THE MANOMETERS. 



41 



threaded plugs 12, 13, 14, 15, and 16. The instrument has been tested 

 and found to be mercury-tight up to 350 atmospheres. 



Pure mercury only is put into the block, but it can not be presumed, 

 under the prevailing conditions, to maintain its purity unimpaired; 

 hence some precautions are necessary to prevent contamination of the 

 mercury in the instruments under investigation or a fouling of the 

 glass walls of the tubes. The usual precaution is to fuse the calibrated 

 portion of the manometer to one end of a glass tube of nearly equal 

 bore, which has been bent to a double U form. In the intermediate 

 limb a bulb is blown that 

 serves as a reservoir of 

 pure mercury for use in 

 the manometer proper. 

 Having filled the instru- 

 ment with pure mercury, 

 it is fastened in place in the steel block. The 

 arrangement for adjusting the height of the 

 mercury in the tube under examination and 

 for determining capillary depression by differ- 

 ence of level consists of a glass tube having 

 an internal diameter of 35 millimeters, which 

 is connected, by means of a rubber tube, with 

 a second glass tube occupying one of the holes 

 in the steel block. In order to render the 

 rubber tube sufficiently rigid, and thereby to 

 avoid unnecessary oscillations of the mercury 

 meniscus, it is tightly wound with several thick= 

 nesses of insulating tape. The remaining hole 

 in the block is usually occupied by a tube whose 

 capillary depression has been investigated in 

 great detail. 



Formerly it was attempted to determine 

 capillary depression by means of comparisons 

 with a standard, i. e., by dispensing with the 



wide tube mentioned above and inserting in one of the holes of the 

 steel block a tube whose capillary depression in every part was known. 

 This is a much more convenient method, but it was abandoned because 

 it was found that the errors of the standard add themselves to those 

 of the other instrument. The same difficulty makes itself felt when 

 it is attempted to compare one manometer with another. In such 

 cases it is impossible to tell to what extent the observed discrepancy 

 is due to the incorrectness of the values assigned to the capillary 

 depression of each instrument. It is as likely to be the sum as the 

 difference of the two. In any event, it is, of course, their algebraic sum. 



Another important instrument in connection with the investigation 

 of manometers is the " tapper " seen in Figure 23. In the measurement 



Fig. 23. 



Electric hammer for tapping 

 manometers. 



