TEMPERATURE. 7 



When the thermometer is not to serve as a standard, the labour of 

 calibration may be saved by comparing the readings with some standard 

 instrument, and observing the deviation from the true reading at 

 various points along the scale. Such comparisons are undertaken by 

 the National Physical Laboratory, and a certificate is issued with each 

 thermometer compared, stating its errors. 



Precautions in Use. In order to obtain consistent values for a given 

 temperature, certain precautions must be observed in using the mercury- 

 glass thermometer. If the instrument is first used for a low tempera- 

 ture, then exposed to a high one, and lastly brought back to the first low 

 temperature, it will give a lower indication than before. The effect is 

 entirely due to the glass, which does not on cooling at once contract to 

 its original volume. In the course of days or weeks, however, it does 

 return to that volume. The effect may be well observed by immersing 

 a thermometer in melting ice till it is at 0, then putting it into steam 

 for twenty minutes or so and then returning it to the ice. The zero 

 point will be found to be depressed by an amount differing with the kind 

 of glass used, ranging from about Ol C to perhaps 0*5 0. With some 

 kinds of glass the depression is nearly proportional to the high tempera- 

 ture reached, but with others the relation is not so simple. If time cannot 

 be allowed to eliminate the effect, the thermometer, if of English make, 

 should have its zero point re-determined immediately before being used for 

 any temperature lower than the high one to which it has been subjected. 

 Abroad it is usual to mark the fixed point 100 before the point in 

 graduation, and with such a thermometer the zero point should be re- 

 determined immediately after the reading of an intermediate temperature. 



We have already mentioned the gradual contraction of the bulb and 

 consequent rise of zero point. Though after a few years this becomes 

 very small, it may still be sensible for delicate instruments, and it is 

 necessary, therefore, to find the zero point at intervals and subtract the 

 rise from the indication on the scale. Dr. Joule observed the rise of 

 zero point on two delicate thermometers at intervals during forty 

 years, and the results obtained for one of them (Scientific Papers, 

 vol. i. p. 358) may be represented by the curve in Fig. 5. It practically 

 coincides with the curve, 



the height y being in arbitrary divisions of the stem, 13 divisions to 

 1 F., and t being the time in years from 1844. 



The total rise in thirty-eight years was 1 F., and if the curve truly 

 represents the results, it appears to show that it had still, in 1882, about 

 Jjyth of a degree Fah. to rise, and that it will halve its distance from 

 the final value about every ten years. 



In taking a temperature the whole instrument should if possible be 

 at that temperature. If, for instance, the bulb alone is in a hot liquid 

 while the stem emerges into the colder air, not only is the stem con- 

 ducting heat from the bulb and keeping it at a lower temperature than 

 the liquid, but the glass and the part of the mercury in the air have too 

 small volumes, and on both accounts the temperature indicated is too low. 

 There are formulae for correcting for the emergence of the stem, but they 

 are unsatisfactory, and where possible their vise should be avoided. 



