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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 profuseness 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 are iron-foundries and flour-mills. Other industries are brewing, dyeing, weaving and brick-making. Before 1891 it was known as Merscheid.

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 ohmmeter 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 is

W

Sh

Se

FIG. 1.

W

W

steadily turned. In
making the test, the
whole of the copper
section of the wiring and
wires belonging to any
the test must be con-
nected together at some
point and then con-
nected through the series
coil of the ohmmeter
with one terminal of the
dynamo. The shunt coil
Sh and the series coil Se
are connected together
at one point, and the
remaining terminals of
the dynamo and shunt coil must be connected
"
to a good
earth," which 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 ohmmeter 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 series coil proportional to the conductivity of the insulation of
the wiring under the electromotive force used. The two coils, the
shunt and the series coil, then produce two magnetic fields, with
iron needle ns placed in their common field therefore takes up a
their lines of force at right angles to one another. The small pivoted
certain position, dependent on the relative value of these fields.
The tangent of the angle of deflection of this needle measured from
of the voltage of the dynamo to the current through the insulator. If
its position, when the shunt coil is disconnected, is equal to the ratio
we call this last resistance R, the voltage of the working dynamo V,
and the current through the insulator C, then tan =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.

OHM, GEORG SIMON (1787-1854), German physicist, was born at Erlangen on the 16th 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 galvanische Kette mathematisch bearbeitet. This work, the germs of which had appeared during the two preceding years in the journals of Schweigger and Poggendorff, 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 The Evershed and Vignoles form of the instrument is much used current " instead of "flux of heat," we have the solution of in testing the insulation resistance of electric wiring in houses. a corresponding problem of electric conduction. The basis In this case the dynamo and ohmmeter are combined in one instruof Fourier's work was his clear conception and definition of ment. The field magnet of the dynamo has two gaps in it. In one conductivity. But this involves an assumption, undoubtedly 250, 500 or 1000 volts. In the other gap are pivoted two coils the exciting armature is rotated, producing the working voltage of true for small temperature-gradients, but still an assumption, wound on an iron core and connected at nearly a right angle to viz. that, all else being the same, the flux of heat is strictly each other. One of these coils is in series with the armature circuit proportional to the gradient of temperature. An exactly similar and with the insulation or high resistance to be measured. The other is a shunt across the terminals of the armature. When the armature assumption is made in the statement of Ohm's law, i.e. that, is rotated, these two coils endeavour to place themselves in certain other things being alike, the strength of the current is at each directions in the field so as to be perforated by the greatest magnetic point proportional to the gradient of potential. It happens, how-flux. The exact position of the core, and, therefore, of an index ever, that with our modern methods it is much more easy to test needle connected with it, is dependent on the ratio of the voltage the accuracy of the assumption in the case of electricity than applied to the terminals of the high resistance or insulator and the current passing through it. This, however, is a measure of the in that of heat; and it has accordingly been shown by J. Clerk insulation-resistance. Hence the instrument can be graduated to Maxwell and George Chrystal that Ohm's law is true, within the show this directly. limits of experimental error, even when the currents are so powerful as almost to fuse the conducting wire.

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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 Wheatstone's Bridge (q.), 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 ohmmeter is more convenient. An ohmmeter in one form consists of two pairs of coils, one pair called the series coil and the other called the shunt coil. These coils are 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 up as shown in fig. 1, where W represents a portion of the wiring of the building and I a portion of the insulating materials surrounding it. The object of the test is to discover the resistance of the insulator I, that is, to determine how much current flows through this insulator

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 (see 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 of the needle and each quadrant, and, therefore, by the ratios of the by the potential difference (P.D.) of the quadrants and the P.D. P.D. of the ends of the insulator and the current flowing through it, that is, by its insulation resistance.

The ohmmeter 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 (see 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 series with a large resistance R' of the order of a megohm, and if the same

battery is applied to the shunted galvanometer, then the current C | Pau, Perigord and other cities were embodied in forty-five MS. passing through the galvanometer will be given by the expression volumes, which were sent by his son Gabriel to Colbert. Twentythree of these are in the Bibliothèque Nationale of Paris (Coll. Duchesne).

SV

C=R'(R+S)+RS'

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 galvanometer in amperes. In applying this method to test the insulation of indiarubber-covered Gor of insulated

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W

C

P

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FIG. 2.

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 the insulated battery B, the other terminal being connected to the metallic conductor CC of the wire under test, through a galvanometer G. 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 fine 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 further information on the measurement of high resistance, see J. A. Fleming, A Handbook for the Electrical Laboratory and Testing Room (2 vols., London, 1904); H. R. Kempe, A Handbook of Electrical Testing (London, 1900); H. L. Webb, A Practical Guide to the Testing of Insulated Wires and Cables (New York, 1902). (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 Constitutionnel in succession. In collaboration with the engineer and dramatist Louis Denayrouze (b. 1848) he produced the play Regina Sarpi, and in 1877 Marthe. He was an admirer of Georges Sand and bitterly opposed to the realistic modern novel. He began a series of novels, Les Batailles de la vie, of a simple and idealistic character, which, although attacked by the critics as unreal and commonplace, were very popular. The series included Serge Panine (1881) which was crowned by the Academy; Le Maître de forges (1882), La Grande Marnière (1885), Volonté (1888), Dernier amour (1891). Many of his novels have been dramatized with great success, Le Mattre de forges, produced at the Gymnase in 1833, holding the stage for a whole year. His later publications indude Le Crépuscule (1902), Le Marchand de poisons (1903), La Conquérante (1905), La Dixième Muse (1906).

OHRDRUF, a town of Germany in the duchy of Saxe-CoburgGotha, 11 m. by rail S.E. of Gotha. Pop. (1905) 6114. It has 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 Obergleichen.

OIHENART, ARNAULD DE (1592-1668), Basque historian and poet, was born at Mauléon, 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 parlement of Navarre. He spent his leisure and his fortune in the search for documents bearing on the old Basque and Bearnese provinces; and the fruits of his studies in the archives of Bayonne, Toulouse,

Oihenart published in 1625 a Déclaration historique de l'injuste usurpation et retention de la Navarre par les Espagnols and a fragment of a Latin work on the same subject is included in Galland's Mémoires pour l'histoire de Navarre (1648). His most important work is Notitia utriusque Vasconiae, tum Ibericae, tum Aquitanicae, qua practer silum regionis et alia scitu digna, Navarrae regum coeterarumque: in iis insignum vetustate et dignitate familiarum Paris, 1638 and 1656), a description of Gascony and Navarre. His collection of over five hundred Basque proverbs, Atsolizac edo Refravac, included in a volume of his poems Ofen Gastaroa NevrthizeAtsotizen Vrrhenquina. The proverbs were edited by Francisque tan, printed in Paris in 1657, was supplemented by a second collection, Michel (Paris, 1847), and the supplement by P. Hariston (Bayonne, 1892) and by V. Stempf (Bordeaux, 1894). See Julien Vinson, Essai d'une bibliographie de la langue basque (Paris, 1891); J. B. E. de Jaurgain, Arnaud d'Oihenart et sa famille (Paris, 1885).

OIL CITY, a city of Venango 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 1000 ft. above the sea, and is divided by the river and the creck into three sections connected by bridges. The business part of the 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 5th 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 thermodynamic 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 compound. It is a mixture of various hydrocarbons of the paraffin and olefine 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 14,240 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 739 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 modern 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 to 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 took 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 for 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. 1 is a diagrammatic section of an early Daimler motor. A is the cylinder, B the piston, C the connecting rod, and D the crank, 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 for 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 f, 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 spurt of petrol passes K round the jet. The petrol breaks up into spray by impact by the jet G', separate air at the same time passing by the passage 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 tube, which is always open to the compression space It is rendered incandescent igniter tube H fires the charge. H is a short platinum 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 does not ignite the charge until the cominto contact with the hot part till it is forced up the tube by the pression takes place, because the inflammable mixture cannot come

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compression. The engine is started by giving the crank-shaft a alone actuated from the valve shaft. The shaft Q is operated by smart turn round by means of a detachable handle. The exhaust is 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 high, 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.

Benz of Mannheim followed close on the work of Daimler, and in France Panhard and Levassor, Peugeot, De Dion, Delahaye and Renault all contributed to the development of the petrol engine, while Napier, Lanchester, 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. 1, 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 oil 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 no 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 Maybach as shown in fig. 1. A diagrammatic section of a carburettor of the Maybach 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 G

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 makes its suction stroke, the air enters from the atmosphere at F and passes to the cylinder through G. The pressure around the jet 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 suction stroke. An inflammable mixture is thus formed, which enters the cylinder by way of G. The area for the passage of air around the petrol jet É 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 limits, the volume of petrol thrown by the jet is fairly proportional range of speed required from the engine is not great; that is, within to the air passing the jet. When, however the speed range is great, such as in modern motors, which may vary from 300 to 1500 revoluimpossible to secure proportionality sufficiently accurate for regular tions per minute under light and heavy loads, then it becomes 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 volume per stroke, which change may occur either at a low or at a complications. Throttle control implies a change of total charge 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 other times the mixture may be so dilute as to miss firing altogether. petrol will be present in excess with no oxygen to burn it, and at 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 chamber to the jet E; and, while the engine is running slowly, the whole supply of air enters by way of the passage F, mixes with the petrol and reaches the cylin- L ders by way of the pipe G. The volume of charge entering the cylinder per stroke is controlled throttle valve H, by the piston operated by the rod ; and so long as the charge volume required remains small, air from the

enters from the float

K

FIG. 3.

atmosphere enters only by F. When speed rises, hewever, 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 proportion of air to petrol is maintained through a comparatively air and petrol entering through F. By this device the required 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 in 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

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