990 



Under these conditions the result was 



Ö, = 0.0091 ') 

 which gives (as 7' ==40.06 and /i:= 3.95 + 26.12 = 30.07) 



L', z= 0,0137, 

 so that the torsional moment of the friction on the sphere alone 

 amounts to : 



L' — L\ —L', = 0.1311. 

 The density of liquid hydrogen at the temperature of observation 

 (20°.43 K) being 0.0708*), it follows, according to equation {a) of 

 Comm. N". I486, that 



7]=: 0.000130 



with a degree of accui'acj which may be placed at about IVo- 



A further experiment was made from which a preliminary value 

 could be derived for the viscosity of the hydrogen vapour. The 

 oscillations of the system carrying the sphere and cylinder 6', were 

 observed, while only a little liquid hydrogen was left on the bottom 

 of the vessel, the sphere thus being just above the liquid surface 

 in the vapour. In these circumstances a damping was found with 

 a decrement of 0.0128, whereas the system without the sphere and 

 with cylinder C^ gave d, = 0.0093 (at the same room-temperature 

 of 18° C and the same pressure of 76.9 cms). The decrement due 

 to the friction on the sphere alone is therefore 6^ = 0.0035 ; hence 

 with r=40, K='^0 and ft = 0.00119. 



ij = 0.000010'). 



1) This result was obtained from a few experiments at different room-temperatures 

 and different pressures, so that a reduction could be made to the same temperature 

 (8.7° G.) and pressure (76.9 cms.) as in the experiment with the sphere in the 

 liquid. These experiments indicated a small increase of 6 with the temperature, 

 the change with pressure being insensibly small 



2) Comp. Gomm. 137a. 



3) This preliminary value agrees well with that found by H. Kamerlingh Onnes, 

 G. Dorpman and S. Weber by the method of transpiration (Gomm. N*^. 134a). The 

 accuracy of this result was further tested by a few observations made in air (at 

 10° C). These gave for the system with cylinder (73 and sphere {\he latter 

 suspended in a large vessel) : J = .0448 and for the system without sphere, but 

 witli cylinder Cj : y = .0154, so that for the sphere by itself: (J = .0294, giving 

 (Z = 30, 2' =40, fx = .00126) », = .000177, in good agreement with known data 

 (comp. Phys. Rev., 8, p. 738, 1916). 



