenomena, in metals as well as in electrolytes, in terms of this very hypothesis. Any statical charge is supposed to be made up of a very great number of electrons, just as a material body is composed of atoms of matter. A metallic conductor is supposed to contain many free electrons, which normally bear much the same relation to the material molecules as a saturated vapour bears to the liquid in equilibrium with it. When an electromotive force is applied, it causes a drift of the electrons in the opposite direction to the force, the charge on the electrons being negative. It is this drift of electrons which constitutes an electric current.

The striking advances that have been made in our knowledge of the nature of electricity since the last years of the nineteenth century have been due chiefly to the study of the electric discharge in gases. Hittorf in 1869 and Crookes in 1879 examined the rays, now called the cathode rays, which stream from cathode to anode in a tube containing gas of very low pressure. The phenomena suggested to Crookes that the rays consist of material particles carrying a negative charge and moving at a high speed; but many physicists rejected this explanation, holding that the rays were due to some form of wave motion in the ether. About 1897 it was conclusively shown by Perrin, Wiechert, and Sir J. J. Thomson that Crookes's view was the correct one. Sir J. J. Thomson measured the velocity of the particles, and also the ratio of the charge e, to the mass m of each. His method was to subject a fine beam to the action of two fields of force, one magnetic, the other electric, and both perpendicular to the line of motion and also to each other. The electric field being X, and the magnetic field H, the forces on a particle were in the same direction, and equal to eX, evH. Either field by itself deflected a fine beam, as was shown by the motion of a spot of light where the beam struck a fluorescent screen. The value of X was adjusted till there was no deflection in the combined fields. Hence X = vH, and v was found from the measured values of X and H. The deflections under the two fields acting separately were also observed. Either of these deflections, when v is known, gives the value of the ratio e/m. The values of the velocity v were found to depend on the E.M.F. between the terminals of the discharge-tube. They varied from 1/30 to 1/3 of the velocity of light. The fraction e/m, however, had always the same negative value, no matter how the material of the cathode and the nature and pressure of the gas were varied.

Many other ways of obtaining these negatively charged particles, or electrons, are now known. The [beta]-rays from radio-active substances (see _Radio-activity_) are simply electrons moving with great speeds, approaching sometimes within 2 or 3 per cent of the velocity of light. Hot metals give off electrons copiously: this property is used in the construction of the Coolidge X-ray tube and of the thermionic valve (q.v.). A metal plate illuminated by ultra-violet light, from an electric arc or spark, for instance, gives off electrons moving at all velocities below a certain maximum (see _Photo-electric Effect_). From whatever source the electrons are derived, their properties are found to be the same.

The determination of e and m separately is a much more difficult matter than the determination of their ratio. The first attempt to measure e directly was made by Townsend, and published in 1897. Townsend obtained his ions in the hydrogen and oxygen given off when caustic potash is electrolyzed. The charged gases when bubbled through water formed a cloud. This cloud could be completely removed by bubbling through concentrated sulphuric acid, but reappeared when the gas came out again into the atmosphere, owing to the condensation of water-vapour on the ions. Townsend determined the weight of the cloud and its total charge. He also found the average weight of the minute spherical drops forming the cloud by observing their rate of fall under gravity, and calculating their radius from a theoretical formula known as Stokes's law, viz. v = 2/9ga^2[rho]/[eta], where a is the radius, [rho] the density, v the velocity of the drop, and [eta] is the viscosity of air. The weight of the cloud divided by the weight of a drop gave the number of drops, which was presumably the same as the number of ions. Finally, dividing the total charge by the number of ions, Townsend found e, the average charge carried by an ion. His value came out about three-fifths of the value accepted now.

This pioneer method of Townsend's has been improved and modified in various ways by C. T. R. Wilson, Sir J. J. Thomson, H. A. Wilson, and notably by Millikan, of Chicago. Millikan's charge carriers were minute oil drops, which were given elementary charges by means of ionizing rays from radium. Observations were made of the equilibrium and motion of these charges under the combined influence of gravity and a strong vertical electric field, the intensity of which could be varied at will. A single drop could be kept in view for several minutes at a time, and note was taken of the effect of each new charge as it was picked up by the drop. On calculation, the charge was found in all cases to have very approximately the same value. It so happened, as a consequence of the method of producing the drops, that they carried a small frictional charge, and incidentally Millikan was able to verify that this was always an integral multiple of the electronic charge e. Millikan's result, which is most probably the best yet found, is that e = 4.774 x 10^{-10} absolute electrostatic units, or 1.591 x 10^{-20} absolute electromagnetic units.

An indirect but very interesting method of determining e was devised independently by Regener and by Rutherford and Geiger. The special feature of this method is the actual counting of the number of [alpha]-particles (see _Radio-activity_) shot out per second through a given solid angle by a small speck of radium. Each [alpha]-particle produces a scintillation on a sensitive screen placed in its path, and these scintillations are counted one by one by the observer. The total quantity of electricity carried by the [alpha]-particles emitted in one second is measured independently. The charge on each particle is then found by simple division. This charge is found to be almost exactly twice Millikan's value for e, as it ought to be, as it is practically certain that the [alpha]-particle is an atom of helium which has lost two electrons.

The value of e/m, as determined by Thomson's method described above, is 1.76 x 10^7 e.m.u. per gramme, or 5.29 x 10^{17} e.s.u. per gramme. Taking this with Millikan's value for _e_, we find n = 0.902 x 10^{-27} grammes. The exact determination of e has made it possible to assign precise values to several other important physical constants, which formerly were only known roughly from data depending on the Kinetic Theory of Gases. Thus Avogadro's constant N, or the number of molecules in one gramme-molecule (molecular weight in grammes) of any gas can be connected with e by the exact measurements of electrolysis, which give Ne = 9650 e.m.u. It follows that N = 6.06 x 10^{23}, and that the number of gas molecules per cubic centimetre at 0deg C and 76 centimetres pressure is 2.70 x 10^{19}. We find at once also the mass of the hydrogen atom as 1.66 x 10^{-24} grammes, the density of hydrogen being known to be .0899 grammes per litre. The mass of the electron is therefore about 1/1840 of the mass of the hydrogen atom, which till the isolation of the electron was the smallest mass known.

It is necessary, however, to scrutinize with some care the meaning of the word mass as applied to an electron. The determination of the mass of the hydrogen atom ultimately depends on weighing, that is, on finding its gravitational inertia. We cannot weigh an electron, but must determine its mass by experiments involving its motion, the word mass here meaning the ratio of the force acting on the electron to the acceleration produced, and the force being calculated from the charge and velocity of the electron on the principles of electrodynamics. An electron being entirely different in its physical nature from ordinary matter, the question arises whether its mass, as calculated in this way, is actually a definite constant, as it is for a material particle, according to the accepted principles of Newtonian dynamics. It can even be shown, as was first done by Sir J. J. Thomson, that a moving charged body possesses inertia in virtue of the mere fact that it carries a charge. The value of this inertia, or electromagnetic mass, when the velocity is small compared with that of light is, in a vacuum, for a small sphere of radius a, 2/3e^2/a, where e is the charge. If the velocity is greater than, say, 1/10 the velocity of light, the formula for the electromagnetic mass is more complicated, and, indeed, cannot be calculated without some assumption as to the internal distribution of charge in the electron itself. Two formulae for the mass have been given, one by Abraham, the other by H. A. Lorentz. Abraham started from the supposition that the electron is a rigid sphere carrying a uniform surface charge. Lorentz showed that a simpler theory could be obtained by the hypothesis that the electron contracts, in the direction of its motion, by a certain definite amount depending on its velocity. On both theories the value found for the mass depends on the relation between the direction of the force and the direction of the motion. On Lorentz's theory the longitudinal mass, or mass when the force is in the direction of the motion, is m_0/(1-[beta]^2)^{3/2}; and the transverse mass, or mass when the force is perpendicular to the velocity, is m_0/(1-[beta]^2)^{1/2}; where m_0 is the mass for very small speeds, and [beta] is v/c, the ratio of the velocity of the electron to the velocity of light. The two theories have been tested by various experimenters, with somewhat conflicting results. On the evidence of experiments by Bucherer, however, Lorentz's theory of the contractile electron is now generally accepted, and it is regarded as highly probable that electrons are devoid of all mass except the electromagnetic mass due to their charge of negative electricity.

No fundamental positive electron has been isolated which at all corresponds to the negative electron, or corpuscle, as it is called by Sir J. J. Thomson. The nearest approach to a positive electron is the nucleus of the hydrogen atom, which carries a positive charge of the same magnitude as the charge on an electron. Practically the whole mass of the atom resides in this nucleus. According to the modern theory of the structure of matter, the neutral atom of any element is built up of a comparatively small number of electrons and an equal number of these positive nuclei. Electrons being present everywhere, and their action influencing all natural phenomena, their properties will naturally come up for consideration from various points of view in other articles. See _Ionization_; _Isotopes_; _Matter_; _Radio-activity_; _Rays_, _Electric._--BIBLIOGRAPHY: J. A. Crowther, _Ions, Electrons, and Ionizing Radiations_; R. A. Millikan, _The Electron_; N. R. Campbell, _Modern Electrical Theory_; O. W. Richardson, _The Electron Theory of Matter_; H. A. Lorentz, _Theory of Electrons_.

ELECTRO-PLATING, the process of depositing a coating of some selected metal on a given surface by means of electrolysis (q.v.). The most important classes of electro-plating commonly carried out are nickel-plating, used very largely for a variety of articles made of iron, steel, &c.; copper-plating, used for facing printing-blocks and as a first coating to non-metallic substances prior to silver- or gold-plating; silver-plating, for imitation silverware and for cutlery, &c.; gold-plating, for ornamental ware, jewellery, &c. Previous to plating it is necessary to remove all grease, dirt, oxide, &c., from the surface, this cleansing of the articles being the first step in the operations necessary. The exact procedure for cleansing varies with the nature of the articles to be plated, but for the removal of grease a strong caustic alkali bath is generally used. To remove oxide and dirt, scratch-brushing is used, also scouring with pumice-stone. Acid-dipping baths are also employed, muriatic acid or sulphuric acid for iron or steel articles, dipping-acid, which is a mixture of sulphuric acid and nitric acid, for brass. For the actual deposition an electrolytic cell is prepared, containing a solution of a suitable salt of the metal to be deposited, with an anode, generally consisting of a plate of the same metal, attached to the positive pole of the battery used, the article to be treated being connected with the negative pole and thus forming the cathode. When a current of electricity is passed through the solution, a thin coating of metal is deposited on the article forming the cathode, and an equivalent portion is carried into solution from the anode. In the case of nickel-plating, the solution used is made from the double chloride or sulphate of nickel and ammonium, to which salt, sal-ammoniac, &c., may be added. The bath is used at a temperature of 100deg F., and cast-nickel plates are used as anodes. For copper-plating the bath used generally consists of an acid solution of sulphate or acetate of copper, cyanide of potash also being added; in case the article is made of zinc, an alkaline bath is used. The bath may be used cold, but is sometimes kept at about 120deg F. For iron simple dipping is sometimes used, as copper is readily deposited on iron without the use of an electric current. For electroplating of copper, anodes of metallic copper, having a surface equal to that of the articles to be coated, are used. For silver-plating the solution consists of the double cyanide of silver and potash, and may be used either hot or cold. An article to be silver-plated is often prepared by a preliminary dip in a solution of nitrate of mercury, which causes a slight amalgamation with mercury. After this preliminary treatment it is placed in the bath and a slight deposit of silver obtained, after which it is removed, well brushed, washed, and replaced in the bath. A silver plate is used as an anode. A density of 1-1/4 to 1-1/2 ounces of silver to the square foot gives an excellent plate about the thickness of common writing-paper. In gold-plating baths, a hot solution of the double cyanide of gold and potash is used at 170deg F., and for the anode platinum foil is frequently used, the strength of the bath being maintained by the addition of fresh quantities of chloride of gold. After all kinds of plating as described above, the goods are thoroughly washed in water, and dried by means of saw-dust or in a drying-chamber. In ordinary circumstances the deposited metal presents a dead or matted appearance, and if a bright polished effect is desired, it is burnished and buff-polished. Certain chemicals added to the solution will cause the original deposit to have a metallic lustre.

ELECTROTYPE. The production of copper facsimiles by the electric current is called _electrotype_, and is the oldest branch of electro-metallurgy. One of its most important applications is the copying of type set up for printing, and of wood blocks for wood-cuts. A mould is first obtained in gutta-percha or some similar material. This, being a non-conductor, is brushed over with plumbago in its interior, so as to give it a conducting surface to receive the deposit. After several hours the deposit is detached from the mould and backed by pouring in melted solder, the surface being first moistened with chloride of zinc to make the solder adhere. In the copying of steel engravings the mould is obtained by electro-deposition of copper on the steel, the surface of which must first be specially prepared to prevent adhesion; and a second electro-deposition of copper, on the mould thus obtained, gives the required copy, from which impressions can be printed.

ELEC'TRUM (Gr. _[=e]lektron_), in antiquity, a term applied to native gold, which frequently contains notable quantities of silver, copper, and other metals. According to Pliny, the term electron was applied to native gold containing at least 20 per cent of silver. The term was afterwards transferred from this native alloy to the artificial alloy of gold and silver on account of its colour and inferior lustre. The word originally meant 'amber', and was given to impure gold on account of a supposed resemblance. Electrum was used since the eighth or seventh century B.C.

ELEC'TUARY, or CONFECTION, is a pharmacopoeial preparation. It is solid, but of soft consistence, and contains sugar or honey, impregnated with some more active body. The best known is the confection of senna.

ELE'GIT, in English law, a writ by which a creditor who has obtained a judgment against a debtor, and is hence called the _judgment-creditor_, may be put in possession of the lands and tenements of the person against whom the judgment is obtained, called the _judgment-debtor_, until the debt is fully paid. The writ is addressed to the sheriff, who enforces it. The writ of elegit was first authorized by the Statute of Westminster the Second, which gave the _judgment-creditor_ the right to choose between a writ against the debtor's land, and until 1883 his goods also, and an execution by writ against the latter's person or chattels. The new writ, representing the choice of the creditor, was therefore called an elegit, Lat., he has chosen. See _Fieri Facias_.

EL'EGY (Gr. _elegos_, mourning, song), a mournful and plaintive poem or funeral song, or any serious poem of a melancholy contemplative kind. In classic poetry what is known as _elegiac verse_ is composed of couplets consisting of alternate hexameter and pentameter lines. In English we generally understand by elegies lyric poems which are laments over the dead, such as Milton's _Lycidas_, or Shelley's _Adonais_.

ELEMEN'TAL SPIRITS, according to a belief common in the Middle Ages, spirits proper to and partaking of the four so-called elements, viz. salamanders or fire spirits, sylphs or aerial spirits, gnomes or earth spirits, and undines or water spirits.

EL'EMENTS, the simplest constituent principles or parts of anything; in a special sense, the ultimate indecomposable constituents of any kind of matter. In ancient philosophies the term was applied to fire, air, earth, and water. The mediaeval chemists, however, absorbed in the study of metals and mineral substances, supposed that the metals consisted of an elemental sulphur and an elemental mercury mixed together more or less perfectly and in different proportions. To these were subsequently added salt and some others, so that about the middle of the seventeenth century the first principles amounted to five, divided into two classes; the active, consisting of mercury or spirit, sulphur or oil, and salt; and the passive, consisting of water or phlegm, and earth or the terrestrial part. The names remained, not so much as denoting substances or ultimate principles as gradually coming to denote functions; the first great modification being the expansion of the idea of elemental sulphur into phlogiston by Stahl, as the result of which the adherents of the phlogistic theory applied the term phlogiston to the gases then discovered, the mineral, vegetable, and animal acids, the alkalies, earths, and metallic calces, oil, alcohol, and water. The substances considered as simple naturally changed with the change of theory introduced by Lavoisier, who considered as elements, oxygen, nitrogen, hydrogen, sulphur, phosphorus, and carbon, the metals and the earths, and, as Boyle had already suggested, practically defined an element as a body not yet decomposed, the definition now commonly adopted. For list of known elements see _Chemistry_.

EL'EMI, the fragrant resinous exudation from various trees, such as the _Canarium commune_, from which the Eastern or Manila elemi is obtained; the _Ic[=i]ca Ic[=i]car[=i]ba_, the source of the American or Brazilian elemi; and the _Elaphrium elemif[)e]rum_, from which the Mexican elemi comes. It is a regular constituent of spirit varnishes, and is used in medicine, mixed with simple ointment, as a plaster.

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EL'EPHANT, the popular name of a genus, family, or sub-order of five-toed proboscidian mammals, usually regarded as comprehending two species, the Asiatic (_Elephas indicus_) and the African (_E. afric[=a]nus_). From a difference in the teeth, however, the two species are sometimes referred to distinct genera (Euelephas and Loxodon). The so-called white elephants are merely albinos. The African elephant is distinguished from the Asiatic species by its greater height, its larger ears, its less elevated head and bulging or convex forehead, the closer approximation of the roots of the tusks, and the greater density of the bone. It has also only three external hoofs on the hind-feet, while the Asiatic has four. All elephants are remarkable for their large, heavy, short bodies supported on columnar limbs, a very short neck, a skull with lofty crown and short face-bones, with the exception of the premaxillaries, which are enlarged to form tusk-sockets. To compensate for the short neck, they have the long proboscis, often 4 or 5 feet in length, produced by the union and development of the nose and upper lip. It is made up of muscular and fibrous tissue. The trunk is of great strength and sensibility, and serves alike for respiration, smell, taste, suction, touch, and prehension. The tusks, which are enormously developed upper incisor teeth, are not visible in young animals, but in a state of maturity they project in some instances 7 or 8 feet. The largest on record (undoubtedly that of an extinct species) weighed 350 lb. Elephants sometimes attain the height of 12 feet or more, but their general height is about 9 or 10 feet. Their weight ranges from 4000 to 9000 lb. The period of gestation is twenty months, and the female seldom produces more than one calf at a birth: this, when first born, is about 3 feet high, and continues to grow till it is sixteen or eighteen years of age. It is said that they live to the age of 150 years. They feed on vegetables, the young shoots of trees, grain, and fruit. They are polygamous, associating in herds of a considerable size under the guidance of a single leader. An elephant leaving or driven from a herd is not allowed to join another, but leads a lonely, morose, and destructive life. Such solitary elephants are known as 'rogues'. Elephants are caught either singly or in herds. In the former case it is necessary to catch adroitly one of the elephant's legs in the noose of a strong rope, which is then quickly attached to a tree; another leg is then caught, until all are securely fastened. His captors then encamp beside him, until under their treatment he becomes tractable. When a herd is to be caught a strong enclosure is constructed, and into this the elephants are gradually driven by fires, noise, &c. With the aid of tame elephants the wild ones are tied to trees and subjected to the taming process. The domesticated elephant requires much care, and a plentiful supply of food, being liable to many ailments. The daily consumption of a working elephant is, according to Sir J. E. Tennent, 2 cwt. of green food, about half a bushel of grain, and about 40 gallons of water. Their enormous strength, docility, and sagacity make them of great value in the East for road-making, building, and transport. They are used by the great on occasions of pomp and show, being often richly caparisoned, and bearing on their back a howdah containing one or more riders, besides the mahout or driver sitting on the animal's neck. Tiger-shooting is often practised from an elephant's back. Several extinct species are known, the most notable being the mammoth (_E. primigenius_), a contemporary of prehistoric man. The allied genus Mastodon was of very wide distribution, and the Tertiary deposits of the Fayum (Egypt) have yielded the remains of types that bridge over the gap between elephants and more typical quadrupeds. See _Mammoth_; _Mastodon_.--BIBLIOGRAPHY: Andersson, _The Lion and the Elephant_; Sir. J. E. Tennent, _The Wild Elephant in Ceylon_; Sanderson, _Wild Beasts of India_; R. Lydekker, _The Game Animals of Africa_.