12 THE ATTAINMENT OF VERY LOW TEMPERATURES 



low pressure by opening the expansion valve wide, in which case there 

 is a steady fall in pressure throughout the length of the coil, a very 

 much smaller yield of liquid is obtained than when the valve is kept 

 partially closed and supply of air is reduced. It appears that though 

 a slight fall of pressure in the gas between the top of the coil and 

 the expansion valve does not materially effect the working of the 

 machine, it is a factor which cannot be neglected in estimating its 

 efficiency. 



VI. Application of the Perforated Discs to the Hydrogen 



LiQUEFIER. 



I now constructed a hydrogen liquefier, of which the regenerator 

 coil A, with the perforated discs, formed part. This machine did not, 

 however, yield satisfactory results, for though I obtained some liquid 

 hydrogen by means of it, the yield was poor. I may add, however, 

 that the liquefaction commenced very soon after the gas was first al- 

 lowed to expand. Further, experiments conducted with compressed 

 air, of which it is not necessary to give the details, pointed to the fact 

 that the conduction of heat down the coil was considerably decreased 

 by the introduction of the perforated discs. 



The probable explanation of the behaviour of this coil is as follows. 

 The effect of the discs in producing eddy currents increases the rate 

 of heat interchange, so long as the temperature difference between the 

 compressed and expanded gas is not very great, thus increasing the 

 rate at which the coil cools down at the commencement of the experi- 

 ment. When, however, the temperature of the expanded gas reaches 

 the liquefaction point (20.5° abs.), the temperature of the compressed 

 gas is still above the critical temperature, and we arrive at a condition 

 similar to that which obtains in the lower part of the coil of an air 

 liquefier. It appears then that the effect of the perforated discs in 

 shielding the coils from contact with the expanded gas is greater than 

 in increasing the eddying and thereby the rate of heat interchange. 



VII. Final Form of Hydrogen Liquefier. 



In reviewing my experiments with the hydrogen liquefier I am 

 driven to the conclusion that the form of regenerator coil used in the 

 construction of my first machine gave better results than any which 

 I afterwards obtained. In this machine the coil was constructed of 

 a single tube wound into a coil 150 mm. long and 50 mm. in diameter, 

 the spaces between the components of each helix being as narrow as 

 possible. 



It appears that the small diameter of the coil and the closeness of 

 the spacing are more effective in producing a rapid heat interchange, 



