37, p. 723). These experiments showed that over moderate ranges | values as the "Swedish Iron," commonly considered best for of induction, such as may be expected in electro-technical work, the hysteresis loss per cycle per cubic centimetre was practically the same when the iron was tested in an alternating field with a❘ periodicity of 100, the field remaining constant in direction, and when the iron was tested in a rotating field giving the same maximum flux density.

papers:

The

dynamo machines are considered in the article DYNAMO, and there | metal and the accompanying slag were to be caught, after leaving is, therefore, no necessity to refer particularly to the numerous the arc and while still liquid, in a hearth fired with ordinary different shapes and types employed in electrotechnics. fuel. Although this primitive furnace could be made to act, its BIBLIOGRAPHY.-For additional information on the above subject efficiency was low, and the use of a separate fire was disadvanthe reader may be referred to the following works and original tageous. In 1878 Sir William Siemens patented a form of furnace1 which is the type of a very large number of those designed by H. du Bois, The Magnetic Circuit in Theory and Practice; S. P. Thompson, The Electromagnet; J. A. Fleming, Magnets and Electric later inventors. Currents; J. A. Ewing, Magnetic Induction in Iron and other Metals; In the best-known form a plumbago crucible was used with a J. A. Fleming, "The Ferromagnetic Properties of Iron and Steel," hole cut in the bottom to receive a carbon rod, which was ground Proceedings of Sheffield Society of Engineers and Metallurgists (Oct. in so as to make a tight joint. This rod was connected with the 1897); J. A. Ewing, "The Magnetic Testing of Iron and Steel," positive pole of the dynamo or electric generator. The crucible Proc. Inst. Civ. Eng., 1896, 126, p. 185; H. F Parshall, was fitted with a cover in which were two holes; one at the side to Magnetic Data of Iron and Steel," Proc. Inst. Civ. Eng., 1896, serve at once as sight-hole and charging door, the other in the 126, p. 220; J. A. Ewing, "The Molecular Theory of Induced centre to allow a second carbon rod to pass freely (without touching) Magnetism," Phil. Mag., Sept. 1890; W. M. Mordey, "Slow Changes into the interior. This rod was connected with the negative pole of in the Permeability of Iron," Proc. Roy. Soc. 57, p. 224; J. A. the generator, and was suspended from one arm of a balance-beam, Ewing, Magnetism," James Forrest Lecture, Proc. Inst. Civ. Eng. while from the other end of the beam was suspended a vertical hollow 138; S. P. Thompson, Electromagnetic Mechanism," Electrician, iron cylinder, which could be moved into or out of a wire coil or 26, pp. 238, 269, 293; J. A. Ewing, "Experimental Researches in solenoid joined as a shunt across the two carbon rods of the furnace. Magnetism," Phil. Trans., 1885, part ii.; Ewing and Klassen, The solenoid was above the iron cylinder, the supporting rod of which Magnetic Qualities of Iron," Proc. Roy. Soc., 1893. (J. A. F.) passed through it as a core. When the furnace with this well-known ELECTROMETALLURGY. The present article, as explained regulating device was to be used, say, for the melting of metals or other conductors of electricity, the fragments of metal were placed under ELECTROCHEMISTRY, treats only of those processes in in the crucible and the positive electrode was brought near them. which electricity is applied to the production of chemical re- Immediately the current passed through the solenoid it caused the actions or molecular changes at furnace temperatures. In iron cylinder to rise, and, by means of its supporting rod, forced the many of these the application of heat is necessary to bring end of the balance beam upwards, so depressing the other end that the negative carbon rod was forced downwards into contact with the the substances used into the liquid state for the purpose of metal in the crucible. This action completed the furnace-circuit, electrolysis, aqueous solutions being unsuitable. Among the and current passed freely from the positive carbon through the earliest experiments in this branch of the subject were fragments of metal to the negative carbon, thereby reducing the those of Sir H. Davy, who in 1807 (Phil. Trans., 1808, current through the shunt. At once the attractive force of the solenoid on the iron cylinder was automatically reduced, and the p. 1), produced the alkali metals by passing an intense cur- falling of the latter caused the negative carbon to rise, starting an rent of electricity from a platinum wire to a platinum dish, arc between it and the metal in the crucible. A counterpoise was through a mass of fused caustic alkali. The action was started placed on the solenoid end of the balance beam to act against the attraction of the solenoid, the position of the counterpoise determinin the cold, the alkali being slightly moistened to render ing the length of the arc in the crucible. Any change in the resist conductor; then, as the current passed, heat was produced ance of the arc, either by lengthening, due to the sinking of the charge and the alkali fused, the metal being deposited in the liquid in the crucible, or by the burning of the carbon, affected the procondition. Later, A. Matthiessen (Quarterly Journ. Chem. Soc. portion of current flowing in the two shunt circuits, and so altered viii. 30) obtained potassium by the electrolysis of a mixture the position of the iron cylinder in the solenoid that the length of arc was, within limits, automatically regulated. Were it not for the of potassium and calcium chlorides fused over a lamp. There use of some such device the arc would be liable to constant fluctuation are here foreshadowed two types of electrolytic furnace-opera- and to frequent extinction. The crucible was surrounded with a tions: (a) those in which external heating maintains the bad conductor of heat to minimize loss by radiation. The positive electrolyte in the fused condition, and (b) those in which a current-carbon was in some cases replaced by a water-cooled metal tube, or ferrule, closed, of course, at the end inserted in the crucible. Several density is applied sufficiently high to develop the heat necessary modifications were proposed, in one of which, intended for the heating to effect this object unaided. Much of the earlier electrometal-of non-conducting substances, the electrodes were passed horizontally lurgical work was done with furnaces of the (a) type, while through perforations in the upper part of the crucible walls, and the nearly all the later developments have been with those of class charge in the lower part of the crucible was heated by radiation. (b). There is a third class of operations, exemplified by the manufacture of calcium carbide, in which electricity is employed solely as a heating agent; these are termed electrothermal, as distinguished from electrolytic. In certain electrothermal processes (e.g. calcium carbide production) the heat from the current is employed in raising mixtures of substances to the temperature at which a desired chemical reaction will take place between them, while in others (e.g. the production of graphite from coke or gas-carbon) the heat is applied solely to the production of molecular or physical changes. In ordinary electrolytic work only the continuous current may of course be used, but in electrothermal work an alternating current is equally available.

a

The furnace used by Henri Moissan in his experiments on reactions at high temperatures, on the fusion and volatilization of refractory materials, and on the formation of carbides, silicides and borides of various metals, consisted, in its simplest form, of two superposed blocks of lime or of limestone with a central cavity cut in the lower block, and with a corresponding but much shallower inverted cavity in the upper block, which thus formed the lid of the furnace. Horizontal channels were cut on opposite walls, through which the carbon poles or electrodes were passed into the upper part of the cavity. Such a furnace, to take a current of 4 H.P. (say, of 60 amperes and 50 volts), measured externally about 6 by 6 by 7 in., and the electrodes were about 0-4 in. in diameter, while for a current of 100 H.P. (say, of 746 amperes and 100 volts) it measured about 14 by 12 by 14 in., and the electrodes were about 1.5 in. in diameter. In the latter case the crucible, which was placed in the cavity immediately beneath the arc, was about 3 in. in diameter (internally), and about 3 in. in height. The fact that energy is being used at so high a rate as too H.P. on so small a charge of material sufficiently indicates that the furnace is only used for experimental work, or for the fusion of metals which, like tungsten or chromium, can only be melted at temperatures attainable by electrical means. Moissan succeeded in fusing about Ib of either of these metals in 5 or 6 minutes in a furnace similar to that last described. He also arranged an experimental tubefurnace by passing a carbon tube horizontally beneath the arc 1 Cf. Siemens's account of the use of this furnace for experimental purposes in British Association Report for 1882.

practically limited to that at which the least easily vaporized material available for electrodes is converted into vapour. This material is carbon, and as its vaporizing point is (estimated at) over 3500° C., and less than 4000° C., the temperature of the electric furnace cannot rise much above 3500° C. (6330° F.); but H. Moissan showed that at this temperature the most stable of mineral combinations are dissociated, and the most refractory elements are converted into vapour, only certain borides, silicides and metallic carbides having been found to resist the action of the heat. It is not necessary that all electric furnaces shall be

descence or resistance type may be worked at any convenient temperature below the maximum. The electric furnace has several advantages as compared with some of the ordinary types of furnace, arising from the fact that the heat is generated from within the mass of material operated upon, and (unlike the blastfurnace, which presents the same advantage) without a large volume of gaseous products of combustion and atmospheric nitrogen being passed through it. In ordinary reverberatory and other heating furnaces the burning fuel is without the mass, so that the vessel containing the charge, and other parts of the plant, are raised to a higher temperature than would otherwise be necessary, in order to compensate for losses by radiation, convection and conduction. This advantage is especially observed in some cases in which the charge of the furnace is liable to attack the containing vessel at high temperatures, as it is often possible to maintain the outer walls of the electric furnace relatively cool, and even to keep them lined with a protecting crust of unfused charge. Again, the construction of electric furnaces may often be exceedingly crude and simple; in the carborundum furnace, for example, the outer walls are of loosely piled bricks, and in one type of furnace the charge is simply heaped on the ground around the carbon resistance used for heating, without containing-walls of any kind. There is, however, one (not insuperable) drawback in the use of the electric furnace for the smelting of pure metals. Ordinarily carbon is used as the electrode material, but when carbon comes in contact at high temperatures with any metal that is capable of forming a carbide a certain amount of combination between them is inevitable, and the carbon thus introduced impairs the mechanical properties of the ultimate metallic product. Aluminium, iron, platinum and many other metals may thus take up so much carbon as to become brittle and unforgeable. It is for this reason that Siemens, Borchers and others substituted a hollow watercooled metal block for the carbon cathode upon which the melted metal rests while in the furnace. Liquid metal coming in contact with such a surface forms a crust of solidified metal over it, and this crust thickens up to a certain point, namely, until the heat from within the furnace just overbalances that lost by conduction through the solidified crust and the cathode material to the flowing water. In such an arrangement, after the first instant, the melted metal in the furnace does not come in contact with the cathode material.

in the cavity of the lime blocks. When prolonged heating is required at very high temperatures it is found necessary to line the furnace-cavity with alternate layers of magnesia and carbon, taking care that the lamina next to the lime is of magnesia; if this were not done the lime in contact with the carbon crucible would form calcium carbide and would slag down, but magnesia does not yield a carbide in this way. Chaplet has patented a muffle or tube furnace, similar in principle, for use on a larger scale, with a number of electrodes placed above and below the muffle-tube. The arc furnaces now widely used in the manufarture of calcium carbide on a large scale are chiefly develop-run at these high temperatures; obviously, those of the incanments of the Siemens furnace. But whereas, from its construction, the Siemens furnace was intermittent in operation, necessitating stoppage of the current while the contents of the crucible were poured out, many of the newer forms are specially designed either to minimize the time required in effecting the withdrawal of one charge and the introduction of the next, or to ensure absolute continuity of action, raw material being constantly charged in at the top and the finished substance and by-products (slag, &c.) withdrawn either continuously or at intervals, as sufficient quantity shall have accumulated. In the King furnace, for example, the crucible, or lowest part of the furnace, is made detachable, so that when full it may be removed and an empty crucible substituted. In the United States a revolving furnace is used which is quite continuous in action. The class of furnaces heated by electrically incandescent materials has been divided by Borchers into two groups: (1) those in which the substance is heated by contact Incan. with a substance offering a high resistance to the descence furnaces. current passing through it, and (2) those in which the substance to be heated itself affords the resistance to the passage of the current whereby electric energy is converted into heat. Practically the first of these furnaces was that of Despretz, in which the mixture to be heated was placed in a carbon tube rendered incandescent by the passage of a current through its substance from end to end. In 1880 W. Borchers introduced his resistance-furnace, which, in one sense, is the converse of the Despretz apparatus. A thin carbon pencil, forming a bridge between two stout carbon rods, is set in the midst of the mixture to be heated. On passing a current through the carbon the small rod is heated to incandescence, and imparts heat to the surrounding mass. On a larger scale several pencils are used to make the connexions between carbon blocks which form the end walls of the furnace, while the side walls are of fire-brick laid upon one another without mortar. Many of the furnaces now in constant use depend mainly on this principle, a core of granular carbon fragments stamped together in the direct line between the electrodes, as in Acheson's carborundum furnace, being substituted for the carbon pencils. In other cases carbon fragments are mixed throughout the charge, as in E.H. and A.H. Cowles's zinc-smelting retort. In practice, in these furnaces, it is possible for small local arcs to be temporarily set up by the shifting of the charge, and these would contribute to the heating of the mass. In the remaining class of furnace, in which the electrical resistance of the charge itself is utilized, are the continuous-current furnaces, such as are used for the smelting of aluminium, and those alternating-current furnaces, (e.g. for the production of calcium carbide) in which a portion of the charge is first actually fused, and then maintained in the molten condition by the current passing through it, while the reaction between further portions of the charge is proceeding. For ordinary metallurgical work the electric furnace requiring as it does (excepting where waterfalls or other cheap sources of power are available) the intervention of the boiler Uses and and steam-engine, or of the gas or oil engine, with a advigLages. consequent loss of energy, has not usually proved so economical as an ordinary direct fired furnace. But in some cases in which the current is used for electrolysis and for the production of extremely high temperatures, for which the calorific intensity of ordinary fuel is insufficient, the electric furnace is employed with advantage. The temperature of the electric furnace, whether of the arc or incandescence type, is

Electrothermal Processes.-In these processes the electric current is used solely to generate heat, either to induce chemical reactions between admixed substances, or to produce a physical (allotropic) modification of a given substance. Borchers predicted that, at the high temperatures available with the electric furnace, every oxide would prove to be reducible by the action of carbon, and this prediction has in most instances been justified. Alumina and lime, for example, which cannot be reduced at ordinary furnace temperatures, readily give up their oxygen to carbon in the electric furnace, and then combine with an excess of carbon to form metallic carbides. In 1885 the brothers Cowles patented a process for the electrothermal reduction of oxidized ores by exposure to an intense current of electricity when admixed with carbon in a retort. Later in that year they patented a process for the reduction of aluminium by carbon, and in 1886 an electric furnace with sliding carbon rods passed through the end walls to the centre of a rectangular furnace. The impossibility of working with just sufficient carbon to reduce the alumina, without using any excess which would be free to

Alumin

lum

form at least so much carbide as would suffice, when diffused
through the metal, to render it brittle, practically restricts the
use of such processes to the production of aluminium
alloys. Aluminium bronze (aluminium and copper)
alloys. and ferro-aluminium (aluminium and iron) have
been made in this way; the latter is the more satis-
factory product, because a certain proportion of carbon is
expected in an alloy of this character, as in ferromanganese and
cast iron, and its presence is not objectionable. The furnace is
built of fire-brick, and may measure (internally) 5 ft. in length
by 1 ft. 8 in. in width, and 3 ft. in height. Into each end wall
is built a short iron tube sloping downwards towards the centre,
and through this is passed a bundle of five 3-in. carbon rods,
bound together at the outer end by being cast into a head of
cast iron for use with iron alloys, or of cast copper for aluminium
bronze. This head slides freely in the cast iron tubes, and is
connected by a copper rod with one of the terminals of the
dynamo supplying the current. The carbons can thus, by the
application of suitable mechanism, be withdrawn from or plunged
into the furnace at will. In starting the furnace, the bottom
is prepared by ramming it with charcoal-powder that has been
soaked in milk of lime and dried, so that each particle is coated
with a film of lime, which serves to reduce the loss of current
by conduction through the lining when the furnace becomes
hot. A sheet iron case is then placed within the furnace, and
the space between it and the walls rammed with limed charcoal;
the interior is filled with fragments of the iron or copper to be
alloyed, mixed with alumina and coarse charcoal, broken pieces
of carbon being placed in position to connect the electrodes.
The iron case is then removed, the whole is covered with charcoal,
and a cast iron cover with a central flue is placed above all.
The current, either continuous or alternating, is then started,
and continued for about 1 to 1 hours, until the operation is
complete, the carbon rods being gradually withdrawn as the
action proceeds. In such a furnace a continuous current, fore
example, of 3000 amperes, at 50 to 60 volts, may be used at first,
increasing to 5000 amperes in about half an hour. The reduction
is not due to electrolysis, but to the action of carbon on alumina,
a part of the carbon in the charge being consumed and evolved
as carbon monoxide gas, which burns at the orifice in the cover
so long as reduction is taking place. The reduced aluminium
alloys itself immediately with the fused globules of metal in
its midst, and as the charge becomes reduced the globules of
alloy unite until, in the end, they are run out of the tap-hole

after the current has been diverted to another furnace. It was
found in practice (in 1889) that the expenditure of energy per
pound of reduced aluminium was about 23 H.P.-hours, a
number considerably in excess of that required at the present
time for the production of pure aluminium by the electrolytic
process described in the article ALUMINIUM. Calcium carbide,
graphite (q.v.), phosphorus (q.v.) and carborundum (q.v.) are now
extensively manufactured by the operations outlined above.

Electrolytic Processes.-The isolation of the metals sodium and potassium by Sir Humphry Davy in 1807 by the electrolysis of the fused hydroxides was one of the earliest applications of the electric current to the extraction of metals. This pioneering work showed little development until about the middle of the

19th century. In 1852 magnesium was isolated electrolytically by R. Bunsen, and this process subsequently received much attention at the hands of Moissan and Borchers. Two years later Bunsen and H. E. Sainte Claire Deville working independently obtained aluminium (q.v.) by the electrolysis of the fused double sodium aluminium chloride. Since that date other processes have been devised and the electrolytic processes have entirely replaced the older methods of reduction with sodium. Methods have also been discovered for the electrolytic manufacture of calcium (q.v.), which have had the effect of converting a laboratory curiosity into a product of commercial importance. Barium and strontium have also been produced by electrometallurgical methods, but the processes have only a laboratory interest at present. Lead, zinc and other metals have also been reduced in this manner.

For further information the following books, in addition to those mentioned at the end of the article ELECTROCHEMISTRY, may be consulted: Borchers, Handbuch der Elektrochemie; Electric Furnaces (Eng. trans. by H. G. Solomon, 1908); Moissan, The Electric Furnace (1904); J. Escard, Fours électriques (1905); Les Industries électrochimiques (1907). (W. G. M.)

ELECTROMETER, an instrument for measuring difference of potential, which operates by means of electrostatic force and gives the measurement either in arbitrary or in absolute units (see UNITS, PHYSICAL). In the last case the instrument is called an absolute electrometer. Lord Kelvin has classified electrometers into (1) Repulsion, (2) Attracted disk, and (3) Symmetrical electrometers (see W. Thomson, Brit. Assoc. Report, 1867, or Reprinted Papers on Electrostatics and Magnetization, p. 261).

Repulsion Electrometers.-The simplest form of repulsion electrometer is W. Henley's pith ball electrometer (Phil. Trans., 1772, 63, p. 359) in which the repulsion of a straw ending in a pith ball from a fixed stem is indicated on a graduated arc (see ELECTROSCOPE). A double pith ball repulsion electrometer was employed by T. Cavallo in 1777.

It may be pointed out that such an arrangement is not merely an arbitrary electrometer, but may become an absolute electrometer within certain rough limits. Let two spherical pith balls of radius r suspended by parallel silk threads of length / so as just to touch each and weight W, covered with gold-leaf so as to be conducting, be other. If then the balls are both charged to a potential V they will repel each other, and the threads will stand out at an angle 20, which can be observed on a protractor. Since the electrical repulsion of either ball, and this force is balanced by the restoring force due of the balls is equal to CV 4/2 sin 20 dynes, where C=r is the capacity to their weight, Wg dynes, where g is the acceleration of gravity, it is casy to show that we have

as an expression for their common potential V, provided that the balls are small and their distance sufficiently great not sensibly to with measurement of the value of I and reckoned in centimetres disturb the uniformity of electric charge upon them. Observation of and W in grammes gives us the potential difference of the balls in absolute C.G.S. or electrostatic units. The gold-leaf electroscope invented by Abraham Bennet (see ELECTROSCOPE) can in like manner, by the addition of a scale to observe the divergence of the gold-leaves, be made a repulsion electrometer.

Attracted Disk Electrometers.-A form of attracted disk

absolute electrometer was devised by A. Volta. It consisted
of a plane conducting plate forming one pan of a balance which
electrified. The attraction between the two plates was balanced
was suspended over another insulated plate which could be
by a weight put in
the opposite pan.
A similar electric
balance was subse-

quently devised by
SirW.Snow-Harris,'
one of whose instru-
ments is shown in
fig. 1. C is an in-
sulated disk over
which is suspended
another disk at-

tached to the arm

of a balance. A

weight is put in the
opposite scale pan
and a measured
charge of electricity
is given to the disk
just sufficient to
FIG. 1.-Snow-Harris's Disk Electrometer.
tipover the balance.
of the weight in the opposite pan, thus showing that the
Snow-Harris found that this charge varied as the square root

1 It is probable that an experiment of this kind had been made as far back as 1746 by Daniel Gralath, of Danzig, who has some claims to have suggested the word "electrometer in connexion with it. See Park Benjamin, The Intellectual Rise in Electricity (London, 1895),

p. 542.