Popular Science Monthly/Volume 18/January 1881/Examination of Thermometers at the Yale Observatory
By Dr. LEONARD WALDO.
ONE of the most useful institutions to science in England is the Kew Observatory of the Royal Society, whose principal work for the last quarter of a century has been to furnish accurate comparisons of thermometers sent there by physicists, meteorologists, physicians, and instrument-makers. The recognized benefits accruing to the scientific world from this well-known and widely popular service at Kew have caused the managing board of the Winchester Observatory of Yale College to organize a service having the same ends in view under the direction of the observatory. Although this work is but fairly commenced, yet it has met with most gratifying success, and there have been so many inquiries as to the methods and scope of this service that the writer has ventured upon a description suitable for the pages of the "Monthly," with the hope that in this form it may the more readily come to the notice of the meteorologists and physicians who are the most likely to be benefited by it.
Few are aware of the errors found to exist even in the thermometers of reputable makers. The well-known change which takes place with age in every thermometer not infrequently amounts to a degree and a half Fahrenheit within two years from the time the thermometer is made. The change depending upon the temperature to which a thermometer is heated, even supposing this to be no greater than the boiling-point of water, may be three fourths of a degree. If we add to these two sources of error the original error in the graduation of the thermometer scale arising from the boiling and freezing points not being properly fixed, and the error arising from the variations in the size of the capillary tube, it is quite within the range of possibility that thermometers, which from their general construction would appear likely to give correct indications, may really be in error two or three degrees Fahrenheit at some part of their scales. Thus the "fever" thermometers in general use by physicians are almost invariably too high in their readings. An analysis of the results of sixty-eight thermometers of this description, verified in June of this year, will show how great this error may come to be: one fifth had errors less than 0·1°; one fifth had errors less than 0·4° but more than 0·2°; two fifths had errors less than 0·7° but more than 0·4°; one fifth had errors less than 1·0° but more than 0·7°; and occasionally a thermometer was found which had errors exceeding 1° and more rarely one exceeding 2°. The thermometers on which the above deductions rest were chosen to represent seven makers, and may be fairly taken to indicate the liability to error in using fever-thermometers which have not been compared with authoritative standards. It is not unlikely that members of the medical profession have been sometimes misled by the readings of inaccurate thermometers, and they may have made such unfavorable statements regarding the chances of recovery of patients whose temperatures were high, that the patient, under the influence of his imagination, has given up the struggle for existence when a little more hopeful view of the case might have imbued him with fresh courage and led to ultimate recovery.
The work at the Yale Observatory divides itself into two parts—the establishing of the standard thermometers with which thermometers sent to the observatory are to be compared, and the-work of comparing thermometers. The investigation of the standards themselves is by far the most tedious of the two; and as the methods used in studying the observatory standards are also the methods used, with greater or less detail, in investigating the higher grades of thermometers sent to the observatory, the methods will be briefly outlined.
It will be necessary to recall some of the fundamental principles of thermometry, however, in order to properly comprehend the methods of procedure in the case of standards:
1. Glass mercurial thermometers slowly increase their freezing-point readings as their age increases after the heating they undergo in filling with mercury in their manufacture.
2. The readings of the boiling-points are also increased, but in a much less degree—perhaps not more than one fifth as much as the freezing-point.
3. Whenever the thermometer is heated at all, the freezing-point is lowered, and the amount of this depression is very nearly proportional to the square of the temperature to which the thermometer is heated.
4. It follows from 3, that if a thermometer is kept at the ordinary temperatures, the freezing-point of water will be indicated by a lower scale reading than if the thermometer is kept at a low temperature. Now, if we suppose the thermometer has been kept at the freezing-point of water for a period of several days, and that the progressive change which takes place in the first years after manufacture has ceased, the freezing-point which is then determined is called the permanent freezing-point, and is the zero of the Centigrade scale, or 32° of the Fahrenheit scale. If we heat the thermometer to the boiling-point of water, and then immediately cool it and immerse it in melting ice, we shall obtain another point on the thermometer scale which we may call the temporary freezing-point, because it will gradually approach the permanent freezing-point, and after a few months, if the instrument is not again heated, it will finally coincide with it. The difference between the permanent and temporary freezing-points is usually about three fourths of a degree Fahrenheit, and, so far as now known, remains constant for the same thermometer.
5. The boiling-point of water at the level of the sea, and with a barometric pressure of 760 mm. 29∙922 inches in the latitude of 45°, is the second point in the thermometer scale to be fixed. To do this the thermometer is exposed to the steam of pure water, and, from the observed height of the barometer, the known elevation and latitude of the place of observation, the true boiling-point is computed from the observed one, and the 100° C. or 212° F. is thus fixed.
6. Having thus located the freezing and boiling points of a standard thermometer, the intermediate points are to be fixed by dividing the scale so that at every part the length of 1° shall measure an equal
volume of mercury. At least, this has been the usual procedure, and for ordinary standards perhaps it is the most convenient. For standards to be used in the highest class of work it would be better to graduate the distance between 0° and 100° C. into one hundred equal parts, and then allow the observer to accurately determine the value of the corrections at each degree. Indeed, it is preferable for many researches that the whole scale be simply a millimetre one, and care only be taken to have the millimetre graduation extremely accurate.
The dividing of the tube so that an equal volume of mercury may occupy the same number of degrees at the various parts of the tube is called the calibration of a thermometer, and on the perfection of this work, if it is attempted at all, largely depends the value of the thermometer. As Pernet has remarked, the labor of determining the errors of a thermometer is much increased by having to determine the errors the maker has introduced in the imperfect calibration of its scale. In observations not requiring an accuracy beyond 0·1° F., it it might be quite safely left to the skill of a reputable maker to free the instrument from errors of this kind. It is accomplished by detaching a small portion of the column and measuring its length at different, and usually consecutive, parts of the tube. Obviously from these results may be computed the value of 1° at successive parts of the thermometer scale, in terms of the dividing engine used by the maker.
The precision attained in the calibration of standards when the greatest care is exercised is surprising; thus, in the three Kew standards of the Yale Observatory, the maximum sum of the errors depending on imperfect calibration is very nearly 0.01° in each of them.
Supposing that several thermometers, by different and equally skillful makers, have been prepared with the greatest care, it is found in comparing them that they differ sensibly among themselves, owing to the difference in the glass used in their construction, their varying sensitiveness to the slight changes caused by the circulation of the water in which they are immersed, and a variety of less obvious causes. It becomes necessary, therefore, that some definite construction of the mercurial standard thermometer should be adopted, and the standard chosen by the Yale Observatory is defined upon the certificates issued with standards compared, as follows:
"The theoretical mercurial standard thermometer to which this instrument has been referred, is graduated by equal volumes upon a glass stem of the same dimensions and chemical constitution as the Kew standards 578 and 584. The permanent freezing-point is determined, by an exposure of not less than forty-eight hours to melting ice, supposing the temperature of the standard has not been greater than 25° Cent. 77° Fahr. during the preceding six months. The boiling-point is determined from the temperature of the steam of pure water at a barometric pressure of 760 mm. 29·922 inches (reduced to 0° Cent.) at the level of the sea and in the latitude of 45°."
This standard has its 0° and 100° C. identical with the standard of the International Commission of Weights and Measures, and the physicists generally have agreed upon the pressure and latitude given as the most advisable. It is practically coincident with a pressure of 29·905 inches in the latitude of London, and at the sea-level—the conditions under which the 212° point of the English standard is determined under act of Parliament.
A description of the Kew standards referred to is given in the accompanying table:
The tubes of which the Kew standards are made are about twelve years old, and belong to the series purchased by the Royal Society and deposited at Kew to be used as standards.
The essential parts of the water comparator in use at Yale for comparing thermometers is shown in outline in Fig. 1, where a a' a" is a bright-tinned iron cylindrical tank 15 X 20 inches, having an aperture for the stopcock f, a lid a a" with various apertures for the insertion of long thermometers, and having a plate-glass window 4 x 14 inches set in the side. Within this outer tank a smaller copper tank 11 x 15 inches is symmetrically placed, and rests upon wooden bars which are supported by the bottom of the outer tank. A window, placed in the same relative position as the outer window, allows the thermometers which are attached by springs at their upper ends to the adjustable brass disk d', to be read. The brass axis c c' turns in a bearing c', and has attached to it two disks, d, d'. A small cathetometer with its telescope is placed before the, windows, and the number of the thermometer under observation is shown by means of the graduated dial at d. Water of a given temperature is admitted through the tubes at e, and after the temperature has been brought to the degree required, it is thoroughly agitated by moving vertically the ring plunger shown at p p p' p'. The disk d', which is perforated, will accommodate sixty-four thermometers. The agitation of the water having subsided, the thermometers to be compared are read as rapidly as possible, first from left to right and then from right to left. Two standards are read at the beginning and end of the series. It is obvious that, if they are read at approximately equal intervals, the mean of the two readings will be free from the error of radiation caused by the slow cooling or warming of the water. The greater part of the work of the observatory upon standards is done with this comparator. For clinical thermometers a smaller apparatus, constructed on the same general principles, is used; but, as in this case a much less degree of precision is desired than in the investigations of standards, the work may be simplified. It is not necessary, for instance, to resort to the somewhat tedious readings with the cathetometer, and since the thermometers are self-registering, they may be lifted from the water to be read.
The space between the outer and inner tanks is filled with cotton-wool which has been picked with the fingers until it has as little body as possible. The object of this wool is to prevent currents of air, which would otherwise cause a much greater conduction of heat to or from the body of water in the inner tank.
The determination of the freezing-points of standards is accomplished by the apparatus shown in Fig. 2, where a a' is a tinned-iron cylindrical vessel 9 x 9 inches, inclosing a smaller one 7½ X 5 inches. A strainer allows the water from the finely crushed ice or snow to escape into the open space b', and the space between the outer and inner vessels is filled with cotton-wool. Close-fitting covers prevent currents of air from the outside, and when in use each thermometer is fitted to a cork which is imbedded partly in the ice.
One boiling-point apparatus is constructed after Regnault's plan, and consists essentially of a brass stand (Fig. 3) supporting a water-tank w w', 6 inches in diameter and 3∙5 inches deep, upon which in turn rests a brass section of double tubing having an inside diameter of 5 and an outside diameter of 6 inches. This section, which extends upward 3·1 inches, has three open tubes each 0∙7 inch in diameter (v v') let into its outer wall. At the place which would be occupied by the fourth there is a small manometer-tube m, with a stopcock s, by which Fig. 2. the difference of the pressure of the steam inside and the air outside may be noted. Any one of a series of four brass double cylinders, ranging in height from three to twelve inches, may be fitted to this first section by a telescope-joint at will. Each of these double cylinders has perforations at its top for the insertion of thermometers. Around the top of the inner cylinder there is a series of ten holes, each three fourths of an inch in diameter, to allow the steam to pass from the inner chamber to the outer, and thus through the vents v v' to make its escape. When in use, the tank 10 w' is filled with pure water, taking the precaution to put several feet of brass ribbon in the bottom to equalize the boiling; and the heat is communicated by means of the Bunsen burner b'. The thermometers are suspended as at t t', with their bulbs at b.
Another boiling-point apparatus, to be used for very long thermometers, and where it is desirable to take the greatest care in the boiling-point determination, is made entirely of glass. The thermometer is completely immersed in steam, and the readings are made with the cathetometer by looking through the glass and steam which surround the thermometer.
A standard barometer, wrapped in cotton-wool and cloth to prevent rapid change, in the temperature of its mercury, and made by James Green, is hung on the same level as the boiling-point apparatus, and the thermometers are read alternately with the barometer. The cathetometer is used for reading thermometers in both the boiling and freezing-point apparatus.
For the calibration of tubes, two microscopes have been mounted so that the position of the two ends of a short mercury-column may be
read at the same time by means of eye-piece micrometers. The observatory is having built a comparator especially for this work, which will soon be mounted in its place.
By far the most valuable apparatus in connection with this work is the collection of foreign standards which have been obtained to represent the work of foreign observatories. This collection comprises seventeen standards of the highest class, eight working standards, and forty-five comparison thermometers. The makers comprise noted artists of Europe, and among them are the Kew Observatory; Baudin, Fastré, Tonnelot, and Alvergniat, of Paris; Fuess, and Greiner & Geissler, of Berlin; and Casella, of London.
The comparison of the important standards was undertaken by the Kew Observatory in England, the Seewarte at Hamburg, and the Imperial Commission of Weights and Measures, under Dr. Foerster, at Berlin. There can be little doubt, therefore, that the observatory of Yale College possesses an accurate copy of the standard thermometers now in use in the prominent observatories in Europe.
It is the object of the observatory to make this service as widely popular as possible; and it particularly desires to be useful to the physicians, meteorologists, and the commercial manufacturers who have occasion to use fairly accurate thermometers. The testing of illuminating oils, the manufacture of spirits and ethers, and the numerous operations of the chemical laboratory, require thermometers of considerable accuracy, and for the benefit of persons using such the observatory has issued a circular which will be mailed on application.
Thermometers may be sent by mail or express, directed to the Winchester Observatory, New Haven, Connecticut. If they are sent by mail (and nothing larger than a clinical thermometer should be), they ought to be packed in a wooden box, in tissue-paper. In whatever manner they are sent, a little care taken in packing them in soft paper will materially lessen the risk of accident. Ordinary thermometers are returned to the senders, with certificates stating their deviation from the true mercurial standard for every ten degrees, within a few days from the date of their reception. Standards require from a week to a month for their investigation, depending upon the degree of precision desired in the final certificates.
The official circular of the observatory contains detailed information relating to the supervision of hospital thermometers and the facilities offered to makers. There is no good reason why any maker should not furnish, with any thermometer sold, a certificate stating the errors of that particular instrument. That the service will be a popular one is shown by the fact that already about five hundred thermometers have been sent to the observatory for verification, and not the least benefit will be that the errors of every thermometer issued with a certificate will be on file at the observatory, and this will be of particular value in cases where a uniformity of data is desired, as in the case of the United States Signal Service, or the observations made by isolated meteorologists in different parts of the country.