192 Sir J. J. Thomson on Conduction 



is a consequence of a change in the mechanism of the dis- 

 charge, as the high speed cathode rays make their appearance 

 with a consequent increase in the amount of Rontgen radia- 

 tion in the tube, that an unknown factor which is of small 

 importance at high pressure predominates at the lower 

 pressure, causing the length of the field to be independent 

 of pressure. 



I have much pleasure in recording my thanks to Professor 

 Sir J. J. Thomson for suggesting this work to me. 



East London College. 



XIV. Conduction of Electricity through Metals. 

 By Sir J. J. Thomson, O.M., F.R.S* 



THE investigations of Kamerlingh Onnes on the resistance 

 of metals at the temperature of liqnid helium have led 

 to results which are of vital importance in the theory of 

 metallic conduction ; they have shown, for example, that 

 some metals can exist in a state where their specific resistance 

 is less than one hundred thousandth millionth part of that 

 at 0° 0. The transition from the state in which the resistance 

 is diminishing normally with the temperature to the one 

 where they possess this super-conductivity takes place 

 abruptly at a definite temperature, and the difference in the 

 electrical properties of the metal above and below this 

 temperature are as well marked as the difference in elastic 

 properties when a solid melts, or in the magnetic ones when 

 a piece of iron passes through the temperature of recalescence. 

 One of the most remarkable effects discovered by Kamer- 

 lingh Onnes is that when a current was started in a small 

 ring of lead at a temperature of about 4° absolute, by 

 bringing a magnet close to it, the current instead of dying 

 away, as it would have done at 0° C, as soon as the magnet 

 was stopped, went on with practically undiminished intensity, 

 its rate of decay being so slow that Kamerlingh Onnes 

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

 initial value. 



This power of transmitting a current for long periods 

 when no external electromotive force acts on the metal is 

 one that has to be accounted for by any theory of metallic 

 conduction : any such theory must indicate that in certain 

 metals a change of electrical state takes place at a definite 

 temperature, that above this temperature the current dies 

 away almost instantaneously after the electromotive force is 



* Communicated by the Author. Read before the Physical Society 

 of London, June 25, 1915. 



