442 SCIENCE PROGRESS 



of liquid helium, and shown that some metals can exist in a 

 state where their specific resistance is less than one hundred 

 thousand millionth part of that at o°C. Thus when a current 

 was induced in a small ring of lead at 4 absolute, by bringing 

 a magnet close to it, the current, instead of dying away when 

 the motion of the magnet ceased, went on with practically 

 undiminished intensity, and its rate of decay was so slow that 

 Onnes estimated it would take four days to fall to half its 

 initial value. The transition from the state in which the 

 resistance is diminishing normally with the temperature to the 

 condition in which a metal possesses this remarkable super- 

 conductivity is abrupt and takes place at a definite temperature, 

 and the difference in electric properties above and below this 

 temperature are as marked as the difference in elastic properties 

 between the solid and fused state, or in magnetic properties 

 above and below the temperature of recalescence. This super- 

 conductivity is another, and apparently fatal, objection to the 

 free electron hypothesis, and in the paper referred to above 

 Thomson puts forward his own special hypothesis once more, 

 and shows how the effects discovered by Onnes are in accordance 

 with it. 



No permissible increase in the number of free electrons or 

 in the mean free path is of any avail to explain superconductivity 

 at low temperatures, but Thomson's hypothesis is one of 

 electrically polarised atoms and has some points of resemblance 

 with the usual theory of residual magnetisation in magnetism. 

 According to it the atoms of some substances, including the 

 metals, contain electric doublets, i.e. pairs of equal and opposite 

 charges at a small distance apart. Under ordinary circum- 

 stances the axes of the doublets in a body are uniformly dis- 

 tributed in all directions, but under the influence of an electric 

 force they tend to set themselves parallel to this force, just as 

 a magnet tends to set its axis parallel to a magnetic force. 

 However, this tendency is opposed by various causes, and so a 

 compromise is effected between the condition of uniform dis- 

 tribution and the extreme condition in which the axes would 

 all point one way. Foremost among these opposing causes is 

 the heat motion of the molecules, which, in the case of gases, 

 gives rise to collisions between the molecules, thus tending to 

 knock the axes of the doublets out of line as fast as they are 

 brought into it by the electric force. In the case of solids and 



