battery is applied to the shunted galvanometer, then the current C passing through the galvanometer will be given by the expression
C＝SVR′(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 potentiometer (q.v.). 
Fig. 2.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
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 or of insulated 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 Maître de forges, produced at the Gymnase in 1883, holding the stage for a whole year. His later publications include 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-Coburg-Gotha, 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 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 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,
Pau, Perigord and other cities were embodied in forty-five MS. volumes, which were sent by his son Gabriel to Colbert. Twenty-three of these are in the Bibliothèque Nationale of Paris (Coll. Duchesne).
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 Memoires pour l’histoire de Navarre (1648). His most important work is Notitia utriusque Vasconiae, tum Ibericae, tum Aquitanicae, qua praeter situm 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, Atsotizac edo Refravac, included in a volume of his poems Oten Gastaroa Nevrthizetan, printed in Paris in 1657, was supplemented by a second collection, Atsotizen Vrrhenquina. The proverbs were edited by Francisque 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; (1906 estimate) 14,662. 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 creek 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 110 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
