( 858 ) 



otlior i-cspects. Putting- (='=: JO -'^ wc «irrivo at a value of M^ wliicli 

 does not deviate very much from that of Thomson. 



Rosa and Cohen') calculate for the self-induction of a circle with 

 /? = 25 cm. and r = 0,05 cm. 



L = 654,40496. 

 For this same circle we find 



Z' = 0,01, 



so if we neglect L' we make in this case only an error of ± 0,002"/^,. 

 This value applies to bismuth, for other metals the correction is 

 [»robably much smaller still. The correction is also relatively smaller 

 when we have not one circle but many windiugo. On the other 

 hand the correction is much greater if we take a thinner wire. 



Notwithstanding the perfect agreement between the numbers of 

 Thomson and of Dkude these values do not seem very reliable to me. 

 It is therefore not superfluous to inquire whether we can find another 

 way in which we might evaluate L'. Perhaps this might be done 

 as follows. 



Hagen and Rubens') have shown that the reflective power of 

 metals for infra-red light of large wave-length can be explained by 

 ascribing to these metals the same conductivity for electric vibrations 

 of the considered frequencies as for stationary currents. This seems 

 to indicate that the mean free path between two collisions of an 

 electron against the atoms of the metal is small compared with those 

 wave-lengths "). As the same does not apply to light of a wave- 

 length smaller than one micron, we should be inclined to deduce 

 from these optical properties that the mean free path is not much 

 smaller than one micron. 



We. cannot deny that this value of the free path is remarkably 

 great, as we find a value for the free path for the molecules of 

 the air at a pressure of one atmosphere, which is about 10 times 

 smaller. But let us notwithstanding assume this value of the path- 

 length to be correct, then it yields a new method to calculate L'. 

 We find, namely, for the conductivity of a metal : 



2AV/ 



Ö =: n- 



where u is the mean velocity of the heat-motion of the electrons, 



^) Hagen and Rubens. Berl. Sitzungsber. 1903, \). 269. Bei'. d. deutschen pbys. 

 Gesellsch. 1903, p. 145. 



~) Comp. H. A. LoRENTZ. These Proceedings V, p. 6ü6, 1903. 



