23g Tables 45 and 46 (concluded) 



REDUCTION OF THE MERCURY COLUMN TO STANDARD TEMPERATURE 



Fixed-cistern barometers. 2 — Many commercial millimeter and millibar mercurial barom- 

 eters make use of a cistern of relatively large cross section in which the level of the 

 mercury changes by an amount which is small compared with the change of level in the 

 tube. Readings are made on the mercury level in the tube only, and the level in the 

 cistern inferred from that in the tube. 



The temperature correction for these barometers is usually somewhat larger than for the 

 Fortin-type barometers. If all dimensions and materials of the barometer are known, it 

 might be possible to calculate this correction. It is usually preferable to determine the 

 temperature correction experimentally by calibrations at two or more temperatures. It 

 has been found to be possible to obtain sufficient accuracy by using the temperature cor- 

 rection tables prepared for the Fortin barometer by entering them with the quantity 

 B -\- k where B is the height of the mercury column above the level in the cistern and k 

 is a quantity characteristic of the design of the barometer. 



In the absence of a temperature test the cistern constant k of a barometer with a metal 

 cistern can be estimated with an uncertainty of 20 percent by the formula 



4A 

 where 



V = total volume of mercury in the barometer, 



A = sum of the horizontal cross-section area of the cistern and of the horizontal 

 cross-section area of the tube at the top of the mercury column. 



Both A and V are to be measured in terms of the units of the barometer scale itself. 



The gravity correction for these barometers is the same as for the Fortin barometer. 



The scale interval of these barometers is often modified so that there will be no cor- 

 rection when the instrument is at some specified temperature and gravity. In this case 

 the correction to be applied is the difference between the usual correction for the ambient 

 temperature and gravity and that for the temperature and gravity for which the barometer 

 is calibrated. 



Table 46 has been extended to 1200 units in order that the method outlined above may 

 be used for fixed-cistern millibar barometers. 



U-shaped manometers. — The corrections for temperature given in Tables 45 and 46 are 

 equally applicable to U-shaped mercurial manometers having brass scales calibrated in 

 metric or English units respectively. In this case the "height of the mercury column" is 

 the difference in height between the two arms of the manometer. Tables 45 and 46 are 

 extended to include corrections for the smaller heights usually encountered in working 

 with manometers. 



' National Bureau of Standards, private communication, April 7, 1949. 



SMITHSONIAN METEOROLOGICAL TABLES 



