( 2(H) ) 



Uy ail elec'li-ic motor '). Hy this means a powerful cireulation is 

 obtained. 



Only tlio upper i)ortion of the cryoslat glass is silvered. At the 

 level of Ihe inlet valve the silvering ceases, so that one is able to 

 control the action of the valve and to observe the level oflheli(|uid 

 meniscus in the cryostat glass. A vertical strip on each side of the 

 vacuum glass A"7> which contains liquid hydrogen and surrounds the 

 the cryostat glass is left unsilvered so that the position of the liquid 

 helium in the cryostat glass may be seen. The vacuum glass A""6' 

 with licpiid air which })rotects the liquid hydrogen is treated in the 

 same way. Vaporization is chiefly due to entrance of heat from 

 above by conduction and by absorption of radiation by uncooled or 

 insuflicienti}' cooled portions of the a|)paratus (e.g. cover, resistance 

 thermometer leads, etc.). 'The radiation, too, which is admitted 

 through the transparent parts of the walls as well as the radiation 

 from above can be absorbed by objects which are in conlaci 

 with the licpiid helium (this seems to be particularly the case with 

 the opaque or metal parts of the apparatus), and can give rise to 

 considerable evaporation '). For many experiments the rapid evaporation 

 occasions great difficulty. 



In the case represented by the accompanying figure (experiments 

 of April 1911) the cryostat contains a. the helium thermometer 77iJ' 

 (with capillary 7%,,"); f>- the gold resistance thermometer i2.i„ (an 

 insulated gold wire wound on a glass cylinder); c. the mercury 

 resistance S2j-/,j''). To make it possible for the mercury to solidify and 



1) It is peculiarly charming to see this litde pump ejecting the light liquid over 

 its upper edge when the level of the liquid helium sinks a little below it. 



2) The whole of these radiation and absorption phenomena at exlreniely 

 low temperatures IcmI to the conviction that in bringing a measuring instrument 

 that receives radiation to a low temperature, a wide field of investigation is m.ade 

 easily accessible. In this connection it may be mentioned that according to the 

 well known formula A™ r=0.294 cm. deg., at the lowest helium temperatures 

 Am becomes approximately 2 mm. (at the melting point of hydrogen a value of 

 0.2 mm. is already realised) ; the heat radiation, therefore, given by the wave- 

 length Km is almost identical wilh the actual Hertz oscillations of small wave 

 length (4 mm.) realised by Lampa. We may also mention that at 4° K. at which 

 temperature vibrators of wave length 0.5 mm which seem to play an important 

 part in electiical resistance come to rest, A,» becomes 0.7 mm. approximately. 



•^) The estimate of the frequennj of the resistance vibrators in mercury (which 

 gave a = 30, see Comm. N'. 119 B, § 3 note under table, Proc. March 1911) 

 was obtained by paying due attention to the fact that the situation of the chief 

 points on the cu)ve defining resistance as a function of temperature seemed to be 

 determined by the melting point — a fact already commented upon in Gomm. 

 N". 99% Sept. 1907). 1 thought it clearly suitable to apply the law of corresponding 



