THE MANOMETERS. 37 



THE UNCALIBRATED PORTIONS OF THE MANOMETERS. 



It will be noticed (Figures 18, 19, 20, and 21) that the uncalibrated 

 portion of all manometers is provided with two or three bulbs, or their 

 equivalents in the form of inserted short pieces of tubing of larger 

 diameter. The bulb nearest the calibrated end (Figures 18 and 19, 3; 

 20 and 21, 2) serves as a reservoir in which the nitrogen, when under 

 diminished pressure, may expand without danger of escaping from the 

 instrument. Its capacity is regulated by the volume of the gas to be 

 accommodated, i. e., by its original or usual volume, and the maximum 

 probable amount of diminished pressure to which it will ever be 

 subjected. The bulbs nearest the cell (Figures 18 and 19, 1 and 2; 20 

 and 21, 1) serve as reservoirs for the mercury which is to be driven 

 forward in compressing the nitrogen, and their total capacity is, there- 

 fore, to be regulated by the volume of the gas under ordinary conditions 

 and the maximum pressures to be measured. 



For reasons which will appear later, none of the bulbs should be 

 made unnecessarily large. The requirements of the situation may be 

 reduced to the simple rule that some mercury must be left in the bulb 

 nearest the manometer proper under the lowest pressure, and some in 

 the bulb nearest the cell under the highest pressure. It will be noticed 

 that bulbs 1 and 2 in Figures 18 and 19, and their equivalents (1 in 

 Figures 20 and 21) in other manometers, are provided with traps. By 

 means of these, the mercury is made to enter the narrow tubes below at 

 points somewhat above the bottom of the bulbs. The purpose of the 

 arrangement will be understood from the following explanation : When 

 the solution in the cell is under pressure, it drives the mercury before it 

 and enters to some extent the upper end of the nearest bulb. When the 

 pressure is afterwards removed, and the mercury which had been 

 expelled returns, it is apt to entangle minute drops of the solution be- 

 tween itself and the wall of the bulb. Occasionally, during the subse- 

 quent movements of the mercury in the tube, one or more of these 

 drops will persistently work its way forward toward the calibrated end 

 of the manometer, making it necessary, sooner or later, to open, cleanse, 

 and refill the instrument. The "traps" are an effectual prevention of 

 such calamities. Before their introduction, it was frequently necessary 

 to inspect the manometers for the presence of these migrating particles 

 of liquid, and it happened at times that, notwithstanding the greatest 

 vigilance, they escaped detection until it was discovered that the ma- 

 nometers were no longer measuring correctly. Straight tubes, because 

 of their greater strength (Figures 20 and 21, 1 and 2), are used for 

 mercury reservoirs instead of bulbs (Figures 18 and 19, 1, 2, and 3) 

 when high pressures are to be measured. 



The manometers shown in Figures 18 and 19 have no vents. They 

 are suitable for use in the arrangements seen in Figures 9, 10, and 11, 



