Page:EB1911 - Volume 05.djvu/17

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readings to a common standard, and may be neglected if relative values only are required. A precisely analogous correction occurs in the case of electrical instruments. A potentiometer, for instance, if correctly graduated or calibrated in parts of equal resistance, will give correct relative values of any differences of potential within its range if connected to a constant cell to supply the steady current through the slide-wire. But to determine at any time the actual value of its readings in volts, it is necessary to standardize it, or determine its scale-value or reduction-factor, by comparison with a standard cell.

EB1911Calibration Curve.jpg
Calibration Curve.

A very neat use of the calibration curve has been made by Professor W. A. Rogers in the automatic correction of screws of dividing machines or lathes. It is possible by the process of grinding, as applied by Rowland, to make a screw which is practically perfect in point of uniformity, but even in this case errors may be introduced by the method of mounting. In the production of divided scales, and more particularly in the case of optical gratings, it is most important that the errors should be as small as possible, and should be automatically corrected during the process of ruling. With this object a scale is ruled on the machine, and the errors of the uncorrected screw are determined by calibrating the scale. A metal template may then be cut out in the form of the calibration-correction curve on a suitable scale. A lever projecting from the nut which feeds the carriage or the slide-rest is made to follow the contour of the template, and to apply the appropriate correction at each point of the travel, by turning the nut through a small angle on the screw. A small periodic error of the screw, recurring regularly at each revolution, may be similarly corrected by means of a suitable cam or eccentric revolving with the screw and actuating the template. This kind of error is important in optical gratings, but is difficult to determine and correct.

Calibration by Comparison with a Standard.—The commonest and most generally useful process of calibration is the direct comparison of the instrument with a standard over the whole range of its scale. It is necessary that the standard itself should have been already calibrated, or else that the law of its indications should be known. A continuous current ammeter, for instance, can be calibrated, so far as the relative values of its readings are concerned, by comparison with a tangent galvanometer, since it is known that the current in this instrument is proportional to the tangent of the angle of deflection. Similarly an alternating current ammeter can be calibrated by comparison with an electrodynamometer, the reading of which varies as the square of the current. But in either case it is neccessary, in order to obtain the readings in amperes, to standardize the instrument for some particular value of the current by comparison with a voltameter, or in some equivalent manner. Whenever possible, ammeters and voltmeters are calibrated by comparison of their readings with those of a potentiometer, the calibration of which can be reduced to the comparison and adjustment of resistances, which is the most accurate of electrical measurements. The commoner kinds of mercury thermometers are generally calibrated and graduated by comparison with a standard. In many cases this is the most convenient or even the only possible method. A mercury thermometer of limited scale reading between 250° and 400° C., with gas under high pressure to prevent the separation of the mercury column, cannot be calibrated on itself, or by comparison with a mercury standard possessing a fundamental interval, on account of difficulties of stem exposure and scale. The only practical method is to compare its readings every few degrees with those of a platinum thermometer under the conditions for which it is to be used. This method has the advantage of combining all the corrections for fundamental interval, &c., with the calibration correction in a single curve, except the correction for variation of zero which must be tested occasionally at some point of the scale.

Authorities.—Mercurial Thermometers: Guillaume, Thermométrie de Précision (Paris, 1889), gives several examples and references to original memoirs. The best examples of comparison and testing of standards are generally to be found in publications of Standards Offices, such as those of the Bureau International des Poids et Mésures at Paris. Dial Resistance-Box: Griffiths, Phil. Trans. A, 1893; Platinum Thermometry-Box: J. A. Harker and P. Chappuis, Phil. Trans. A, 1900; Thomson-Varley Potentiometer and Binary Scale Box: Callendar and Barnes, Phil. Trans. A, 1901.  (H. L. C.) 

CALICO, a general name given to plain cotton cloth. The word was spelt in various forms, including “calicut,” which shows its derivation from the Indian city of Calicut or Kolikod, a seaport in the presidency of Madras, and one of the chief ports of intercourse with Europe in the 16th century, where cotton cloths were made. The name seems to have been applied to all kinds of cotton cloths imported from the East. In England it is now applied particularly to grey or bleached cotton cloth used for domestic purposes, and, generally, to any fairly heavy cotton cloth without a pattern. In the United States there is a special application to printed cloth “of a coarser quality than muslin.” In England “printed calico” is a comprehensive term.

CALICUT, a city of British India, in the Malabar district of Madras; on the coast, 6 m. N. of Beypur. In 1901 the population was 76,981, showing an increase of 14% in the decade. The weaving of cotton, for which the place was at one time so famous that its name became identified with its calico, is no longer of any importance. Calicut is of considerable antiquity; and about the 7th century it had its population largely increased by the immigration of the Moplahs, a fanatical race of Mahommedans from Arabia, who entered enthusiastically into commercial life. The Portuguese traveller Pero de Covilham (q.v.) visited Calicut in 1487 and described its possibilities for European trade; and in May 1498 Vasco da Gama, the first European navigator to reach India, arrived at Calicut. At that time it was a very flourishing city, and contained several stately buildings, among which was especially mentioned a Brahminical temple, not inferior to the largest monastery in Portugal. Vasco da Gama tried to establish a factory, but he met with persistent hostility from the local chief (zamorin), and a similar attempt made by Cabral two years later ended in the destruction of the factory by the Moplahs. In revenge the Portuguese bombarded the town, but no further attempt was made for some years to establish a trading settlement there. In 1509 the marshal Don Fernando Coutinho made an unsuccessful attack on the city; and in the following year it was again assailed by Albuquerque with 3000 troops. On this occasion the palace was plundered and the town burnt; but the Portuguese were finally repulsed, and fled to their ships after heavy loss. In the following year they concluded a peace with the zamorin and were allowed to build a fortified factory on the north bank of the Kallayi river, which was however again, and finally, abandoned in 1525. In 1615 the town was visited by an English expedition under Captain Keeling, who concluded a treaty with the zamorin; but it was not until 1664 that an English trading settlement was established by the East India Company. The French settlement, which still exists, was founded in 1698. The town was taken in 1765 by Hyder Ali, who expelled all the merchants and factors, and destroyed the cocoa-nut trees, sandal-wood and pepper vines, that the country reduced to ruin might present no temptation to the cupidity of Europeans. In 1782 the troops of Hyder were driven from Calicut by the British; but in 1788 it was taken and destroyed by his son Tippoo, who carried off the inhabitants to Beypur and treated them with great cruelty. In the latter part of 1790 the country was occupied by the British; and under the treaty concluded in 1792, whereby Tippoo was deprived of half his dominions, Calicut fell to the British. After this event the