THE ATTAINMENT OF VERY LOW TEMPERATURES 7 



the air liquefied. To determine the temperature gradient in the coil 

 the ends of thermo-electric junctions, carefully insulated, were placed 

 at the top of the coil, near the valve, and at two points equi-distant 

 from the end and well within the coil. Copper-constantan junctions 

 were used; the potential difference between each of the junctions in 

 the coil and a junction immersed in ice was measured by the usual 

 potentiometer method ; the results were interpreted with the aid of 



+ 20 



-20 -40 -60 -80 -100 -120 -140 -160 -180 

 Fig. 4- — Temperalure gradients. 



-200 



comparative measurements of the electromotive force at temperatures 

 corresponding to — 190°, — 78°, and 100° on the hydrogen scale.^ 



The exact arrangement of the apparatus is shown in figure 3. The 

 regenerator coil A was completely enclosed within the vacuum vessel 

 B, the space between the coil and the glass wall of the vessel being 

 carefully filled with flannel, which was wound in thin strips round the 

 former. The brass tube C surrounding the valve rod D, the supply 

 pipe G from the compresser, and the tube H through which the ex- 

 panded air escaped, were soldered into a brass cap F which was con- 

 nected with the vacuum vessel bv means of the rubber sleeve K. The 



'Bradley and Hale (loc. cit.), working independently, obtained similar results 

 for a Hampson machine with two coaxial coils. They used platinum resistance 

 thermometers inserted between the coils. 



