THE MANOMETERS. 



39 



CAPILLARY DEPRESSION. 



Before joining the calibrated to the uncalibrated portion of the ma- 

 nometer, the former must be subjected to a thoroughgoing investigation 

 of its capillar depression. The mean diameter of the bore of the 

 whole tube is known, that having been calculated from the length 

 and weight of the long thread of mercury which is used in the calibra- 

 tion ; also the mean diameters of a considerable number of short spaces, 

 these having been calculated in the same manner from the weight of 

 the short thread and its length in different parts of the tube. But, 

 though such data are useful as a means of judging the excellence of the 

 tube for manometric purposes, they can not be relied upon for the 

 derivation of the capillary depression. 



The mean capillary depression of the mercury in the manometer of 

 smallest bore amounts to 18 millimeters, i. e., to more than 0.023 

 atmosphere. In the remaining instruments, the average depression is 

 about 15 millimeters, or 0.02 atmosphere. The real difficulty with 

 the capillary depression is due to the fact that in most tubes it varies 

 frequently and largely within short distances. In addition to these 

 sharp local fluctuations, there is nearly always a gradual increase or 

 diminution of the depression due to a corresponding general change in 

 the diameter of the bore, the diameter at one end of the tube being 

 usually larger than at the other. 



Owing to the large changes which may occur within short distances, 

 it is necessary to determine the amount of the capillary depression at 

 a great many points in a tube. By way of illustrating the importance 

 of doing so, the following partial record of the capillary depressions 

 which were found at different places in one manometer is given. In 

 one column of the table, there are recorded the distances above the 

 lower " scratch" at which observations were made; and in the other, 

 the depressions which were found at these points. 



Table 2. 



A difference of 1 millimeter in the capillary depression is equivalent 

 to about one calibration unit in determining the volume of the nitrogen 

 in the manometer. Suppose now the capillary depression of this tube 

 had been determined only at nine points, beginning with the second 

 one, 22.7 millimeters above the scratch. The mean of the values 



