447 



G and H glass valves as introduced hy Karovodink in Töpt,er- 

 H A GEN 's mercury pump. 



Fig. 2 shows one of these valves ; n? is a ground 

 sui-face. Inside the glass tube a small piece of iron 

 is fastened : without a current through the electro- 

 magnet M the communication is open to the gas; 

 when the curr-cnt is closed the vapours can only 

 diffuse through the narrow interstices left open at 

 a. The measurements seem to confirm that hy the 

 use of these valves a better vacuum may be obtained. 

 The calibration was performed in the following 

 manner. By means of the high-pressure taps D and 

 C some carbon dioxide was taken from the cylinder 

 A containing carbon dioxide which had been tho- 

 roughly dried and distilled several times. This gas 

 was solidified in E by means of liquid air and sub- 

 sequently strongly exhausted with a GAEDE-pump. It was then distilled 

 into pipette p^ and the pressure read with the manometer F. The 

 pipettes Pj, p^ and P.^, which communicated with the heated-wire 

 manometer were evacuated and the vacuum was measured with the 

 aid of this manometer, which had been previously compared with 

 an absolute manometer. The gas in p^ was then distributed overy^^, 

 Pj, and p^. If stop-cock 3 is now closed and 4 opened, the gas in 

 P2 is distributed over p^ and P.^, by which the pressure in P,^ in- 

 creases by a known amount. B3" connecting p^ with P^ and with P^ 

 successively the pressure in P^ again increases by an amount which 

 is known, at least if the dimensions of the apparatus are known. 

 They were : 



Fiff. 2. 



Each time when the pressure had been raised by a known amount 

 the loss of heat of the WoLLASTON-wire under definite conditions 

 was measured. The following table may serve as an instance of the 

 calibration : q is proportional to the loss of heat per second and p 

 is the pressure in baryes computed by means of the system of 

 pipettes. The initial i)ressure is taken as zero in the table ; in reality 

 it amounted to 0.029 baryes. 



