16 



For the very finest effects the barometer tube can be exhausted 

 by a Sprengel or other high vacuum pump, but in this case the 

 rubber-tube connections must be replaced by glass and fused joints. 



38. It may be remarked here that the very high vacua with which 

 we are familiar nowadays, in X-ray and other such tubes, are by no 

 means essential, except in the highest grade of " normal " iDarometers 

 (see par. 45), where results depend upon the absolute height of 

 the mercurial column.. In the case of instruments in which a cor- 

 rection is found by comparison with a normal, and especially in baro- 

 graphs where the results depend entirely upon differences in the posi- 

 tion of the mercurial column, simple methods of filling give entirely 

 satisfactory results. In these cases the gaseous pressure in the 

 vacuum is so nearly constant that no serious error is involved. 



Suppose, for example, that the residual air in a barograph tube 

 exerts a pressure of 0.1 inch, which would be inexcusably bad fill- 

 ing. Now, since we set the pen of the barograph to agree, from time 

 to time, with a standard barometer, the only effect the air can have 

 is such as results from changes in pressure due either to changes 

 in temperature or changes in the volume of the vacuum chamber. 

 A 20° change of temperature between settings of the pen is not 

 usual but would introduce an error in this case of only about 0.003 

 inch, whence, with reasonably good filling, the errors from imperfect 

 vacua are entirely insignificant. 



39. Errors of harometers. — No matter how carefully a barometer 

 may be made certain errors due to various causes can hardly be 

 eliminated. In the first place, if any residual air or vapor or any 

 kind of gaseous matter remains in the top of the barometer tube, 

 the column of mercury will not rise as high as it should. We know, 

 likewise, from physical laws, that the capillary forces acting between 

 the free surface of mercury and the glass walls at the top of the 

 column also operate to prevent the mercury from rising as high as it 

 should in the tube. Still other errors arise from faults in the 

 graduation of the scale and from failure to place it and the vernier 

 at exactly the positions they should occupy. 



It is not practicable, or necessary, as a rule, to determine these 

 errors separately. When an instrument is completed, its readings 

 are carefully compared with those of a standard barometer. The 

 differences found in this way represent the outstanding effect of the 

 several sources of error mentioned above and are commonly called 

 the " correction for instrumental error and capillarity." 



40. There is still another source of considerable variation in the 

 readings of mercurial barometers, namely, the influence of temper- 

 ature, a rise of temperature expanding both the metal scale and the 

 mercurial column. If both mercury and scale expanded equally no 

 correction would be necessary, but the mercury expands much more 

 than the scale, so that a large correction is required for temperature. 



41. The following detailed discussion of the several errors men- 

 toned above will make the matter more clear. 



(1) Corrections for capillarity. — In all barometers having com- 

 paratively small tubes, that is, of less diameter than from 0.7 to 1 

 inch, the top of the mercurial column, or the meniscus, as the rounded 

 surface is generally called, will nearly always be quite convex on 

 account of the capillary action between the mercury and the glass. 



