3. From time to time a zero-point test should 

 be made in order to determine casual variations 

 of the freezing point. 



4. If some defect of the instrument has been 

 discovered and according to our directions has 

 been corrected, we recommend for absolute 

 safety with regard to perfect functioning, the 

 check of the freezing point mentioned under 

 "3." 



5. When the reversing thermometer is in a 

 reversing frame or is mounted on a water 

 sampler, do not permit this to be tripped in the 

 air, as the thermometer can be damaged by the 

 hard impact. 



Safe function of reversing thermometers is 

 only assured in water. 



IV. Ice-point determinations 



The undertaking of an ice-point test is from 

 time to time necessary in order to establish 

 random changes of the instruments. These 

 tests become especially important if a thermom- 

 eter in disorder has been successfully repaired. 

 The following description gives a reliable ice- 

 point determination to within 0.015° C. 



The thermometer will be stood up in finely 

 shaved, pounded, melting ice up to the begin- 

 ning of the graduations. Air space surround- 

 ing the reservoir is to be guarded against; 



the ice should therefore be stirred from time to 

 time. A standard thermometer for the ice 

 point, graduated in %o° or Koo° C. is recom- 

 mended in order to establish an exact zero 

 point of the ice. 



A one-fourth hour rest of the thermometer 

 suffices. Then the thermometer will be re- 

 versed with the ice chamber and extracted. 

 In this reversed position it is again placed into 

 the ice and the main and auxiliary thermom- 

 eters are read off after one-half hour. In the 

 ice-point reading, it is necessary that the 

 thermometer be lifted and shaken in the ice 

 to avoid adhesion of the mercury in the capil- 

 lary; especially with a narrow capillary ()2o° 

 and }^o° C.) and such with a widening above 

 zero, it is important to ascertain the accurate 

 level while tapping. 



For repeated ice-point determinations of a 

 large number it is recommended that one 

 supply oneself with a special apparatus con- 

 sisting of a reversible, double-walled and in- 

 solated cylinderlike ice bath. The ice-point 

 determination of the D. A. E. showed an 

 individual curve of ice point changes for each 

 instrument, so that for exact investigations 

 extending over a long time this test cannot be 

 ignored. 



170 



H. O. 607 



