A. L. Day and J. K. Clement — Gas Thermometer. 417 



of the room near the ceiling and connected with the lower 

 reservoir by a flexible iron tube. Cocks conveniently arranged 

 admitted mercury whenever required. The tine adjustment of 

 the mercury level was obtained by the use of a nickel diaphragm 

 which formed the bottom of the lower steel reservoir. This 

 diaphragm was about 12 cm in diameter and could be raised 

 slightly by the upward pressure upon its center produced by 

 turning a milled hand screw convenient to the hand of the 

 operator. The lower reservoir was dome-shaped within and 

 opened into a tube and stop-cock at its highest point, through 

 which any air which might chance to become imprisoned within 

 the reservoir might be allowed to escape. 



Gas was admitted to the bulb by means of the three-way 

 cock A (fig. 3) leading to a supply of pure nitrogen, the pres- 

 sure of which could be varied at convenience. It was also 

 possible to exhaust the bulb through the same cock for the 

 purpose of testing for leakage or rinsing the bulb. 



Unhealed Space. — From the point of view of the errors of 

 the instrument, the most important part of the manometer is 

 the nickel cap at the top of the short arm which carries the 

 fixed point for defining the constant volume. This cap 

 is sealed into the glass manometer tube with ordinary seal- 

 ing wax of good quality, some care being taken that the seal- 

 ing wax fills all the cracks, which might otherwise retain gas 

 and become a part of the unheated space. The under side of 

 the cap is hollowed out slightly to conform to the shape of the 

 rising mercury meniscus, and in the center a somewhat rounded 

 point of nickel projects downward about -3 mm . When the 

 column of mercury is raised in the arm until it becomes tan- 

 gent to this point, the constant volume of the system is deter- 

 mined. The setting is. made through a fixed magnifying 

 microscope of some twenty diameters power. The portion of 

 the " unheated space " included above the column is about 'S mm 

 thick, l cm in diameter, and corresponds in form to the mercury 

 meniscus. 



The outlet leading to the bulb is a small opening beside the 

 contact point containing a tiny valve of nickel about l-5 mm in 

 diameter and 2 mm long, with a ground joint at the top which 

 slides loosely in such a way that if an accidental rise of the 

 mercury column should tend to drive the mercury over into 

 the bulb, this little nickel ping will be lifted by the mercury 

 and automatically close the opening at the ground joint. This 

 tiny valve opens into the capillary (/5 mm diameter) leading 

 outward to the bulb. Fig. 3 will show the construction more 

 clearly. Where the space above the mercury column requires 

 to be reduced absolutely to minimum volume some such pro- 

 tection is essential. If mercury once passes this opening, 



