BODIES SMALLER THAN ATOMS. 239 



exposed to an electric force will be sent drifting alon^- in tiie direction 

 opposite to the force; this drifting of the corpuscles will be an elec- 

 tric current, so that we could in this way explain the electrical con- 

 ductivity of metals. 



The amount of electricity carried across unit area under a given 

 electric force will depend upon and increase with (1) the number of 

 free corpuscles per unit volume of the metal; {'2) the freedom with 

 which these can move under the force between the atoms of the 

 metal. The latter will depend upon the average velocity of these cor- 

 puscles, for if they are moving with very great rapidit}' the electric 

 force will have very little time to act before the corpuscle collides 

 with an atom, and the effect produced by the electric force annulled. 

 Thus the average velocit}' of drift imparted to the corpuscles b}^ the 

 electric held will diminish as the average velocity of translation, 

 which is tixed l^y the temperature, increases. As the average velocity 

 of translation increases with the temperature, the corpuscles will 

 mo^e more freely under the action of an electric force at low tem- 

 peratures than at high, and thus from this cause the electrical 

 conductivity of metals Avould increase as the temperature diminishes. 

 In a paper presented to the International Congress of Physics at Paris 

 in the autumn of last3'ear, I described a method I)}' which the number 

 of corpuscles per unit volume and the velocity with which they moved 

 under an electric force can be determined. Applying this method to 

 the case of bismuth, it appears that at the temperature of 20 ' C. 

 there are about as many corpuscles in a cubic centimeter as there are 

 molecules in the same volume of a gas at tLe same temperature and at 

 a pressure of about one-fourth of an atmosphere, and that the cor- 

 puscles under an electric field of 1 volt per centimeter would travel at 

 the rate of about TO meters per second. Bismuth is at present the 

 only metal for which the data necessar}' for .the application of this 

 method exists, but experiments are in progress at the Cavendish lab- 

 oratory which it is hoped will furnish the means for applying the 

 method to other metals. We know enough, however, to be sure that 

 the corpuscles in good conductors, such as gold, silver, or copper, 

 must be much more numerous than in bisuuith, and that the corpus- 

 cular pressure in these metals must amount to many atmospheres. 

 These corpuscles increase the specific heat of a metal and the specific 

 heat gives a superior limit to the number of them in the metal. 



An interesting application of this theor}" is to the conduction of 

 electricity through thin films of metal. Longden has recently shown 

 that when the thickness of the film falls below a certain value the 

 specific resistance of the film increases rapidly as the thickness of the 

 film diminishes. This result is readih' explained by this theory of 

 metallic conduction, for when the film gets so thin that its thickness is 

 comparable with the mean free path of a corpuscle the number of col- 



