414- STANDARDISATION AND CALIBRATION OF APPAEATUS. 



"VI. Lactometers.— Lactometers are graduated by taking the specific 

 gravity of several samples of milk which have had the density determined 

 by a pycnometer ; tlie range of specifio gravities should be fairly vpide ; 

 no lactometer showing differences of more than 0-0002 (0 2°) should be used, 

 unless the differences are constant, when a constant correbtion may be 

 applied. 



VII. Thermometers.— One thermometer should be specially calibrated, 

 and this will then serve as a standard of comparison for others. The 

 calibration is divided into two parts. 



(a) Calibration of scale. 



(6) Determination of fixed points. 



(a) Oalibration of Scale. — By means of a finely-divided rule the distances 

 between the marks on the scale (e.g., to 10, 10 to 20, &c. ; or to 5, 5 to 

 10, &c. ) are measured and tabulated. 



The mercury is allowed to flow into the stem, and at a point, which 

 should be as nearly as possible 10° from the end, the tip of a fine flame 

 is carefully appUed ; by a gentle jerk a thread about 10° in length 

 can be separated from the main portion, which is now allowed to flow 

 back into the bulb. By gently tapping the tube, the thread is brought 

 BO that one end coincides with the zero mark, and the length of the thread 

 is carefully measured ; the thread is next brought to the 10° mark, and its 

 length carefully measured again ; and so on throughout the whole scale. 



By dividing the lengths of the thread when it is between each pair of 

 points (0 to 10, 10 to 20, &c.) by the distance between the same pair of 

 points, the length of the thread in apparent degrees will be obtained ; the 

 average of these lengths will give the mean length of thread in mean 

 degrees. By dividing the length of thread between each pair of points by 

 the mean length, the value of a degree between each pair of points in 

 terms of a mean degi-ee will be obtained ; and, on multiplying by ten, the 

 distance between each pair of points in mean degrees will be obtained. 

 The values in mean degrees of the scale from to 10, to 20, &c., should 

 now be calculated, and also the lengths of scale between the same points. 

 A curve of conicality can be plotted for the thermometer in the same way 

 tliat a similar curve was plotted for a burette [q. c. ). 



(6) Determination of fixed Poiiitn. — A flask with a long neck is partially 

 filled with water and placed over a flame ; a shallow cork, with two holes, 

 is fitted to the neck, and through one of the holes the thermometer is 

 passed ; in the other a short bent tube to take the steam away from the 

 operator is placed. The water is boiled briskly, and the thermometer 

 pushed in till only the top of the mercury is visible, and left in this position 

 for several minutes. The exact point on the scale where the top of the 

 mercury rests is now noted ; the atmospheric pressure is read, and, from 

 the table below, the boiling point of water is taken ; the difference between 

 this and 100° (or 212° if a Fahrenheit thermometer is used) is now added to 

 (or subtracted from) the scale reading of the thermometer, and the value 

 thus obtained noted as the true value of 100° C. (or 212° F. ). 



The thermometer is now removed, allowed to cool, and placed in melting 

 ice ; when the mercury is .stationary, the position of the top of the 

 mercury is noted as the freeziu(j point. 



The difference between the observed boiling and freezing points is taken, 

 divided by 100 (or 180 if a Fahrenheit thermometer is used) ; the values in 

 mean degrees of the scale from to 10, 10 to 20, &c., are multiplied by the 

 value thus obtained, and the corrected value tabulated. The differences 

 between these and the nominal values of the scale are now plotted on 

 squared paper, and will serve as a curve of correction of the instrument. 



It is advisable to redetermine the boiling and freezing points from time 

 to time, as they are liable to slight alteration. 



Other thermometers may be standardised by comparison with this one. 



