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CALIBRATION
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CALIBRATION , a See also:term primarily signifying the determination of the " calibre " or See also:bore of a See also:gun. The word calibre was introduced through the See also:French from the See also:Italian calibro, together with other terms of gunnery and warfare, about the 16th See also:century. The origin of the Italian See also:equivalent appears to be uncertain. It will readily be understood that the calibre of a gun requires accurate See also:adjustment to the See also:standard See also:size, and further, that the bore must be straight and of See also:uniform See also:diameter throughout. The term was subsequently applied to the accurate measurement and testing of the bore of any See also:kind of See also:tube, especially those of thermometers. In See also:modern scientific See also:language, by a natural See also:process of transition, the term " calibration " has come to denote the accurate comparison of any measuring See also:instrument with a standard, and more particularly the determination of the errors of its See also:scale. It is seldom possible in the process of manufacture to make an instrument so perfect that no See also:error can be discovered by the most delicate tests, and it would rarely be See also:worth while to See also:attempt to do so even if it were possible. The cost of manufacture would in many cases be greatly increased without adding materially to the utility of the apparatus. The scientific method, in all cases which admit of the subsequent determination and correction of errors, is to economize See also:time and labour in See also:production by taking pains in the subsequent verification or calibration. This process of calibration is particularly important in laboratory See also:research, where the observer has frequently to make his own apparatus, and cannot afford the time or outlay required to make See also:special tools for See also:fine See also:work, but is already provided with apparatus and methods of accurate testing. For non-scientific purposes it is generally possible to construct See also:instruments to measure with sufficient precision without further correction. The See also:present See also:article will therefore be restricted to the scientific use and application of methods of accurate testing. See also:General Methods and Principles.—The process of calibration of any measuring instrument is frequently divisible into two parts, which differ greatly in importance in different cases, and of which one or the other may often be omitted. (I) The determination of the value of the unit to which the measurements are referred by comparison with a standard unit of the same kind. This is often described as the Standardization of the instrument, or the determination of the Reduction See also:factor. (2) The verification of the accuracy of the subdivision of the scale of the instrument. This may- be termed calibration of the scale, and does notnecessarily involve the comparison of the instrument with any See also:independent standard, but merely the verification of the accuracy of the relative values of its indications. In many cases the process of calibration adopted consists in the comparison of the instrument to be tested with a standard over the whole range of its indications, the relative values of the subdivisions of the standard itself having been previously tested. In this See also:case the distinction of two parts in the process is unnecessary, and the term calibration is for this See also:reason frequently employed to include both. In some cases it is employed to denote the first See also:part only, but for greater clearness and convenience of description we shall restrict the term as far as possible to the second meaning. The methods of standardization or calibration employed have much in See also:common even in the cases that appear most diverse. They are all founded on the See also:axiom that " things which are equal to the same thing are equal to one another." Whether it is a question of comparing a scale with a standard, or of testing the equality of two parts of the same scale, the process is essentially one of interchanging or substituting one for the other, the two things to he compared. In addition to the things to be tested there is usually required some See also:form of See also:balance, or comparator, or See also:gauge, by which the equality may be tested. The simplest of such comparators is the instrument known as the callipers, from the same See also:root as calibre, which is in See also:constant use in the workshop for testing equality of linear dimensions, or uniformity of diameter of tubes or rods. The more complicated forms of See also:optical comparators or measuring See also:machines with scales and See also:screw adjustments are essentially similar in principle, being finely adjustable gauges to which the things to be compared can be successively fitted. A still simpler and more accurate comparison is that of See also:volume or capacity, using a given See also:mass of liquid as the gauge or test of equality, which is the basis of many of the most accurate and most important methods of calibration. The common balance for testing equality of mass or See also:weight is so delicate and so easily tested that the process of calibration may frequently with See also:advantage be reduced to a See also:series of weighings, as for instance in the calibration of a burette or measure-See also:glass by weighing the quantities of See also:mercury required to fill it to different marks. The balance may, however, be regarded more broadly as the type of a general method capable of the widest application in accurate testing. It is possible, for instance, to balance two electromotive forces or two See also:electrical resistances against each other, or to measure the refractivity of a See also:gas by balancing it against a See also:column of See also:air adjusted to produce the same retardation in a See also:beam of See also:light. These See also:equilibrium," or " null," or " balance " methods of comparison afford the most accurate measurements, and are generally selected if possible as the basis of any process of calibration. In spite of the See also:great diversity in the nature of things to be compared, the fundamental principles of the methods employed are so essentially similar that it is possible, for instance, to describe the testing of a set of weights, or the calibration of an electrical resistance-See also:box, in almost the same terms, and to represent the calibration correction of a mercury thermometer or of an ammeter by precisely similar curves. Method of Substitution. In comparing two See also:units of the same kind and of nearly equal magnitude, some variety of the general method of substitution is invariably adopted. The same method in a more elaborate form is employed in the calibration of a series of multiples or submultiples of any unit. The details of the method depend on the See also:system of subdivision adopted, which is to some extent a See also:matter of See also:taste. The simplest method of subdivision is that on the binary scale, proceeding by multiples of 2. With a pair of submultiples of the smallest See also:denomination and one of each of the See also:rest, thus 1, 1, 2, 4, 8, 16, &c., each weight or multiple is equal to the sum of all the smaller weights, which may be substituted for it, and the small difference, if any, observed. If we See also:call the weights A, B, C, &c., where each is approximately See also:double the following weight, and if we write a for observed excess of A over the rest of the weights, b for that of B over C+D+&c., and so on, the observations by the method of substitution give the series of equations, A - rest = a, B - rest = b, C - rest = c, &c. . . (1) Subtracting the second from the first, the third from the second, and so on, we obtain at once the value of each weight in terms of the preceding, so that all may be expressed in terms of the largest, which is most conveniently taken as the standard B=A/2+(b-a)/2, C=B/2+(c-b)2, &c. . . (2). The advantages of this method of subdivision and comparison, in addition to its extreme simplicity, are (I) that there is only one possible See also:combination to represent any given weight within the range of the series; (2) that the least possible number of weights is required to See also:cover any given range; (3) that the smallest number of substitutions is required for the See also:complete calibration. These advantages are important in cases where the accuracy of calibration is limited by the constancy of the conditions of observation, as in the case of an electrical resistance-box, but the See also:reverse may be the case when it is a question of accuracy of estimation by an observer. In the See also:majority of cases the ease of numeration afforded by familiarity with the decimal system is the most important See also:consideration. The most convenient arrangement on the decimal system for purposes of calibration is to have the units, tens, hundreds, &c., arranged in See also:groups of four adjusted in the proportion of the See also:numbers 1, 2, 3, 4. The relative values of the weights in each See also:group of four can then be determined by substitution independently of the others, and the See also:total of each group of four, making ten times the unit of the group, can be compared with the smallest weight in the group above. This gives a sufficient number of equations to determine the errors of all the weights by the method of substitution in a very See also:simple manner. A number of other equations can be obtained by combining the different groups in other ways, and the whole system of equations may then be solved by the method of least squares; but the equations so obtained are not all of equal value, and it may be doubted whether any real advantage is gained in many cases by the multiplication of comparisons, since it is not possible in this manner to eliminate constant errors or See also:personal See also:equation, which are generally aggravated by prolonging the observations. A common arrangement of the weights in each group on the decimal system is 5, 2, I, I, or 5, 2, 2, I. These do not admit of the independent calibration of each group by substitution. The arrangement 5, 2,I, I, I, or 5, 2, 2, I, 1, permits independent calibration, but involves arlarger number of weights and observations than the I, 2, 3, 4, grouping. The arrangement of ten equal weights in each group, which is adopted in " See also:dial " resistance-boxes, and in some forms of chemical balances where the weights are mechanically applied by turning a handle, presents great advantages in point of quickness of manipulation and ease of numeration, but the complete calibration of such an arrangement is tedious, and in the case of a resistance-box it is difficult to make the necessary connexions. In , all cases where the same total can be made up in a variety of ways, it is necessary in accurate work to make sure that the same weights are always used for a given combination, or else to See also:record the actual weights used on each occasion. In many investigations where time enters as one of the factors, this is a serious See also:drawback, and it is better to avoid the more complicated arrangements. The accurate adjustment of a set of weights is so simple a matter that it is often possible to neglect the errors of a well-made set, and no calibration is of any value without the most scrupulous See also:attention to de-tails of manipulation, and particularly to the correction for the air displaced in comparing weights of different materials. Electrical resistances are much more difficult to adjust owing to the See also:change of resistance with temperature, and the calibration of a resistance-box can seldom be neglected on See also:account of the changes of resistance which are liable to occur after adjustment from imperfect See also:annealing. It is also necessary to remember that the See also:order of accuracy required, and the actual values of the smaller resistances, depend to some extent on the method of connexion, and that the box must be calibrated with due regard to the conditions under which it is to be used. Otherwise the method of See also:procedure is much the same as in the case of a box of weights, but it is necessary to pay more attention to the constancy and uniformity of the temperature See also:condition's of the observing-See also:room. Method of Equal Steps.—In calibrating a continuous scale divided into a number of divisions of equal length, such as a See also:metre scale divided in millimetres, or a thermometer tube divided in degrees of temperature, or an electrical slide-See also:wire, it is usual to proceed by a method of equal steps. The simplest method is that known as the method of See also:Gay Lussac in the calibration of See also:mercurial thermometers or tubes of small bore. It is essentially a method of substitution employing a column of mercury of constant volume as the gauge for comparing the capacities of different parts of the tube. A precisely similar method, employing a pair of microscopes at a fixed distance apart as a standard of length, is applicable to the calibration of a divided scale. The See also:interval to be calibrated is divided into a whole number of equal steps or sections, the points of See also:division at which the corrections are to be determined are called points of calibration. Calibration of a Mercury Thermometer.—To facilitate description, we will take the case of a fine-bore tube, such as that of a thermometer, to be calibrated with a See also:thread of mercury. The bore of such a tube will generally vary considerably even in the best standard instruments, the tubes of which have been specially See also:drawn and selected. The correction for inequality of bore may amount to a See also:quarter or See also:half a degree, and is seldom less than a tenth. In See also:ordinary chemical thermometers it is usual to make See also:allowance for See also:variations of bore in graduating the scale, but such instruments present discontinuities of division, and cannot be used for accurate work, in which a finely-divided scale of equal parts is essential. The calibration of a mercury thermometer intended for work of precision is best effected after it has been sealed. A•thread of mercury of the desired length is separated from the column. The exact adjustment of the length of the thread requires a little manipulation. The thermometer is inverted and tapped to make the mercury run down to the See also:top of the tube, thus See also:collecting a trace of residual gas at the end of the bulb. By quickly See also:reversing the thermometer the bubble passes to the See also:neck of the bulb. If the instrument is again inverted and tapped, the thread will probably break off at the neck of the bulb, which should be previously cooled or warmed so as to obtain in this manner, if possible, a thread of the desired length'. If the thread so obtained is too See also:long or not accurate enough, it is removed to the other end of the tube, and the bulb further warmed till the mercury reaches some easily recognized division. At this point the broken thread is rejoined to the mercury column from the bulb, and a microscopic bubble of gas is condensed which generally suffices to determine the subsequent breaking of the mercury column at the same point of the tube. The bulb is then allowed to cool till the length of the thread above the point of separation is equal to the desired length, when a slight tap suffices to See also:separate the thread. This method is difficult to work with See also:short threads owing to deficient inertia, especially if the tube is very perfectly evacuated. A thread can always be separated by See also:local See also:heating with a small See also:flame, but this is dangerous to the thermometer, it is difficult to adjust the thread exactly to the required length, and the mercury does not run easily past a point of the tube which has been locally heated in this manner. Having separated a thread of the required length, the thermometer is mounted in a See also:horizontal position on a suitable support, preferably with a screw adjustment in the direction of its length. By tilting or tapping the instrument the thread is brought into position corresponding to the steps of the calibration successively, and its length in each position is carefully observed with a pair of See also:reading microscopes fixed at a suitable distance apart. Assuming that the temperature remains constant, the variations of length of the thread are inversely as the variations of See also:cross-See also:section of the tube. If the length of the thread is very nearly equal to one step, and if the tube is nearly uniform, the See also:average of the observed lengths of the thread, taking all the steps throughout the interval, is equal to the length which the thread should have occupied in each position had the bore been uniform throughout and all the divisions equal. The error of each step is therefore found by subtracting the average length from the observed length in each position. Assuming that the ends of the interval itself are correct, the correction to be applied at any point of calibration to reduce the readings to a uniform tube and scale, is found by taking the sum of the errors of the steps up to the point considered with the sign reversed. In the preceding example of the method an interval of ten degrees is taken, divided into ten steps of 1 ° each. The distances of the ends of the thread from the nearest degree divisions are estimated by the aid of micrometers to the thousandth of a degree. The error of any one of these readings probably does not exceed half a thousandth, but they are given to the nearest thousandth only. The excess length of the thread in each position over the corresponding degree is obtained by subtracting the second reading from the first. Taking the average of the numbers in this See also:line, the mean excess-length is -10.4 thousandths. The error of each step is found by subtracting this mean from each of the numbers in the previous line. Finally, the corrections at each degree are obtained by adding up the errors of the steps and changing the sign. The errors and corrections are given in thousandths of I °. Complete Calibration.—The simple method of Gay Lussac does very well for short intervals when the number of steps is not excessive, but it would not be satisfactory for a large range owing to the See also:accumulation of small errors of estimation, and the variation of the personal equation. The observer might, for instance, consistently over-estimate the length of the thread in one half of the tube, and under-estimate it in the other. The errors near the See also:middle of the range would probably be large. It is evident that the correction at the middle point of the interval could be much more accurately determined by using a thread equal to half the length of the interval. To minimize the effect of these errors of estimation, it is usual to employ threads of different lengths in calibrating the same interval, and to See also:divide up the fundamental interval of the thermometer into a number of subsidiary sections for the purpose of calibration, each of these sections being treated as a step in the calibration of the fundamental interval. The most symmetrical method of calibrating a section, called by C. E. See also:Guillaume a "Complete Calibration," is to use threads of all possible lengths which are No. of 1 2 3 4 5 6 7 8 9 10 Step. Ends of L +•o,o —.016 —•020 —.031 +'o16 +•oo8 +•013 +•017 +•004 —•o88 thread. $) +038 +•017 —.003 — *022 -}-•010 -{-•005 +•033 +•o18 +013 — '903 Excess- —.028 -.033 — *017 —.009 +oo6 —.003 — .020 — .001 —.004 +•005 length. -17.6 -22.6 — 6.6 + 1.4 +16.4 + 7'4 — 9'6 + 9'4 + 6.4 +15'4' Error of step. +17.6 +40.2 +46'8 +45'4 +29.o +21.6 +31.2 +21.8 +15.4 0 Correc- tion. ampoules, were calibrated by Chappuis in five sections of 20° each, to determine the corrections at the points 20°, 40°, 6o°, 8o°, which may be called the " See also:principal points " of the calibration, in terms of the fundamental interval. Each section of 2o° was subsequently calibrated in steps of 2°, the corrections being at first referred, as in the example already given, to the mean degree of the section itself, and being afterwards expressed, by a simple transformation, in terms of the fundamental interval, by means of the corrections already found for the ends of the section. Supposing, for instance, that the corrections at the points o° and to° of Table III. are not zero, but C° and C' respectively, the correction Cn. at any intermediate point n will evidently be given by the See also:formula, C„=C°+6,+(C'-C°)n/lo . . . (3) where en. is the correction already given in the table. If. the corrections are required to the thousandth of a degree, it is necessary to tabulate the results of the calibration at much more frequent intervals than 2°, since the correction, even of a See also:good thermometer, may change by as much as 20 or 30 thousandths in 2°. To See also:save the labour and difficulty of calibrating with shorter threads, the corrections at inter-mediate points are usually calculated by a formula of See also:interpolation. This leaves much to be desired, as the section of a tube often changes very suddenly and capriciously. It is probable that the graphic method gives equally good results with less labour. Slide-Wire.--The calibration of an electrical slide-wire into parts of equal resistance is precisely analogous to that of a capillary tube into parts of equal volume. The See also:Carey See also:Foster method, employing short steps of equal resistance, effected by transferring a suitable small resistance from one See also:side of the slide-wire to the other, is exactly analogous to the Gay Lussac method, and suffers from the same defect of the accumulation of small errors unless steps of several different lengths are used. The calibration of a slide-wire, however, is much less troublesome than that of a thermometer tube for several reasons. It is easy to obtain a wire uniform to one part in 500 or even less, and the section is not liable to capricious variations. In all work of precision the slide-wire is supplemented by See also:auxiliary resistances by which the scale may be indefinitely extended. In accurate electrical See also:thermometry, for example, the slide-wire itself would correspond to only I °, or less, of the whole scale, which is less than a single step in the calibration of a mercury thermometer, so that an accuracy of a thousandth of a degree can generally be obtained without any calibration of the slide-wire. In the rare cases in which it is necessary to employ a long slide-wire, such as the See also:cylinder See also:potentiometer of See also:Latimer See also:Clark, the calibration is best effected by comparison with a standard, such as a See also:Thomson-See also:Varley slide-box. Graphic See also:Representation of Results.—The results of a calibration are often best represented by means of a correction See also:curve, such as that illustrated in the See also:diagram, which is plotted to represent the corrections found in Table III. The See also:abscissa of such a curve is the reading of the instrument to be corrected. The See also:ordinate is the correction to be added to the observed reading to reduce to a uniform scale. The corrections are plotted in the figure in terms of the whole section, taking the correction to be zero at the beginning and end. As a matter of fact the corrections at these points in terms of the fundamental interval were found to be -29 and -9 thousandths respectively. The correction curve is transformed to give corrections in terms of the fundamental interval by ruling a straight line joining the points +2g and -1-9 respectively, and reckoning the ordinates from this line instead of from the See also:base-line. Or the curve may be replotted with the new ordinates thus obtained. In See also:drawing the curve from the corrections obtained at the points of calibration, the exact form of the curve is to some extent a matter of taste, but the curve should generally be drawn as smoothly as possible on the See also:assumption that the changes are See also:gradual and continuous. The ruling of the straight line across the curve to See also:express the corrections in terms of the fundamental interval, corresponds to the first part of the process of calibration mentioned above under the term " Standardization." It effects the reduction of the integral multiples of the calibration step. In the example already given nine different threads were used, and the length of each was observed in as many positions as possible. Proceeding in this manner the following numbers were obtained for the excess-length of each thread in thousandths of a degree in different positions,' starting in each case with the beginning of the thread at o°, and moving it on by steps of I °. The observations in the first column are the excess-lengths of the thread of I° already given in See also:illustration of the method of Gay Lussac. The other columns give the corresponding observations with the longer threads. The 'simplest and most symmetrical method of solving these observations, so as to find the errors of each step in terms of the whole interval, is to obtain the See also:differences of the steps in pairs by subtracting each observation from the one TABLE II.—Complete Calibration of Interval of to° in to Steps. Lengths of Threads. I° 2° 3° 4° 5° 6° 7° 8° 9° Observed excess- o° -28 +-32 -67 -62 -II -15 -48 - 2 - 8 lengths of threads,i° -33 ~ 2I -47 -28 +14 -' g -22 +21 +24 in various posi- 2 ° -17 + 2 - 8 + I +26 +23 + 6 +58 tions, the begin- 3° - 9 +26 + 5 -` 3 +41 +36 +28 ping of the thread 4° + 6 +31 - 7 + 4 +45 +49 being set near the 5° - 3 + 5 -15 - 6 +43 points. 6° -20 + 7 -16 + 2 7° - I +23 +10 8° - 4 +29 9 ° + 5 above it. This method eliminates the unknown lengths of the threads, and gives each observation approximately its due weight. Subtracting the observations in the second line from those in the first, we obtain a series of numbers, entered in column i of the next table, representing the excess of step (1) over each of the other steps. The sum of these differences is ten times the error of the first step, since by See also:hypothesis the sum of the errors of all the steps is zero in terms of the whole interval. The numbers in the second column of Table III. are similarly obtained by subtracting the third line from the second in Table II., each difference being inserted in its appropriate See also:place in the table. Proceeding in this way we find the excess of each interval over those which follow it. The table is completed by a See also:diagonal See also:row of zeros representing the difference of each step from itself, and by repeating the numbers already found in symmetrical positions with their signs changed, since the excess of any step, say 6 over 3, is evidently equal to that of 3 over 6 with the sign changed. The errors of each step having been found by adding the columns, and dividing by to, the corrections at each point of the calibration are deduced as before. Step I 2 3 4 5 6 7 8 9 10 No. 1 0 -' 5 +11 +20 +34 +25 + 7 -~ 26 +23 +32 2 + 5 0 +16 +23 +39 +29 +12 +31 +28 +37 3 -II -16 0 + 8 +24 +13 - 4 +15 +13 +22 4 - 20 - 23 - 8 0 + 15 + 5 - 12 + 7 + 4 +13 5 -34 -39 -24 -15 0 - 9 -26 - 8 -lo - 2 6 -25 -29 -13 - 5 + 9 0 -17 + 2 - I + 8 7 - 7 -12 + 4 +12 +26 +17 0 +19 +16 +26 8 -26 -31 -15 -7 +8 -2 -19 0 -3 +6 9 -23 -28 -13 - 4 +IO + I -16 + 3 0 + 9 Io -32 -37 -22 -13 + 2 - 8 -26 - 6 - 9 0 Error of -17.3 -22.0 - 6.4 + I•9 +16.7 + 7•I -10.1 + 8.9 + 6.1 +15.1 step. Correc- +17.3 +39.3 +45.7 +43.8 +27.1 +20.0 +30.1 +21.2 +15•I 0 tions. The advantages of this method are the simplicity and symmetry of the work of reduction, and the accuracy of the result, which exceeds that of the Gay Lussac method in consequence of the much larger number of independent observations. It may be noticed, for instance, that the correction at point 5 is 27.1 thousandths by the complete calibration, which is 2 thousandths less than the value 29 obtained by the Gay Lussac method, but agrees well with the value 27 thousandths obtained by taking only the first and last observations with the thread of 5°. The disadvantage of the method lies in the great number of observations required, and in the labour of adjusting so many different threads to suitable lengths. It is probable that sufficiently good results may be obtained with much less trouble by using fewer threads, especially if more care is taken in the micrometric determination of their errors. The method adopted for dividing up the fundamental interval of any thermometer into sections and steps for calibration may be widely varied, and is necessarily modified in cases where auxiliary bulbs or " ampoules " are employed. The See also:Paris mercury-See also:standards, which read continuously from o° to too° C., without intermediate 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 3 4 3 6, 7 4 9 10 CALIBRATION CURVE. potential within its range if connected to a constant See also:cell to See also: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. A very neat use of the calibration curve has been made by See also:Professor W. A. See also:Rogers in the automatic correction of screws of dividing machines or lathes. It is possible by the process of grinding, as applied by See also: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 See also:object a scale is ruled on the See also:machine, and the errors of the uncorrected screw are determined by calibrating the scale. A See also:metal template may then be cut out in the form of the calibration-correction curve on a suitable scale. A See also:lever projecting from the See also:nut which feeds the See also:carriage or the slide-rest is made to follow the See also:contour of the template, and to apply the appropriate correction at each point of the travel, by turning the nut through a small See also: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 See also:cam or See also: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 See also: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 See also: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 See also: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 2500 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 See also:stem exposure and scale. The only See also:practical method is to compare its readings every few degrees with those of a See also:platinum thermometer under the condi- OALICUT tions for which it it 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. AuTnoRITIEs.—Mercurial Thermometers: Guillaume, Thermometrie de Precision (Paris, 1889), gives several examples and references to See also:original See also: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 See also:Bureau See also:International See also:des Poids et Mbsures 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 See also:Barnes, Phil. Trans. A, 1901. (H. L. Additional information and CommentsThere are no comments yet for this article.
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