THE TESTING OF CLINICAL THERMOMETERS.

By C. W. WAIDNER and L. A. FISCHER.

Soon after the organization of the Bureau of Standards work preparatory to the testing of clinical thermometers was taken up." At the same time all of the prominent manufacturers of clinical thermometers were requested to submit samples of their product in order that the Bureau could determine what accuracy might be expected and what defects were most common.

The design and construction of the apparatus required for this purpose was undertaken with two main objects in view: first, that the highest accuracy should be obtained, and second, that the tests could be made so rapidly that the fees for testing would be reduced to a minimum. The apparatus finally adopted has been in use for some time, and the methods of testing have been subjected to exhaustive trials. As both the apparatus and methods used by the Bureau have been the subject of numerous inquiries, it is deemed advisable to publish a full description for the use of those interested and also an account of the experiments upon which the regulations finally adopted are based.

The first tests made showed that many of the clinical thermometers had errors as large as 0.5 or 0.6~ F., and in some cases even larger errors were found. Moreover, a study of groups of thermometers suggested that the standards used were in error. The manufacturers were accordingly requested to submit their standards to the Bureau for examination. Without exception this request was complied with, and, as suspected, the conclusion that the standards were in error was confirmed.

a The work was begun in the section of weights and measures under the direction of Mr. Fischer, and was subsequently transferred to the section of thermometry, upon its organization, under the direction of Dr. Waidner.

CLINICAL STANDARD THERMOMETERS.

Some of the standards submitted were found to be radically defective in construction. Some were so constructed that when used in the water bath for the pointing of ordinary clinical thermometers the stems projected above the surface of the bath by amounts corresponding to 60F. and even more. Since changes of 20° F. or more occur in the temperature of rooms where clinical thermometers are graduated, and since the temperature of the exposed portion of the stem is largely controlled by the temperature of the room, there is here presented a possible source of error of about 0.1° F. Another defect common to most of the clinical standards submitted was the absence of the ice point (32 F.) from the scale. The presence of this point is important, since it enables one immediately to detect any change in the indications of a thermometer by determining the ice point in a mixture of finely crushed pure ice and distilled water. If any change is detected in this point the reading of the thermometer may then be corrected. It is true that most of the standards used by manufacturers are quite old, and hence changes in these thermometers would probably be small. Nevertheless changes might occur on account of the development of minute air bubbles or other causes, which, as before stated, would be readily detected by a determination of the ice point.

A form of clinical standard well adapted for the pointing or testing of clinical thermometers is shown in fig. 1.

Immediately above the ice point is an enlargement of the bore, which makes it possible to obtain on a short stem the ice point and also the range of temperature required in clinical thermometry. Above the enlargement there are graduations corresponding to 0.1 and covering the range 90 to 110 F. In ordinary use these standard thermometers are immersed in the water bath to about the 93 mark, so that not more than 17 is emergent from the bath, and hence the variations in the temperature of the room may be neglected. The corrections to these thermometers are carefully determined by comparing them with the primary standards of the Bureau.

These latter are made of the usual thermometric glasses, namely, French hard glass or Jena 16 normal glass, and are artificially aged. before final filling, by exposing them to a temperature of about 850 F. for at least 60 hours, after which they are allowed to cool slowly. When treated in this manner subsequent time changes are extremely small. The graduations on these thermometers are very fine, whereas some of the standards submitted to the Bureau for examination were only graduated into 09.2 F., and the graduation lines were nearly as wide

FISCHER.

as half an interval. Under such conditions it is easily possible to make an error of several hundredths of a degree in reading the thermometer. It might seem at first sight that errors of this magnitude are negligible in clinical thermometry, but they are sufficient to make the corrections to large numbers of clinical thermometers 0°.1, while if the correct temperature of the bath had been known when the clinical thermometers were pointed the corrections resulting from the tests of the Bureau would have been given as 0°.0. To illustrate, if the observed correction is 0.05 or less it is given as 0°.0, while if 0°.06 had been observed the correction would have been given as 0°.1.

To facilitate the use of standards of proper construction and of a uniform scale of temperature the Bureau has loaned a few of these specially constructed standards to a number of manufacturers for brief periods. The Bureau has also pointed a number of standards made and submitted by the manufacturers for this purpose. The result of this work has been greatly to reduce the errors of clinical thermometers, those now submitted for test having, on the average, errors less than one-half as great as those first submitted.

SCALE OF TEMPERATURE.

The advantages resulting from the universal use of one and the same scale of temperature are almost self-evident, but notwithstanding this fact a number of slightly different scales were found in use by manufacturers, all of whom supposed their standards to be correct. Since frequent inquiries are made as to the scale of temperature used by the Bureau, the question will be briefly considered here.

The interval covering the ordinary range of temperature is fixed by the temperature of melting ice and by the temperature of steam under normal pressure." These two temperatures are universally used to determine the so-called fixed points of thermometers of various kinds.

It is well known that the indications of mercury thermometers constructed of different kinds of glass may differ very materially from one another, after making all necessary corrections to their readings for errors in the fixed points, and for nonuniformity in the caliber of the tubes. These differences, due to the fact that different glasses have different coefficients of expansion, may amount to more than 0°.2 F. Thermometers made up from the same pot of glass, or even from the same piece of tubing that have been worked differently in the blast flame, will often show very appreciable differences in their temperature scales. For these reasons mercury in glass thermometers are not suited for establishing a standard scale of temperature.

a The pressure of 760 mm of mercury at 0° C. and at sea level and latitude 45°.