صور الصفحة
النشر الإلكتروني

Muskingum, Kanawha, Little Kanawha, Big Sandy, Wabash, and Green now afford a total of about 060 m. of slack-water navigation.

See the Board of Engineers' Report a! Examination of Ohio Rim with a Vwd to obtaining Channel Depths of 6 and o ft. respectively (Washington, 1908); A. B. Hulbert, Waterways of Westward Expansion (Cleveland, 1903) and The Ohio River, a Course of Empire (New York, 1906); also K. C. Thwaites, Afloat on Ike Ulna (New York. 1900).

OHLAU. a town of Germany, in the Prussian province of Silesia, :... m. by rail S.E. of Breslau, on the left bank of the Oder. Pop. (1005) 9233. It has two Roman Catholic and two Evangelical churches, and a castle. Ohlau is the centre of a tobacco-growing district and has manufactures of tobacco and cigars, machinery, beer, shoes and bricks. It became a town in 1291 and passed to Prussia in 1742. In the i/th and.iSth centuries it was often the residence of the dukes of Brieg and of the Sobieski family.

See Sxhult.AusOUausVergaagenkeit (Ohlau, 1902).

OHLENSCHLAGER, ADAM GOTTLOB (1770-1850), Danish poet, was born in Vesterbro, a suburb of Copenhagen, on the I4th of November 1779. His father, a Schleswiger by birth, was at that time organist, and later became keeper, of the royal palace of Frederiksberg; he was a very brisk and cheerful man. The poet's mother, on the other hand, who was partly German by extraction, suffered from depressed spirits, which afterwards deepened into melancholy madness. Adam and his sister Sofia were allowed their own way throughout their childhood, and were Uught nothing, except to read and write, until their twelfth year. At the age of nine Adam began to make fluent verses. Three years later, while walking in Frederiksberg Gardens, heattracted the notice of the poet Edvard Storm, and the result of the conversation was that he received a nomination to the college called" Posterity's High School," an important institution of which Storm was the principal. Storm himself taught the class of Scandinavian mythology, and thus Ohlenschlager received his earliest bias towards the poetical religion of his ancestors. He was confirmed in 1795, and was to have been apprenticed to a tradesman in Copenhagen. To his great delight there was a hitch in the preliminaries, and he returned to his father's bouse. He now, in his eighteenth year, suddenly took up study with great zeal, but soon again abandoned his books for the stage, where a small position was offered him. In 1797 he actually made his appearance on the boards in several successive parts, but soon discovered that he possessed no real histrionic talent. The brothers Orsted, with whom he had formed an intimacy fruitful of profit to him, persuaded him to quit the stage, and in iSoo he entered the university of Copenhagen as a student. He was doomed, however, to disturbance in his studies, first from the death of his mother, next from his inveterate tendency towards poetry, and finally from the attack of the English upon Copenhagen in April 1801, which, however, inspired a dramatic sketch (April lac Second iSoi) which is the first thing of the kind by Ohlenschlager that we possess. In the summer of 1802, when Ohlenschlager had an old Scandinavian romance, as well as a volume of lyrics, in the press, the young Norse philosopher, Henrik Steffens, came back to Copenhagen after a long visit to Schelting in Germany, full of new romantic ideas. His lectures at the university, in which Goethe and Schiller vere for the first time revealed to the Danish public, created a great sensation. Steffens and Ohlenschlager met one day at Dreier's Club, and after a conversation of sixteen hours the latter went home, suppressed his two coming volumes, and wrote at a sitting his splendid poem Guldtwrncnc, in a manner totally aew to Danish literature. The result of his new enthusiasm speedily showed itself in a somewhat hasty volume of poems, published in 1803, now chiefly remembered as containing the lovely piece called Sanct-H ansaften-Spit. The next two years saw the production of several exquisite works, in particular the epic of Thors Reise til Jotunheim, the charming poem in hexameters called Langelandsrcisen, and the bewitching piece of fantasy Aladdin's Lampe (1805). At the age of twenty-six Ohlenschlager was universally recognized, even by the opponents of the romantic revival, as the leading poet of Denmark. He

now collected his Poetical Writings in two volumes. He found no difficulty in obtaining a grant for foreign travel from the government, and he left his native country for the first time, joining Steffens at Halle in August 1805. Here he wrote the first of his great historical tragedies, Hakon Jarl, which he sent off to Copenhagen, and then proceeded for the winter months to Berlin, where he associated with Humboldt, Fichte, and the leading men of the day, and met Goethe for the first time. In the spring of 1806 he went on to Weimar, where he spent several months in daily intercourse with Goethe. The autumn of the same year he spent with Tieck in Dresden, and proceeded in December to Paris. Here he resided eighteen months and wrote his three famous masterpieces, Baldur liin Code (1808), Palnalote (1809), and Aid og Valborg (1810). In July 1808 he left Paris and spent the autumn and winter in Switzerland as the guest of Madame de Stajfl-Holstein at Coppet, in the midst of her circle of wits. In the spring of 1809 Ohlenschlager went to Rome to visit Thorwaldscn, and in his house wrote his tragedy of Correggio. He hurriedly returned to Denmark in the spring of 1810, partly to take the chair of aesthetics at the university of Copenhagen, partly to marry the sister-in-law of Rahbck, to whom he had been long betrothed. His first course of lectures dealt with his Danish predecessor Ewald, the. second with Schiller. From this time forward his literary activity became very great; in 1811 he published the Oriental tale of AH og CMyndi, and in 1812 the last of his great tragedies, Slacrkoddtr. From 1814 to 1819 he, or rather his admirers, were engaged in a long and angry controversy with Baggesen, who represented the old didactic school. This contest seems to have disturbed the peace of Ohlenschlager's mind, and to have undermined his genius. His talent may be said to have culminated in the glorious cycle of verse-romances called Hclge, published in 1814. The tragedy of Hagbarlk og Signe, 1815, showed a distinct falling-off in style. In 1817 he went back to Paris, and published Hroars Saga and the tragedy of Fustbrodrene. In 1818 he was again in Copenhagen, and wrote the idyll of Den lille Hyrdedrcng and the Eddaic cycle called Nordens Cudtr. His next productions were the tragedies of Erik og Abel (1820) and Vacringcrne i Uitlagaard (1826), and the epic of Hrolf Krake (1829). It was in the last-mentioned year that, being in Sweden, Ohlcnschla'gcr was publicly crowned with laurel in front of the high altar in Lund cathedral by Bishop Esaias Tcgn£r, as the "Scandinavian King of Song." His last volumes were Tordensltjold (1833), Dronning Hargrctlic (1833), Sokralcs (1835), Ola}den Hcllige (1836), Knvd den Store (1838), Dina (1842), Erik Clipping (1843), and Kiartan og Gudrun (1847). On his seventieth birthday, i<jth November 1849, a public festival was arranged in his honour, and he was decorated by the king of Denmark under circumstances of great pomp. He died on the 2oth of January 1850, and was buried in the cemetery of Fredcriksbcrg. Immediately after his death his Recollections were published in two volumes.

With the exception of Holberg, there has been no Danish writer who has exercised so wide an influence as Ohlenschlager. His great work was to awaken in the breasts of his countrymen an enthusiasm for the poetry and religion of their ancestors, and this he performed to so complete an extent that his name remains to this day synonymous with Scandinavian romance. He supplied his countrymen with romantic tragedies at the very moment when all eyes were turned to the stage, and when the old-fashioned pieces were felt to be inadequate. His plays, partly, no doubt, in consequence of his own early familiarity with acting, fulfilled the stage requirements of the day, and were popular beyond all expectation. The earliest are the best—Ohlenschlager's dramatic masterpiece being, without doubt his first tragedy, Hakon Jarl. In his poems and plays alike his style is limpid, elevated, profuse; his flight is sustained at a high pitch without visible excitement. His fluent tenderness and romantic zest have been the secrets of his extreme popularity. Although his inspiration came from Germany, he is not much like a German poet, except when he is consciously following Goethe; his analogy is much rather to be found among the English poets, his contemporaries. His mission towards antiquity reminds us of Scott, but he is, as a poet, a better artist than Scott; he has sometimes touches of exquisite diction and of overwrought sensibility which recall Coleridge to us. In his wide ambition and profuscncss he possessed some characteristics of Southey, although his style has far more vitality. With all his faults he was a very great writer, and one of the principal pioneers of the romantic movement in Europe. (E. G.)

OHLIGS, a town of Germany, in the Prussian Rhine Province, 17 m. by rail N. of Cologne, on the railway to Elberfeld. Pop. (1905) 24,264. Its chief manufactures are cutlery and hardware, and there arc iron-foundries and flour-mills. Other industries arc brewing, dyeing, weaving and brick-making. Before 1891 it was known as Merscheid.

OHM, GEORG SIMON (1787-1854), German physicist, was born at Erlangen on the i6th of March 1787,and was educated at the university there. He became professor of mathematics in the Jesuits' college at Cologne in 1817 and in the polytechnic school of Nuremberg in 1833, and in 1852 professor of experimental physics in the university of Munich, where he died on the 7th of July 1854. His writings were numerous, but, with one important exception, not of the first order. The exception is his pamphlet published in Berlin in 1827, with the title Die galwnische Kette mathtmalisck bcarbtitct. .This work, the germs of which had appeared during the two preceding years in the journals of Schweigger and PoggcndorfiT, has exerted most important influence on the whole development of the theory and applications of current electricity, and Ohm's name has been incorporated in the terminology of electrical science. Nowadays " Ohm's Law," as it is called, in which all that is most valuable in the pamphlet is summarized, is as universally known as anything in physics. The equation for the propagation of electricity formed on Ohm's principles is identical with that of J. B. J. Fourier for the propagation of heat; and if, in Fourier's solution of any problem of heat-conduction, we change the word " temperature " to " potential" and write " electric current" instead of " flux of heat," we have the solution of a corresponding problem of electric conduction. The basis of Fourier's work was his clear conception and definition of conductivity. But this involves an assumption, undoubtedly true for small temperature-gradients, but still an assumption, viz. that, all else being the same, the flux of heat is strictly proportional to the gradient of temperature. An exactly similar assumption is made in the statement of Ohm's law, i.e. that, other things being alike, the strength of the current is at each point proportional to the gradient of potential. It happens, however, that with our modern methods it is much more easy to test the accuracy of the assumption in the cas*- of electricity than in that of heat; and it has accordingly been shown by J. Clerk Maxwell and George Chrystal that Ohm's law is true, within the limits of experimental error, even when the currents are so powerful as almost to fuse the conducting wire.

OHMMETER, an electrical instrument employed for measuring insulation-resistance or other high electrical resistances. For the purpose of measuring resistances up to a few thousand ohms, the most convenient appliance is a Wheatstonc's Bridge (7.5), but when the resistance of the conductor to be measured is several hundred thousand ohms, or if it is the resistance of a so-called insulator, such as the insulating covering of the copper wires employed for distributing electric current in houses and buildings for electric lighting, then the ohmmcter is more convenient. An ohmmcter in one form consists of two pairs of coils, one pair called the scries coil and the other called the shunt coil. These coils arc placed with their axes at right angles to one another, and at the point where the axes intersect a small pivoted needle of soft iron is placed, carrying a longer index needle moving over a scale.

Suppose it is desired to measure the insulation-resistance of a system of electric house wiring; the ohmmeter circuits are then joined iii) as shown in fie. I, where VV represents a portion of the wiring of the building ana 1 a portion of the insulating materials surrounding it. The object of the test is to discover the resistance of the insulator I, that 13, to determine how much current flows through this insulator


by leakage under a certain electromotive force or voltage which must not be less than that which will be employed in practice when the electric lights supplied through these wires are in operation. For this purpose the ohmmcter is provided with a small dynamo D, contained in a box, which produces a continuous electromotive force of from 200 to 500 volts when the handle of the instrument U steadily turned. In making the test, the whole of the copper wires belonging to any section of the wiring and the test must be con- , nectcd together at some point and then connected through the series coil of the ohmmcter with one terminal of the dynamo. The shunt coil Sn and the series coil Se arc connected together at one point, and the Fig. I.

remaining terminals of

the dynamo and shunt coil must be connected to a "good J!TM! *TM> is .generally the gas or water pipes w of the building. On setting the dynamo in operation, a current passes through the shunt coil of the ohmmcter proportional to the voltage of the dynamo, and, if there is any sensible leakage through the insulator to earth, at the same time another current passes through the scries coil proportional to the conductivity of the insulation of the wiring under the electromotive force used. The two coils, the shunt and the scries coil, then produce two magnetic fields, with their lines of force at right angles to one another. The small pivoted iron needle ns placed in their common field therefore takes up a certain position, dependent on the relative value of these fields. The tangent of the angle of deflection 9 of this needle measured from its position, when the shunt coil is disconnected, is equal to the ratio of the voltage of the dynamo to the current through the insulator. If we call thi&last resistance R, the voltage of the working dynamo V, and the current through the insulator C, then tan 0=C/V = R. Hence the deflection of the needle is proportional to the insulation resistance, and the scale can be graduated to show directly this resistance in megohms.

The Evershed and Vignotes form of the instrument is much used in testing the insulation resistance ol electric wiring in houses. In this case the dynamo and ohmmcter arc combined in one instrument. The field magnet of the dynamo has two gaps in it. In one the exciting armature is rotated, producing the working voltage of 25°. 5°° or looo volts. In the other gap are pivoted two coils wound on an iron core and connected at nearly a right angle to each other. One of these coils is in series with the armature circuit and with the insulation or high resistance to be measured. The other is a shunt across the terminals of the armature. When the armature is rotated, these two coils endeavour to place themselves in certain directions in the field so as to be perforated by the greatest magnetic flux. The exact position of the core, and, therefore, of an index needle connected with it, is dependent on the ratio of the voltage applied to the terminals of the high resistance or insulator and the current passing through it. This, however, is a measure of the insulation-resistance. Hence the instrument can be graduated to show this directly.

In the Nalder ohmmeter the electrostatic principle is employed. The instrument consists of a high-voltage continuous - current dynamo which creates a potential difference between the needle and the two quadrants of a quadrant electrometer (sec ElectroMeter). These two quadrants are interconnected by the high resistance to be measured, and, therefore, themselves differ in potential. The exact position taken up by the needle is therefore determined by the potential difference (P.D.) of the quadrants and the P.D. of the needle and each quadrant, and, therefore, by the ratios of the P.D. of the ends of the insulator and the current flowing through it, that is, by its insulation resistance.

The ohmmctcr recommends itself by its portability, but in default of the possession of an ohmmeter the insulation-resistance can be measured by means of an ordinary mirror galvanometer (sec Galvanometer) and insulated battery of suitable voltage. In this case one terminal of the battery is connected to the earth, and the other terminal is connected through the galvanometer with the copper wire, the insulation of which it is desired to test. If any sensible current flows through this insulator the galvanometer will show a deflection.

The meaning of this deflection can be Interpreted as follows; If a galvanometer has a resistance R and is shunted by a shunt of resistance S, and the shunted galvanometer is placed in scries with a large resistance R' of the order of a megohm, and if the samebattery is applied to the shunted galvanometer, then the current C passing through the galvanometer will be given by the expression

.. _§v

where V is the electromotive force of the battery. It is possible so to arrange the value of the shunt and of the high resistance R' that the same or nearly the same deflection of the galvanometer is obtained as when it is used in series with the battery and the insulation-resistance. In these circumstances the current passing through the galvanometer is known, provided that the voltage of the battery is determined by means of a potentiometer (q.v.). Hence the resistance of the insulator can be ascertained, since it is expressed in ohms by the ratio of the voltage of the battery in volts to the

current through the

C C galvanometer in

amperes. In applying this method to test the insulation of indiarubbcr-covered or of i nsulatcd copper wire, before employing it for electrical purposes, it is usual to place the coil of wire W (fig. 2) in an insulated tank of water T, which is connected to one terminal of

[ocr errors]

tbe insulated battery B, the other terminal being connected to the metallic conductor CC of the wire under test, through a galvanometer C. To prevent leakage over the surface of the insulating covering of the wire which projects above the surface of the water, it is necessary to employ a "guard wire" P, which consists of a piece of 6ne copper wire, twisted round the extremity of the insulated wire and connected to the battery. This guard wire prevents any current which leaks over the surface of the insulator from passing through the galvanometer G, and the galvanometer indication is therefore only determined by the amount of current which passes through the insulator, or by its insulation-resistance.

For farther information on the measurement of high resistance, sec J. A. Fleming, A Handbook for the Electrical Laboratory and Testing Room (2 vols., London. 1904); H. R. Kempe. A Handbook of Eltclrital Testing (London, 1900); H. L. Webb, A Practical Guide to Ac Testing ofJnsulated Wires and Cables (New York, 1002),

(J. A. F.)

OHNET, GEORGES (1848- ), French novelist and man of letters, was born in Paris on the 3rd of April 1848. After the war of 1870 he became editor of the Pays and the Constitutionnd in succession. In collaboration with the engineer and dramatist Louis Denayrouze (b. 1848) he produced the play Regina Sarpi, and in 1877 Martkt. He was an admirer of Georges Sand and bitterly opposed to the realistic modern noveL He began a series of novels, Les Balaffles dc la vie, of a simple and idealistic character, which, although attacked by the critics as unreal and commonplace, were very popular. The scries included Serge PaviKe (rSSi) which was crowned by the Academy; Le Maltrc Ac forges (1882), La Grande Marniere (1885), Volontt (1888), DfTKier amour (1891). Many of his novels have been dramatized with great success, Lc&fattre deforces, produced at the Gymnase in 1883, holding the stage for a whole year. His later publications include Le Crlpuscule (1902), Le Marckand de poisons (1903), La Conqtttranie (1005), La Dixicmc Muse (1006).

OHRDRUF, a town of Germany in the ducby of Saxe-CoburgGotha, ii m. by rail S.E. of Got ha. Pop. (1905) 6114. It bas a castle, two Evangelical churches, a technical and other schools, and manufactures of porcelain, paper, copper goods, shoes and small wares. Close by is the summer resort of Luisenthal As early as 725 there was a monastery at Ohrdruf, which received municipal rights in 1399. With six neighbouring villages it forms the county of Obergleichcn.

OIHENART, ARNAULD DE (1592-1668), Basque historian tad poet, was born at Mauleon, and studied law at Bordeaux, where he took his degree in 1612. He practised first in his native town, and after his marriage with Jeanne d'Erdoy, the heiress of a noble family of Saint-Palais, at the bar of the parlcmcnt of Navarre, He spent his leisure and his fortune in the search for documents bearing on the old Basque and Bearnese provinces; tod the fruits of bis studies in the archives of Bayonne, Toulouse,

Pau, Perigord and other cities were embodied in forty-five MS. volumes, which were sent by his son Gabriel to Colbert. Twentythree of these arc in the Bibliothcquc Nationals of Paris (Coll. Duchesne).

Oihcnart published in 1625 a Declaration kistorique de I'injuste luurtoilion et retention de la Navarre far Its Espagnols and a fragment of a Latin work on the same subject is included in Galland's Mimoirtt Pour I'kistoire de Navarre (1648). His most important work is ffotitia utriusque Vasconiat, turn Ibericae, turn Aquitanicae, qua fraeler situm regionis el alia scitu digna, Navarrae return coeterarumque: in Us imignum vetustate et dignitate familiarum . . . (Paris, 1638 and 1656), a description of Gascony and Navarre. His collection of over five hundred Basque proverbs, Atsotizac edo Rtfravac, included in a volume of his poems 0"* Ga$taroa Nevrtkizetan, printed in Paris in 1657, was supplemented by a second collection, Atsotiztn Vrrkenquina. The proverbs were edited by Francisque Michel (Paris, 1847), and the supplement by P. Hnriston (Bayonne, 1892) and by V. btempf (Bordeaux, 1894). Sec Julicn Vinson, Essai d'une bibliographie de la langue basque (Paris, 1891); J. B. E. de Jaurgain, Arnaud d'Oihenart et safamUie (Paris, 1885).

OIL CITY, a city of Vcnango county, Pennsylvania, U.S.A., on the Allegheny river, at the mouth of Oil Creek, about 55 m. S.S.E. of Erie and about 135 m. N. of Pittsburg. Pop. (1890) 10,932; (1900) 13,264, of whom 2001 were foreign-born and 184 were negroes; (1910 census) 15,657. It is served by the Pennsylvania (two lines), the Erie, and the Lake Shore & Michigan Southern railways. The city lies about loco ft. above the sea, and is divided by the river and the creek into three sections connected by bridges. The business part of tlic city is on the low ground north of the river; the residential districts are the South Side, a portion of the flats, the West Side, and Cottage Hill and Palace Hill on the North Side. Oil City is the centre and the principal market of the Pennsylvania oil region. It has extensive oil refineries and foundries and machine shops, and manufactures oil-well supplies and a few other commodities. The city's factory products were valued at $5,164,059 in 1900 and at $3,217,208 in 1905, and in the latter year foundry and machine-shop products were valued at $2,317,505, or 72% of the total. Natural gas is used for power, heat and light. Oil City was founded in 1860, incorporated as a borough in 1863 and chartered as a city in 1874. The city was partially destroyed by flood in 1865, and by flood and fire in 1866 and again in 1892; on this last occasion Oil Creek was swollen by a cloud-burst on the $th of June, and several tanks farther up the valley, which seem to have been struck by lightning, gave way and a mass of burning oil was carried by the creek to Oil City, where some sixty lives were lost and property valued at more than $1,000,000 was destroyed.

OIL ENGINE. Oil engines, like gas engines (q.v.}, are internal combustion motors in which motive power is produced by the explosion or expansion of a mixture of inflammable material and air. The inflammable fluid used, however, consists of vapour produced from oil instead of permanent gas. The thcrmodynamic operations are the same as in gas engines, and the structural and mechanical differences are due to the devices required to vaporize the oil and supply the measured proportion of vapour which is to mix with the air in the cylinders.

Light and heavy oils are used; light oils may be defined as those which are readily volatile at ordinary atmospheric temperatures, while heavy oils are those which require special heating or spraying processes in order to produce an inflammable vapour capable of forming explosive mixture to be supplied to the cylinders. Of the light oils the most important is known as petrol. It is not a definite chemical cohlpound. It is a mixture ofvarioushydrocarbonsofthcparaffinandolcfine series produced from the distillation of petroleum and paraffin oils. It consists, in fact, of the lighter fractions which distil over first in the process of purifying petroleums or paraffins,

The specific gravity of the standard petrols of commerce generally ranges between 0-700 to about 0-740; and the heat value on complete combustion per ^\ gallon burned varies from 14,24° to 14,850 British thermal units. The thermal value per gallon thus increases with the density, but the volatility diminishes. Thus, samples of petrol examined by Mr Blount of from -700 to -730 specific gravity showed that 98% of the lighter sample distilled over below 120° C. while only 88% of the heavier came over within the same temperature range. The heavier petrol is not so easily converted into vapour. The great modem development of the motor car gives the light oil engine a most important place as one of the leading sources of motive power in the world. The total petrol power now applied to cars on land and to vessels on sea amounts to at least two million H.P. The petrol engine has also enabled aeroplanes lo be used in practice.

The earliest proposal to use oil as a means to produce motive power was made by an English inventor—Street—in 1794, but the first practical petroleum engine was that of Julius Hock of Vienna, produced in 1870. This engine, like Lenoir's gas engine, operated without compression. The piston look in a charge of air and light petroleum spray which was ignited by a flame jet and produced a low-pressure explosion. Like all non-compression engines, Hock's machine was very cumbrous and gave little power. In 1873, Brayton, an English engineer, who had settled in America, produced a light oil engine working on the constant pressure system without explosion. This appears to have been the earliest compression engine to use oil fuel instead of gas.

Shortly after the introduction of the "Otto" gas engine in 1876, a motor of this type was operated by an inflammable vapour produced by passing air on its way to the cylinder through the light oil then known as gasolene. A further air supply was drawn into the cylinder to form the required explosive mixture, which was subsequently compressed and ignited in the usual way. The Spiel petroleum engine was the first Otto cycle motor introduced into practice which dispensed with an independent vaporizing apparatus. Light hydrocarbon of a specific gravity of not greater than 0.725 was injected directly into the cylinder on the suction stroke by means of a pump. In entering it formed spray mixed with the air, was vaporized, and on compression an explosion was obtained just as in the gas engine.

Until the year 1883 the different gas and oil engines constructed were of a heavy type rotating at about 150 to 250 revolutions per minute. In that year Daimler conceived the idea of constructing very small engines with light moving parts, in order to enable them to be rotated at such high speeds as 800 and 1000 revolutions per minute. At that time engineers did not consider it practicable to run engines at such speeds; it was supposed that low speed was necessary to durability and smooth running. Daimler showed this idea to be wrong by producing his first small engine in 1883. In 1886 he made his first experiment with a motor bicycle, and on the 4th of March 1887 he ran foi the first time a motor car propelled by a petrol engine. Daimler deserves great credit for realizing the possibility of producing durable and effective engines rotating at such unusually high speeds; and, further, for proving that his ideas were right in actual practice. His little engines contained nothing new in their cycles of operation, but they provided the first step in the startlingly rapid development of petrol motive power which we have seen in the last twenty years. The high speed of rotation enabled motors to be constructed giving a very large power for a very small weight.

Fig. I is a diagrammatic section of an early Daimler motor. A is the cylinder, B the piston, C the connecting rod, and D the cr.ink, which is entirely enclosed in a casing. A small fly-wheel is carried by the crank-shaft, and it serves the double purpose of a flywheel and a clutch, a is the combustion space, E the single port, which serves both for inlet of the charge and lor discharge of exhaust. W is the exhaust valve, F the charge inlet valve, which is automatic in its action, and is held closed by a spring /, G the carburettor, H the igniter tube, I the igniter tube lamp, K the charge inlet passage, L the air filter chamber, and M an adjustable air inlet cap for regulating the air inlet area. The light oil—or petrol, as it is commonly called—is supplied to the float chamber N of the vaporizer by means of the valve O. So long as the level of the petrol is high, the float n. acting by levers about it, holds the valve O closed against oil forced by air pressure along the pipe P When the level falls, however, the valve opens and more petrol is admitted. When the piston B makes its suction stroke, air passes from the atmosphere by the

passage K through the valve F, which it opens automatically. The pressure falls within the passage K, and a spun of petrol passes by the jet Gl, separate air at the same time passing by the ;•-•;,•«• K1 round the jet. The petrol breaks up into spray by impact against the walls of the passage K, and then it vaporizes and passes into the cylinder A as an inflammable mixture. When the piston B returns it compresses the charge into a, and upon compression the incandescent igniter tube H fires the charge. 11 is a short platinum tube, which is always open to the compression space It is rendered incandescent by the burner I, fed with petrol from the pipe supplying the vaporizer. The open incandescent tube is found to act well for small engines, and it docs not ignite the charge until the compression takes place, because the inflammable mixture cannot come into contact with the hot part till it is forced up the tube by the


Fig. i.

compression. The engine is started by giving the crank-shaft a smart turn round by means of a detachable handle. The exhaust is alone actuated from the valve shaft. The shaft Q is operated -by pinion and a spur-wheel Q* at half the rate of the crank-shaft. The governing is accomplished by cutting out explosions as with the gas engine, but the governor operates by preventing the exhaust valve from opening, so that no charge is discharged from the cylinder, and therefore no charge is drawn in. The cam R operates the exhaust valve, the levers shown are so controlled by the governor (not shown) that the knife edge S is pressed out when speed is too hi, ... and cannot engage the recess T until it falls. The engine has a water jacket V, through which water is circulated. Cooling devices are used to economize water.

Bcnz of Mannheim followed close on the work of Daimler, and In France Panhard and Levassor, Peugeot, De Dion, Dclahaye and Renault all contributed to the development of the petrol engine, while Napier, Lanchcster, Royce and Austin were the most prominent among the many English designers.

The modern petrol engine differs in many respects from the Daimler engine just described both as to general design, method of carburetting, igniting and controlling the power and speed. The carburettor now used is usually of the float and jet type shown in fig. i, but alterations have been made to allow of the production of uniform mixture in the cylinder under widely varying conditions of speed and load. The original form of carburettor was not well adapted to allow of great change of volume per suction stroke. Tube ignition has been abandoned, and the electric system is now supreme. The favourite type at present is that of the high-tension magneto. Valves are now all mechanically operated; the automatic inlet valve has practically disappeared. Engines are no longer controlled by cutting out impulses; the governing is effected by throttling the charge, that is by diminishing the volume of charge admitted to the cylinder at one stroke. Broadly, throttling by reducing charge weight reduces pressure of compression and so allows the power of the explosion to be graduated within wide limits while maintaining continuity of impulses. The object of the throttle control is to keep up continuous impulses for each cycle of operation, while graduating the power produced by each impulse so as to meet the conditions of the load.

Originany three types of carburettor were employed for dealing with light oil; first, the surface carburettor; second, the wick carburettor; and third, the jet carburettor. The surface carburettor has entirely disappeared. In it air was passed over a surface of light oil or bubbled through it; the air carried off a vapour to form explosive mixture. It was found, however, that the oil remaining in the carburettor gradually became heavier and heavier, so that ultimately no proper vaporization took place. This was due to the fractional evaporation of the oQ which tended to carry away the light vapours, leaving in the vessel the oil, which produced heavy vapours. To avoid this fractionation the wick carburettor was introduced and here a complete portion of oil was evaporated at each operation so that Do concentration of heavy oil was possible. The wick carburettor is still used in some cars, but the jet carburettor is practically universal. It has the advantage of discharging separate portions of oil into the air entering the engine, each portion being carried away and evaporated with all its fractions to produce the charge in the cylinder.

The modern jet carburettor appears to have originated with Butler, an English engineer, but it was first extensively used in the modification produced by May bach as shown in fig. i.

A diagrammatic section of a carburettor of theMaybach type is shown in a larger scale in fig. 2.

Petrol is admitted to the chamber A by the valve B which is controlled by the float C acting through the levers D, so that the valve


Fig. 2.

B is closed when the float reaches a determined level and opened when it falls below it. The petrol flows into a jet E and stands at an approximately constant level within it. When the engine piston nukes its suction stroke, the air enters from the atmosphere at F and passes to the cylinder through G. The pressure around the jct.E thus falls, and the pressure of the atmosphere in the chamber A forces the petrol through E as a jet during the greater part of the •action stroke. An inflammable mixture is thus formed, which enter* the cylinder by way of G. The area for the passage of air around the petrol jet E is constricted to a sufficient extent to produce the pressure fall necessary to propel the petrol through the jet E, and the area of the discharge aperture of the petrol jet E is pro

portioned to give the desired volume of petrol to form the proper mixture with air. The device in this form works quite well when the range of speed required from the engine is not great; that is, within limits, the volume of petrol thrown by the jet is fairly proportional to the air passing the )ct. When, however the speed range is great, such as in modern motors, which may vary from 300 to ijjoo revolutions fjer minute under light and heavy loads, then it becomes impossible to secure proportionality sufficiently accurate for regular ignition. This implies not only a change of engine speed but a change of volume entering the cylinder at each stroke as determined by the position of the throttle. This introduces further complications. Throttle control implies a change of total charge volume per stroke, which change may occur either at a low or at a high speed. To meet this change the petrol jet should respond in such manner as to give a constant proportionality of petrol weight to air weight throughout all the variations- -otherwise sometimes petrol will be present in excess with no oxygen to burn it, and at other times the mixture may be so dilute as to miss firing altogether. To meet these varying conditions many carburettors have been produced which seek by various devices to maintain uniformity of quality of mixture by the automatic change of throttle around the jet. Fig. 3 shows in diagrammatic section one of the simplest of these contrivances, known as the Krebs carburettor. The petrol enters from the float « chamber to the jet "* E; and, while the engine is running slowly, the whole suppry of air enters by way of the passage F, mixes with the petrol and reaches the cylinders by way of the pipe G. The volume of charge entering the cylinder per stroke is controlled by the piston throttle valve H. operated by the rod I; and so long as the charge volume required remains small, air from the


Fie. 3.

atmosphere enters only by F. When speed rises, however, and the throttle is sufficiently opened, the pressure within the apparatus falls and affects a spring-pressed diaphragm K, which actuates a piston valve controlling the air passages L, so that this valve opens to the atmosphere more and more with increasing pressure reduction, and additional air thus flows into the carburettor and mixes with the air and petrol entering through F. By this device the required proportion of air to petrol is maintained through a comparatively large volume range. This change of air admission is rendered necessary because of the difference between the laws of air and petrol flow. In order to give a sufficient weight of petrol at low speeds when the pressure drop is small, it is necessary to provide a somewhat large area of petrol jet. When suction increases owing to high speed, this large area discharges too much petrol, and so necessitates a device.! such as that described, which admits more air.

A still simpler device is adopted in many carburettors—that of an additional air inlet valve, kept closed until wanted by a spring. Fig. 4 shows a diagrammatic section as used in the Vauxhall carburettor. Here the petrol jet and primary and secondary air passages are lettered as before.

The same effect is produced by devices which alter the area of the petrol jet or increase or diminish the number of petrol jets exposed as required. Although engine designers have succeeded in proportioning mixture through a considerable range of speed and charge demand, so as to obtain effective power explosions under all these conditions, yet much remains to be done to secure constancy of mixture at all speeds. Notwithstanding much which has been said as to varying mixture, there is only one mixture of air and petrol which gives the best results—that m which there is some excess of oxygen, more than sufficient to burn all the hydrogen and carbon present. It is necessary to secure this mixture under all conditions, not only to obtain economy^ in running but also to maintain purity of exhaust gases. Most engines at certain speeds discharge considerable quantities of carbonic oxide into the atmosphere with their exhaust gases, and some discharge so much as to give rise to danger in a closed garage. Carbonic oxide is an extremely poisonous gas which should be reduced to the minimum in the interests of the health of our large cities. The enormous increase of motor traffic makes it important to render the exhaust gases as pure and innocuous as possible. Tests were made by the Royal Automobile Club some years ago which clearly showed that carbonic oxide should be kept down to 2% and under when carburettors were properly adjusted. Subsequent experiments have been made by Hopkinson, Clerk and

« السابقةمتابعة »