ATMOMETRY AND THE ATMOMETER 107 



the time' when it ceased operating. It is therefore very essential 

 that the evaporating surface be not allowed to alter between the 

 time when the cup is removed from operation and that at which 

 it is restandardized. Cups to be restandardized should be 

 dried, carefully wrapped in clean paper, and stored in a dry place 

 until they can be restandardized. 



If ^standardization is to be attempted by the user of the 

 instrument, it is absolutely essential that there be at hand a 

 cup of known coefficient, of the same form as those to be re- 

 standardized. This cup must never have been used since it came 

 from its own standardization, having been kept dry and prop- 

 erly wrapped meanwhile. All other methods of standardization, 

 excepting by means of such a cup, have proved unsatisfactorj^. 

 If possible, the restandardization should be performed by means 

 of a rotating table, preferably indoors with an electric fan to 

 increase the rate of evaporation. The operation is the same 

 as that already described. After restandardization the cups are 

 air dried before being wrapped for preservation. 



Where a rotating table is not available, the cups may be 

 standardized by placing them (with the cup of known coefficient) 

 in the open, in a place where all will receive equal exposure to 

 sun and wind and where they will be as free from dust as pos- 

 sible. Rain falling upon them vitiates the results, and may 

 alter cups which have easily removable material adhering to the 

 outer surface. It is best not to allow rain to fall upon the cups 

 during standardization. 



Cups may also be standardized indoors, though not satisfac- 

 torily, by arranging them about 50 cm. apart in each direction, 

 in a room which receives no direct sunshine and in which the air 

 conditions are as nearly equable as possible. The group of in- 

 struments should be in the center of the room, not in the vicinity 

 of furniture or other objects, and the room is to be kept closed 

 to avoid drafts. A small group may be placed near the center 

 of a large table, but it is not desirable to have any cups near 

 vertical air currents such as occur about the margin of a table; 

 it is better to place the instruments on the floor. No prevent- 

 able air movement is to be allowed; it would surely affect the 



