efficient. Sometimes the secondary windings are joined to heavy short-circuiting rings at both ends, resulting in the squirrel-cage type of motor; and in other cases the secondary windings are taken out through collector rings, if the secondary be the rotating element, and starting resistances are inserted in series to lessen the reaction due to excessive starting current and thus improve the starting torque. When up to speed these resistances are cut out and the terminals short-circuited as in the squirrel-cage type.

The Asynchronous Generator. If the motor be driven by power from an outside source up to true synchronism, no current will flow in the secondary, and the primary current or field current will be wholly made up of the wattless exciting current, just as in a transformer at no load. The slip, or amount by which the motor speed at full load differs from synchronous speed, may be as little as 2 to 21⁄2 per cent of the speed of synchronism in large, motors, and in small motors may be 5 per cent or more. If the motor above mentioned be forced above synchronism the motor becomes a generator, provided the connection to the mains is left closed, and when a negative slip of the same amount as full load slip as a motor is reached, the generator will be giving out its full output at the same frequency as the exciting circuit. The possibilities of this system are interesting.

The Synchronous Motor.-The synchronous motor is merely an alternating current generator of special design. Both motors and alternators have a direct current field and an alternating current armature. The operation of a synchronous motor, when once brought up to speed and thrown into circuit, is the same as that of an alternator in parallel with one or more alternators. When the back pressure of the motor is equal and directly opposed to that of the line no current can flow. The friction, however, causes the revolving element to lag slightly behind the line pressure, and a current is driven through the motor by the generator. This current increases directly with the lag behind the central-phase position caused by increased load. A good synchronous motor, while always revolving at the same polar speed as the alternator supplying the line current, will carry a load of five or six times full load before it breaks out of step, and becomes practically a short circuit on the system. The current which passes through such a motor on short circuit, while held down by the inductance of the windings, is yet sufficient to rapidly damage the insulation if not cut off. The great advantage of the synchronous over the induction type of motor is that the power factor can be raised or lowered at will. By raising the field strength of a synchronous motor the current taken by the motor may be made leading and hence help keep up the line voltage on a heavy inductive load. This is of the greatest importance in practice. It is good practice to set the field strength for a good power factor at full load. At light loads the motor is assisting the generator to maintain the required pressure. Another advantage of the synchronous motor is that it can easily be built for very high voltage, especially the revolving field type a 12,000 volt motor is not at all unusual

practice thus the use of transformers may be dispensed with.

The Rotary Converter. The rotary converter is a specially designed direct-current generator provided, at proper points in the winding, with taps to collector rings, from which, if the machine is run as a motor from the direct-current side, an alternating current may be taken. Usually the alternating current is taken from the secondaries of suitable transformers and supplied to the rings, driving the rotary as a synchronous motor, the direct current being taken from the brushes on the commutator. As the reaction of the incoming alternating current about balances that caused by the outgoing direct current, the armature reaction of such a machine is very small and the brushes can be always kept in one fixed position. If the taps from the armature are taken off at points differing 180 degrees from each other, electrically, we have a single-phase rotary. If connections are made 90 degrees apart we have a two-phase rotary, using four collector rings. Taking 120 degrees around the armature for our taps we have a three-phase rotary, using three collector rings. By adding to the number of taps and therefore to the number of rings we may have a six-phase rotary. The output of a rotary is greater than its output as a direct-current generator, chiefly on account of the absence of armature reaction and because at certain positions the current flows straight from collector ring to commutator and thus avoids the loss due to heating. The rotary converter, with its step-down transformer, is the most efficient means we now have of transforming the high tension polyphase currents of our large central stations to direct current for the Edison system, and for railway purposes. This piece of apparatus is wound either shunt or compound, in accordance with the use for which it is intended. As in the case of the synchronous motor, the rotary is a valuable help to the central station by running at a very high power factor. By overexciting the fields the current taken by the rotary becomes leading and helps to hold up the voltage of the central station in case of a heavy load of induction motors by means of the armature reaction of the generators. Owing to very high commutator speeds at the higher frequencies, rotaries are not much used on frequencies above 60 degrees. At this frequency they operate satisfactorily. At lower frequencies, however, rotaries are at their best, and will stand enormous overloads, sudden changes in load and other disturbances, with perfect satisfaction. The voltage of the direct current end of a rotary is that of the peak of the sine wave of the alternating pressure, and thus a voltmeter E across the collector rings would read

where

the alternating current side, it is not good practice, excepting in certain special cases. Generally they are started up exactly like a shunt motor, synchronized, and then thrown upon the alternating current line. When a rotary is started up from the alternating current side, on closing the field switch it is impossible to tell what the polarity will be. Rotaries operate in parallel with perfect satisfaction, as a rule, on both the alternating current and the direct sides. The storage battery is always used in a large rotary installation to ensure against any possible contingencies. On compound rotaries the equalizer must be used, just as in the case of direct-current compound generators. See ELECTRIC MACHINE.

A. R. CHEYNEY,

Station Superintendent Philadelphia Electrical Company.

ELECTRIC ANNEALING, a process of annealing by the heat generated by the passage of an electric current through the body to be annealed, or in which heat generated by an electric current is used in place of ordinary heat. The heat developed in a conductor by an electric current is equal to the product of the square of the current by the resistance of the conductor C'R. An interesting experiment showing the fusing power of the electric current is made in the following manner: Provide a glass or porcelain vessel containing a mixture of sulphuric acid and water. Introduce a lead plate electrode suitably connected with the positive pole of a continuous-current generator. Connect by a flexible wire the negative pole to a stout pair of metal pliers. When, by means of the pliers, a metal rod is immersed in the acid solution, the liquid is seen to boil near the rod, which is brought to a dazzling whiteness in a few moments, and presently begins to melt. The heating is so quickly produced that the liquid or the body of the rod has not time to become hot. In a short time a temperature of 7,000° F. may be developed, and with a very strong current a temperature of 14,000° F. has been produced.

ELECTRIC ANNUNCIATOR, a form of annunciator used in private houses, offices and hotels. They are used to call messengers, to announce an alarm and to indicate the source of the alarm in connection with electric burglar-alarm apparatus, and for numerous other purposes. In some forms of annunciator the source of the call is indicated by the movement of a needle on the face of the case opposite a given number; in others a shutter drops, disclosing the number or name of the room or office. See ELECTRIC SIGNALING.

ELECTRIC ARC, the intensely hot bright flame that forms where an electric current jumps a gap between two electrodes: called also voltaic arc. It tends to curve in an arc following the lines of force, hence the name. This is the source of light in an arc lamp. (See ELECTRIC LIGHTING). The lamp carbons, which constitute the electrodes, are usually enclosed to retain the carbon vapors. The carbons have to be set at a slight distance apart, and as they burn down require to be moved so as to maintain the correct distance for permitting a good arc. In burning, the carbons create carbon vapor, which is a conductor, and the

current flows along this vapor conductor in an arc of visible flame. If the electrodes are impregnated with metallic salts and a powerful current passed across the gap, the so-called "flaming arc" results, of varying color, according to the salts employed. Vacuum tube lighting is also accomplished in a somewhat similar principle. See ELECTRIC LIGHTING and ELECTRIC FURNACES-Arc Furnace.

ELECTRIC AURA, a current or breeze of electrified air employed as a mild stimulant in electrifying delicate parts, as the eye.

ELECTRIC AUTOMATIC FIREALARM. See ELECTRIC SIGNALING.

ELECTRIC BALANCE, an instrument for measuring the attractive or repulsive forces of electrified bodies; a form of electronometer, consisting of a graduated arc supported by a projecting plate of brass which is attached to a perpendicular column. A wheel, the axis of which is supported on anti-friction rollers, and is concentric with that of the graduated arc, carries an index. Over this wheel, in a groove on its circumference, passes a line, to one end of which is attached a light ball of gilt wood, and to the other a float, which consists of a glass tube about one-fifth of an inch in diameter, terminating in a small bulb, so weighted that the index may point to the centre of the graduated arc. The difference between the weights of the float when in and out of water is known, and the diameter of the wheel carrying the index is such that a certain amount of rise or fall of the float causes the index to move over a certain number of graduations on the arc. See ELECTROMETER.

ELECTRIC BATH, a solution in a vat or tank containing a salt of some metal, as copper, silver, gold, etc., and connected with the negative pole of a battery or dynamo. A current being passed through, the metallic salt is deposited on the negative pole, or more strictly speaking, on the object to be plated, connected with the pole. The process is called electrodeposition. (See ELECTRO-PLATING; ELECTROCHEMISTRY). The name electric bath is also sometimes applied to a hot water bath through which a weak electric current is sent, for the treatment of patients. Its therapeutic value is questioned by many.

ELECTRIC BATTERIES. The electric battery is a device by which electric energy is derived directly from chemical action. There are two types of electric batteries: (1) primary, and (2) secondary. Secondary batteries are usually called "storage batteries" or "accumulators" and are discussed in another article.

The battery unit is called a "cell." The simple primary cell, or Voltaic cell, as it is often called, from its inventor, Volta, consists of two different metals immersed in a weak water solution of some acid which will act with unequal intensity upon the two metals. The greater this inequality of action, the larger will be the difference of electric potential between the two metals; and, as the current excited in the cell depends upon this difference of potential, the greater will be the strength of the current. The two metals form the electrodes of the battery cell, and the solution is the electrolyte.

The chemistry of the primary cell is thus

explained: When a piece of metallic zinc is placed in sulphuric acid diluted with water, a chemical union takes place, the acid and the zinc combining to form the new substance, zinc sulphate. In order that this may be brought about a certain amount of oxygen must be obtained to complete the combination, and as neither the acid nor the zinc can supply it, it is taken from the water lying next to the zinc, and which is thus decomposed-the hydrogen formerly in combination with the oxygen being set free in little bubbles which cling to the zinc. These bubbles eventually cover the zinc and slow down the formation of zinc sulphate until it nearly ceases. If now a

strip of copper be placed in the same vessel, but not in contact with the zinc, the conditions remain as they were; but if the ends of the pieces of zinc and copper above the level of the water are leaned together so as to touch above the water, the chemical action is vigorously renewed, but the hydrogen bubbles now appear on the copper. The action of the acid upon the zinc so reduces its electric potential that when contact is made with the copper an electric current immediately moves to restore the electrical equilibrium. This being restored, the chemical action - the formation of zinc sulphate is free to go on, and thus the cycle continues until the zinc has been entirely transformed into sulphate. And all the time the action is proceeding the electric current is continually flowing to preserve the equilibrium. If, instead of tipping the two metals in the cell until they touch, a metallic conductor is placed so that one end touches the zinc and the other the copper, the current will traverse the whole length of the conductor; and this conductor may be cut and a machine inserted, so that the passing current may be made to expend part of its energy in work. The current is observed to flow from the zinc anode to the copper cathode within the battery. Outside of the battery the current flows through the conductor from the copper toward the zinc. From this external movement the copper has received the title of the positive pole and the zinc of the negative pole.

The force or pressure which causes the current to flow is called the electromotive force (commonly abbreviated to E.M.F.). It should be understood that the words "current" and "flow" are not used in the same sense as with a liquid like water. A better understanding is obtained from the illustration of a steam boiler carrying a high pressure of steam. When a valve into an empty pipe is opened, the pressure of steam in the boiler is transmitted to the further end of the pipe. In this case the steam fills the pipe carrying the pressure with it. In the case of the electric current passing along a conductor only the pressure (E.M.F.) is transmitted, there being no flow of any known material substance.

The electromotive force of a cell is dependent to a large degree upon the kind of acid used to dissolve the zinc. When the two plates are immersed in other acids than sulphuric, a considerable variation is found in the difference of electric potential set up in the cell, and it is to be borne in mind that it is upon this difference that the strength of the current depends. The electric energy produced or released by a cell depends on the number of pounds of zinc

and acid consumed in the formation of zinc sulphate. The zinc is the battery fuel and is oxidized just as coal is oxidized in a furnace. The sulphuric acid does not dissolve the zinc itself, but dissolves the oxide as fast as it forms, thus making the action of the cell continuous.

If a simple cell is put in circuit with a galvanometer, it is observed that the current gradually diminishes in strength, due to the film of hydrogen bubbles which adhere to the copper. This condition is called "polarization.” If the bubbles are brushed away, the current resumes (nearly) its former strength. It becomes necessary then to establish some mechanical means of removing the hydrogen or to use some chemical substance in the cell which will combine with it and so remove it as fast as it forms. Mercuric chloride is sometimes used for this purpose. In the bichromate cell the oxygen of the bichromate seizes upon the hydrogen and combines with it to form water. In the Leclanché cell the depolarizer is manganese dioxide. Another method of avoiding polarization is a cell construction which admits of using two separate liquids, the metal on which the hydrogen collects being placed in a solution of some chemical which combines with the hydrogen as it forms.

Another phenomenon which affects the strength of current passing through a cell and thence through the conductor which connects the two dry poles of the battery is what is called "resistance." This is of two kinds or divisions internal and external. The former is that within the cell itself the metals and the liquids; the latter in the outside conductor. If this conductor is of some substance which has a low degree of electric conductivity, like lead; or even if of high conductivity, like copper, but is very long, or of very small circumference, or both, the electric current will move along it very sluggishly, as if being held back by some obstacle-a resistance. This has the effect of slowing down the chemical action in the cell, and the result is what is termed a "weak current." With a short and comparatively large conductor of a high degree of conductivity the external resistance is reduced to a minimum. The internal resistance of a cell is increased by polarization, as already mentioned, and this is remedied by using a depolarizer. The internal resistance may also be further reduced by giving the metallic components large areas and placing them quite close together, making the travel of the current through the electrolyte as short as possible.

Primary electric batteries are classified as wet batteries and dry batteries. In the first group liquids are used as electrolytes; in the second, chemicals which retain moisture for a long time take the place of electrolytes.

WET BATTERIES.

Wet batteries are divided into one-liquid batteries and two-liquid batteries. The former are those which contain one homogeneous electrolyte; the latter have two distinct electrolytes, and the cell is usually divided into two parts by a porous cup which contains one of the metallic electrodes and one of the electrolytes.

One-Liquid Batteries.- Among the principal one-liquid batteries now in use for

economic purposes and in general laboratory work are the following:

Smee. A cell consisting of a platinum plate hung between two zinc plates in an electrolyte of dilute sulphuric acid. The platinum plate is roughened by an electro-deposit of platinum, forming a surface to which hydrogen bubbles will not adhere. The platinum is often substituted by silver, which, however, is roughened by depositing a skin of platinum. A variation of this cell has a grid of carbon rods instead of the platinum plate, the surface of the rods being made hydrogen-proof by carbonizing on them jackets of velveteen.

Bichromate, consisting of a zinc plate suspended between two carbon plates, which are gripped together at the top above the jar. The electrolyte used is a mixture of separately prepared solutions of sulphuric acid and of potassium bichromate. With this cell the zinc plates have to be raised out of the electrolyte when the battery is not in use, to prevent continuous chemical action, and therefore waste of

energy.

The

Leclanché has a solution of sal-ammoniac (ammonium chloride) as the electrolyte, and into this dips a zinc bar in one corner of the square glass jar. The other pole is a bar of carbon within a porous jar, the space within being closely packed with a mixture of manganese dioxide and powder coke. whole is immersed in the electrolyte. The oxygen escaping from the dioxide prevents polarization of the cell, and the porous jar prevents the oxygen from reaching the zinc, while opposing no barrier to the passage of the electric current. This cell is useful only for intermittent work, such as ringing bells and buzzers.

Harrison.-The negative (internal) pole or cathode is a rod of hard lead around which is compressed a jacket of lead peroxide. The other pole or anode is of zinc, cast in the form of a very thick tumbler which is supported by an amalgamated copper rod running down through it and riveted in the centre of the bottom. Around this rod the tumbler is partly filled by pouring in melted zinc amalgam. The electrolyte is dilute, sulphuric acid, or a solution of potassium bisulphate, or of sodium bisulphate. This is a very powerful battery,

Caustic Alkali or Copper Oxide Cells.This type of cell was introduced in 1881 by Lalande and Chaperon, France. Their cell consisted of a glass jar, in the bottom of which the oxide of copper was contained in an iron cup; the zinc plate was supported in the solution of caustic potash by a wire, from the cover of the jar. To prevent the carbonic acid gas of the air from combining with the caustic potash, the solution was covered with a layer of petroleum oil. This cell has undergone many modifications at the hands of Edison, Gordon and others.

Edison Primary Battery-An oxide of copper battery. The elements employed in it are zinc and black oxide of copper. The solution is of high grade caustic soda, in the proportion of 25 parts of caustic soda to 100 parts of water. The initial electromotive force of these cells is 98 volt; on closed circuit, 0.7 volt. Their internal resistance varies with the size of the plates from 09 ohm to .02 ohm. The capacity of these cells, as commercially

constructed, ranges from 50 to 600 amperehours. The oxide of copper cell has the advantage that its internal resistance falls with use, inasmuch as the continued reduction of metallic oxide from the oxide of copper increases the conductivity of the plate; in practice, however, a film of metallic copper is deposited in advance on the copper oxide plate to ensure a low resistance at the start.

The containing vessel of the Edison cell is a porcelain jar having a porcelain cover, through which the connecting wires or rods of the plates enter the cell. The copper oxide plate is obtained by roasting copper turnings, which are then ground to a fine powder and mixed with 5 to 10 per cent of magnesium chloride. The oxide is then molded into plates, which are held in a copper frame in the cell, as at cc, Fig. 1; this frame being attached to the cover of the cell and forming one of the terminals, zz are the zinc plates, one on each side of the copper oxide plates. Batteries of the oxide of copper

FIG. 1.- Edison Oxide of Copper Battery. type are extensively employed in connection with spark coils for gas-engine work, and for numerous other purposes requiring continuous current, as these are eminently closed circuit batteries. They can also be used as open circuit batteries.

Gordon.-A copper oxide cell used extensively for working fire, police and railway signals, and of economical use anywhere. Although designed for closed circuit work it does well also on open circuits. The outer jar is of glass, porcelain or enameled ware, with a cover of the same materials, or of tin, or compressed fibre. A perforated cylinder of tinplate is suspended in the centre of the cell by an iron rod. This cylinder is filled with black oxide of copper. On the outer circumference of the cylinder at equal distances are attached three L-shaped lugs of porcelain which support a heavy zinc ring, and at the same time insulate it from the tin cylinder. The electrolyte is a solution of caustic soda, and the surface of the cell is covered with a layer of heavy paraffin oil, which prevents the creeping over

of the caustic. As commonly used these cells give six months' service before renewal is necessary.

Two-Liquid Batteries.- Although the oneliquid cells have proved to be adequately nonpolarizing, several forms of two-liquid cells retain a hold on the market, and for some of them there is a large demand.

Daniell, an annular vessel of copper in the bottom of a jar is piled with crystals of copper sulphate, and within it stands a jar of porous earthenware in which is suspended a zinc bar. The electrolyte in the porous jar is dilute sulphuric acid, or, sometimes, zinc sulphate. The electrolyte in the outer jar is a saturated solution of copper sulphate. This is a closedcircuit battery which has been used extensively for telegraph work.

Gravity, a cell with the same components as the Daniell cell, but without the porous jar. The copper element is a spider-like form of sheet copper spreading its legs over the bottom of the jar. This copper form is completely covered with crystals of copper sulphate, or sometimes there is a perforated copper disc laid upon the spider and the copper sulphate is piled upon the disc. In the upper part of the jar is hung a thick seven-toed crowfoot of zinc-from which this form of cell is often called the crowfoot battery. The electrolyte is of dilute sulphuric acid. A concentrated solution of copper sulphate will soon occupy the lower part of the jar, and above it will float the lighter zinc sulphate solution-with, however, some little diffusion where the two solutions meet.

Minotto, a cell in all respects like the Gravity cell, but with a flat mat of cloth stuffed with sand or sawdust fitted snugly above the copper sulphate to keep the two solutions quite separate. This battery is much used for railway signal work, in spite of the fact that its internal resistance is very high-from four to six times that of the Gravity cell.

Fuller, the approved cell for telephone work consisting of a carbon cathode hung in a depolarizing liquid, a combination of dilute sulphuric acid and a solution of potassium bichromate. A porous inner cup or jar has suspended in it the zinc anode and a little mercury is placed in the bottom. The electrolyte in the porous cup is usually pure water; occasionally a very little sulphuric acid is added.

Grove, a cell consisting of a hollow cylinder of zinc, within which is a porous jar containing a strip of platinum. The electrolyte in the porous cup is strong nitric acid, and in the outer jar is dilute sulphuric acid. This cell is used chiefly in laboratory work.

Bunsen, a cell very similar to the Grove cell except that it has a bar of carbon in the porous jar instead of the strip of platinum.

DRY BATTERIES.

The ordinary commercial dry cell is virtually a Leclanché cell in which the electrolyte is in the form of paste instead of a liquid. It is, therefore, not accurately a dry cell but a moist cell. It is made inside of a cup or cylinder of sheet zinc which forms the anode. This zinc cylinder is lined generally with absorbent pulpboard or layers of blotting paper which are saturated with the electrolyte, a con

VOL. 10-7

centrated solution of sal-ammoniac. Through the centre of the cell runs a carbon bar constituting the cathode, and around this is packed the depolarizing paste. The composition of this paste is a trade secret, each manufacturer having his own formula. It is pretty well understood that the absolutely essential ingredients and their usual proportions are as follows: manganese peroxide, 100 parts; powdered coke, 80 parts; vitrified graphite, 20 parts; sal-ammoniac, 20 parts; zinc chloride (30° Baumé), 7 parts. Other ingredients which are known sometimes to enter the composition are glucose, dextrine, common salt, lime, arsenic, mercury bichloride, hydrochloric acid and plaster of Paris.

After the paste is firmly packed in nearly to the top, the lining is folded down upon it, a thin layer of sawdust is laid in, a snug collar of corrugated pulpboard is fitted, a layer of sand is spread on; and upon this is melted in the asphalt cover or seal. Some makers of dry. cells place next to the zinc, instead of the pulpboard lining, a prepared paste of flour, dextrine and gum tragacanth, with possibly other ingredients.

A strictly dry cell is made in the same way of dry materials without moisture. This cell is inert until it is wet, and provision is made for the wetting by making the carbon bar hollow and perforating its sides. The end is closed with a rubber cork. When the battery is wanted for use, the cork is removed and water poured in. These absolutely dry batteries are made particularly for shipping long distances across the ocean, as in the Egyptian and African trade. (See ELECTRIC STORAGE BATTERIES). Consult Cooper, W. G., Primary Batteries: Their Theory, Construction and Use' (London 1916); Schneider, N. H., Modern Primary Batteries' (London 1905).

ELECTRIC BELL, any bell made to ring by the making and breaking of an electric circuit. Common forms are here illustrated. A familiar design has two electro-magnets, parallel and in series, having at their extremity a vibrating armature in close proximity pivoted between them; fixed to this armature is a clapЗІЯТОЯ ІН

НА ЭТЯ PAO

Magneto Bell.

Door open.

Electric Door Bell.

per vibrating between two gongs. The current passes through the fields, magnetizing the cores, and in generating an alternating current vibrates the armature and rings the bell. A battery bel! employs a small cell battery for