30 minutes)^ and the 20 gallons of water in the cold supply 

 tank of the auxiliary system is cooled by the refrigerator to 

 a temperature substantially below that in the main tank. 



At the start of the calibration the Mueller bridge 

 is checked and the ratio arms of the bridge are adjusted to 

 equality if necessary. A warmup time of one-half hour is 

 allowed for the null detector. 



SCALE CALIBRATION 



The thermometers are allowed to stabilize in the 

 constant temperature bath for a period of at least 10 minutes 

 before they are reversed. During this time the needle valve 

 in the supply line is manually adjusted by the operator to 

 permit continuous introduction of cold water at a rate that 

 will exactly offset heat gain from the room through the tank 

 walls. Temperatures during this last 10-minute period are 

 routinely held to ±0. 001 °C. The rack is reversed and the 

 thermometers, both auxiliary and main, are read. The 

 correction for the auxiliary is immediately logged in the 

 AUXILIARY INDEX column of the data sheet, whereas the 

 exact reading of the main is entered in the FIRST REVERSAL 

 column. 



After the reading, the rack is righted, and the 

 drainage of mercury back to the original reservoir is aided, 

 if necessary, by lifting the tank lid and bumping the ther- 

 mometers in their respective spring-loaded cups. The 

 temperature is again stabilized for 10 minutes, the rack 

 reversed, and only the main stems read and recorded in 

 the SECOND REVERSAL column. Tank temperature is 

 then raised to the next desired point and the procedure is 

 repeated. 



V DETERMINATION 







The volume of mercury in the reservoir when the 

 reading is 0°C is determined after the scale corrections 

 have been completed. The thermometers are righted in the 

 tank and the temperature lowered to 0°C. After stabilizing 

 they are reversed and read, then righted, the mercury is 



23 



