ELECTRICAL PROPERTIES OF CONDUCTORS 587 



p = RT/v, as these latter have been assumed for the electrons 

 in the theories of Rieke, Lorentz, Plank, etc. At high tempera- 

 tures they all approximate, so that at very high temperatures 

 the difference between them may be put equal to zero. At 

 low temperatures, however, the pressure in an ideal gas 

 approaches a value which depends on the molecular weight, 

 and the density, but is independent of temperature. This 

 value is called the zero point pressure. Its value for helium at 

 normal density is found to be 0^25 mm., but it increases more 

 rapidly than the density. The theory shows that in the ideal 

 monatomic gas there will be a zero point energy, which has been 

 assumed by Plank for his newest ideas in the quantum theory of 

 electric conduction, in which the electric oscillators of the atoms 

 absorb radiation energy continuously, but only emit it by whole 

 units or quanta. This theory has been of considerable service in 

 explaining what seemed to be difficulties in the accepted electron 

 theory of conduction. According to this theory, conduction of 

 electricity is due to the motion of the free electrons between the 

 atoms of the conductor. The electrons are continually colliding 

 with the atoms, and in doing so give up current energy. It was 

 formerly suggested that the electrons could be treated as 

 molecules of a condensible monatomic gas, which would all 

 freeze down to the atoms of the conductor at very low tempera- 

 tures. This would naturally result in an enormous increase 

 in the resistance of the conductor, which, as Kelvin indeed 

 supposed, would have an infinitely large resistance at the 

 absolute zero. 



The very careful measurements which have been made with 

 platinum at various temperatures for the purpose of thermometry 

 have shown that the resistance at continually decreasing tem- 

 peratures was taking a course which, if continued, would tend 

 to become zero at some point above the absolute zero of 

 temperature, and hence that it was necessary to suppose that 

 it would attain a minimum, and then increase to infinity at 

 the absolute zero of temperature. In considering the motion 

 of the electrons it follows that the resistance to their free 

 migration increases with their velocity and their number per 

 unit volume, and is inversely proportional to their mean free 

 path. Assuming the two latter factors constant, the velocity 

 will decrease with decreasing temperature, and will become 

 zero when this is zero. However, if with decreasing tempera- 



