﻿On the Theory of the Metallic State. 127 



from wind, and as the temperature should be as uniform as 

 possible, probably the best site could be found in some tropical 

 country with steep limestone cliffs sheltered from winds by 

 jungle; and it would be advisable to use an invar wire. The 

 actual form of the tape could be studied by attaching light 

 paper marks to it at known intervals to which reciprocal 

 vertical angles could be observed by theodolites mounted at 

 the upper and lower ends. The writer concludes by hoping 

 that some reader may have the will and opportunity to make 

 such tests. 



University Observatory, Oxford. 

 June 1914. 



XII. Note on the Theory of the Metallic State. 

 By F. A. Lindemann, Ph.D.* 



Tj^HE outstanding physical properties of metals have 

 JL hitherto been attributed to the presence of a number of 

 so-called free electrons in the interstices between the atoms. 

 These electrons, which are supposed to behave like a perfect 

 gas, may be made to explain most electrical phenomena in 

 metals with fair accuracy. 



The conduction of electricity is considered to be due to a 

 drift of electrons caused by the field. The electrical resis- 

 tance is accounted for by the collisions of the electrons with 

 the atoms, and Ohm's law is explained by the large number 

 of collisions in unit time. The resistance may be expected 

 to change if an alternating current of such a high frequency 

 is induced that there are no longer a large number of 

 collisions during one phase. This has been observed to be 

 the case if currents of about 10 14 periods per second are 

 induced, as is done when infra-red light is reflected. From 

 this one may conclude that an electron must undergo at 



least 10 14 collisions per second. As the resistance is ~^-.',- 



where m is the mass, N the number per cm. 3 , e the charge, 

 and t the time between the two collisions, a lower limit for 

 N may be determined. Putting in the known values one 

 finds about three times as many electrons as atoms. This is 

 approximately the same number as one finds from a con- 

 sideration of the dispersion of metals. 



Various theories have been proposed to explain the fact 

 that the electrical resistance of a metal at different tem- 

 peratures is approximately proportional to the heat-content. 



* Communicated by Prof. J. W. Nicholson. , 



