HOGG. — FRICTION IN GASES AT LOW PRESSURES. 13 



gauge measurements reliable for hydrogen. Recently Scheel and 

 Heuse 9 have applied a membrane manometer, devised by them, to test 

 Boyle's Law, and McLeod gauge measurements for air, and they state 

 that they find both valid clown to about 0.0001 mm., provided proper 

 care is taken in drying the gas. 



An examination of the results at pressures less than 0. 1 mm. showed 

 that the equation given above, viz. : 



l)p = c, 



I 



served the purpose exceedingly well, if the experimentally determined 

 value of yu. given above was used, and if the constants c and /• were 

 determined by means of values of p between 0.1 mm. and 0.01 mm. of 

 mercury, and the corresponding values of /. 



The pressures chosen for the determination of these constants were 

 0.0239 mm. and 0.0150 mm., the former of these being a pressure 

 actually measured by the gauge, while the latter was chosen arbitrarily. 

 The measurement of I corresponding to the former was 0.00936, while 

 the value of / corresponding to the latter was 0.00620, and was obtained 

 from the curve as described above. The values of the constants calcu- 

 lated from the above data are 



c — 0.1491 

 k = 0.0676. 



The equation now takes the form 



0.0676 



( 



/ — 0.000020 



- 1 j» = 0.1491. 



How well the equation gives the relation existing between p and / can 

 be seen by a comparison of the numbers in the second and third columns 

 of the table. A number in the third column is obtained by choosing 

 a value of / from the first column, inserting it in the equation and 

 deducing the value of p, which is then placed in the third column in the 

 same horizontal row as the chosen value of I. It will be seen that the 

 various numbers in this column agree very well with the corresponding 

 numbers in the second column, except at the very lowest pressure, 

 0.000016 mm. used, when the difference is about twenty-five per cent. 



9 Verhandl. d. Deutsch. Physikal. Gesellsch., 11, 1 (1909). 



