4 THE ATTAINMENT OF VERY LOW TEMPERATURES 



which we may theoretically divide the stream no longer form straight 

 lines, but break up into eddies. As the result of this the heat passes 

 more rapidly from the gas to the surface in contact with it, or vice 

 versa, than it does when the " stream line " flow is maintained. It has 

 been stated that for the efficient working of a boiler both the furnace 

 gases and the water must " scrub " the surface separating them. 



The formation of eddies in a fluid depends upon its velocity, viscos- 

 ity, density and on the form of the aperture through which it is flow- 

 ing. The greater the velocity when once the " critical velocity " is 

 passed, the greater the tendency to form eddies, which also increase 

 as the space through which the gas is flowing diminishes. Further, 

 it is well known that when a gas strikes against a sharp edge or angu- 

 lar obstacle eddies are formed much more readily than when the stream 

 of gas meets a curved surface. 



The difference of the behaviour of the two regenerator coils re- 

 ferred to on page 2 may now be explained. The first coil consisted 

 of a single copper tube, so that the velocity of the compressed gas flow- 

 ing through it was twice as great as that of the gas in the second coil, 

 each component of which was half the length. Further, since it was 

 impossible to wind the double coil as closely as the single coil, the 

 velocity of the expanded gas passing over the outside of the pipes was 

 greater in the case of the first coil than in that of the second. As the 

 tendency to form eddies in the gas increases with the velocity, particu- 

 larly when the stream passes through narrow openings, such as exist 

 between the flat spirals of the coil, it is not surprising that the efficiency 

 of the second coil was lower than that of the first. 



I was fortunately able to test this theory by means of trials carried 

 out on two air liquefiers of the Hampson type. In one of these two 

 copper tubes were wound together to form the regenerator coil, and 

 in the other four copper pipes of the same diameter were similarly 

 wound. The difference in the behaviour of the two liquefiers, which 

 were otherwise identical, under exactly similar conditions was suffi- 

 ciently marked to prove my point. The two-coil liquefier gave from 

 2 to 3 per cent, more liquid air than the four-coil liquefier, and whereas 

 in the first case the difference between the temperatures of the air as 

 it entered and left the apparatus was 0.4°, it rose in the second case 

 to 1.4°. 



It appeared, therefore, that in constructing regenerator coils the 

 pipe should have as small an internal diameter as is possible, and that 

 the coil should be closely wound. It is practically impossible to use 

 copper tube of a diameter less than 2 mm. inside and 3.5 mm. outside 

 on account of the mechanical difficulties which arise in joining the 

 sections, and in winding the coil. The spacing of the spirals is theoret- 

 ically limited by the fact that the glass vacuum-vessel enclosing the 



